WB8NBS
This Blog will document projects I have
in progress or completed. You can search for my
handle on Flickr, Facebook, YouTube, or Picasa
for photos. I will link some of those here.
WB8NBS
My Projects
Arduino Based Voltage Logger
Twice this year I have gone out to my garage and found the car’s battery dead. It’s a C-Max hybrid and when that happens, nothing works. You don’t start a hybrid, you boot it up, the computers control everything – even the door locks. Based on googling, this is a common problem with the 2013 model. Apparently, some times the radio doesn’t shut off when you park the car and the battery is soon dead. I have seen this mode twice, once the radio wouldn’t turn on and once it wouldn’t turn off. The fix is fairly simple, pull and replace the fuse. A power cycle brings the radio back to sanity. Maybe a coincidence, but the software was written by Microsoft.
So now it’s winter, I’m recuperating from surgery and the bicycle is hung up until spring. I needed a COVID lockdown project I could work on indoors. I decided to build something to monitor the state of the car’s low voltage battery. A hybrid has a high voltage battery to drive it’s motors and a low voltage battery to run accessories. There is no alternator as on a normal gasoline powered vehicle, instead the engine turns a generator (one of the two motors reversed by magic) to pump up the 300 volt traction battery. There is a DC-DC converter that steps the high voltage down to charge the conventional 12 volt battery. My project would need a measurement range of 0 to 18 volts, measure periodically and log the results with a time stamp to an SD card. I will leave the device connected for months at a time. It would be better to measure idle current drain but I don’t know how.
I bought an Arduino Logging Shield from Adafruit (product 1141). It plugs onto an UNO, provides an SD card socket and a Real Time Clock based on the NXP PCF8523 chip. I also bought a 128×64 pixel OLED display (product 326). I have used the OLED before in Arduino projects and like the flexibility. The display is small but very readable. The UNO I’m using is a Sparkfun RedBoard. It has a perfectly flat bottom which made mounting easier.
I tested the three main outboard components separately by using demonstration programs included with each respective library. The RTC and OLED worked out of the box but I had a hard time with the SD card. I had missed soldering on a six pin female connector that plugs on to the Arduino ICSP pins. Adafruit steals an SPI connection from that port, which is needed to talk to the SD card. Oh well…
On to software. A sketch was quickly put together that would read the RTC and write a time stamp to the SD card. No problem. But when I added the OLED to the mix the Arduino crashed. Apparently, the OLED library and the SD card library won’t fit together in the meager 2k of RAM on the ATMega328 processor. The SD library has a lot of code to handle a FAT file system. The OLED bit maps each pixel in RAM which by itself is 128 * 64 / 8 = 1024 bytes of memory. No wonder the processor barfed. So lowering my expectations, I hogged out the one inch hole I’d made in the plastic case to 3 inches wide and fitted a more familiar 16×2 Liquid Crystal Display.
I had an I2C adapter for these LCD modules ($3 from All Electronics catalog LCD-SI). Soldered it onto the LCD and connected to the I2C terminals on the Logger Shield. Didn’t work. After an entire afternoon checking wires and downloading different LCD libraries (apparently the All Electronics part has a different pinout than the Adafruit equivalent), I ran the I2C_scanner program in the Wire library examples. Turns out that the All Electronics board has a different I2C address, 0x3F instead of 0x20 which was in the documentation. Once the correct address was set, the LCD worked as advertised. All Electronics’ backpack has one advantage over some of the other vendors, it has a switch transistor on board that can turn off the back light on the LCD module saving some power.
Operation:
Three buttons are installed, Function, Up, and Down. These let me move around in the settings menus. The switch componants can be seen just above the LCD module. My usual practice is to solder a 0.1 uFd capacitor across the switch, ground one side, then wire a 10k pullup on the Arduino side. I have a function that analog reads the switch state and returns high or low. It is a very effective de-bouncer which is a good thing with these cheap button switches.
On a reset, or applying power, the logger boots up by reading the last log interval and file name from nonvolatile memory. Unless the settings menu is requested, it will enter continuous timed measurement mode. The display will light for two seconds then goes dark and the processor sleeps for the rest of the current log interval.
This is what the records look like on the SD card. Here the interval is set to thirty minutes. The Comma Separated Value format is easy to import into a spread sheet. It only takes about six mouse clicks to produce a usable graph of the measurements.
jbh@junkbox-2:~/Desktop$ cat /media/jbh/LOGGER1/G.CSV 2020/12/06 20:34:42,09.1 2020/12/06 21:04:00,09.1 2020/12/06 21:34:00,09.1 2020/12/06 22:04:00,09.1 2020/12/06 22:34:00,09.1
It turned out that programming read and save voltage readings was the easy part – about 65 lines in the sketch. More fun was doing setup menus. I wanted:
- to set the Real Time Clock
- to set the logging interval
(5 sec, 10 sec, 30 sec, 1 min, 5 min, 10 min, 30 min) - to set the log file name
restricted to letter.CSV where letter is A-Z to make things easier - to list a directory of the SD card and to browse any file
This forms the rest of the sketch, about 800 lines (so far). There were serious memory problems getting it all running. I have in fact ordered an Adafruit Metro M0 which has much more RAM, but with moving some code into functions it is working OK now on the RedBoard. (update: The sketch will require minor changes to compile on the M0).
Setup mode is triggered by holding the Function button down on reset. Arduino will not check the buttons while sleeping, pressing the reset button wakes everything up. In general, a short press of the Function button selects things, a long press exits the task. Up and Down buttons move through the menu items.
At this point, releasing the Function button displays the setup menu categories. Pressing the Up or Down button will move through the list. Pressing Function will enter the option currently in the display. There are four choices:
SD Directory:
This section allows scrolling through the SD card directory and optionally examining saved records in a selected file.
The Up/Down buttons scroll through the directory of files on the SD card. File names and sizes are shown:
A short press of Function will open the file in the display and begin displaying records, the most recent shown first. This screen is displayed on entry for two seconds:
Followed immediately by the contents of the last file record. The information on the top line is: record number, first letter of file name, and the voltage measured. The second line shows the date and time the measurement was recorded. I couldn’t fit in the year.
A Function long press exits the file browser. Another long press exits the directory option and returns to the main menu. You can select another menu category or do a third long press to exit Setup altogether and begin timed measuring.
New Logfile Name:
Twenty six files are allowed, A.CSV through Z.CSV. Only the first letter changes which simplified programming. A short press on Function while “New Logfile Name” is displayed enters the change code, displaying the currently selected file. Pressing Up or Down will scroll through all names possible. If a named file is already in the SD card directory, the second line will show “Exists”. Short pressing Function at that point will select the file and append future readings.
If the displayed name is not currently on the SD card, the second line will show “Available”. Short pressing Function will then create the file.
In either case a long press of Function will exit the change code, returning to the main Setup menu.
Set Log Interval:
Seven different logging intervals can be selected.
- 5 seconds
- 10 seconds
- 30 seconds
- 1 minute
- 5 minutes
- 10 minutes
- 30 minutes
Up and Down buttons will scroll through the seven choices, a short press on Function will select one of the values and automatically return to the main Setup menu. A long press will return without changing the previous setting.
Intervals are implemented in hardware. A selection is sent to the RTC chip which generates a pulse when the interval has expired. That pulse on the Square Wave output is hard wired to Arduino digital pin 3 which triggers an interrupt to wake up the processor. Selecting different intervals obviously has an effect on file size. Examples: 30 Minute interval – 37.4 kilobytes in a month, 5 Second interval – 13.5 megabytes in a month. My sixteen gigabyte SD card could easily store 5 second measurements for three years. Might be tough loading that into a spread sheet.
Set Clock:
This should not have to be done often. Code in the Adafruit RTC library detects a virgin clock (no battery backup installed for some time) and sets the RTC chip to the compile time of the sketch – which in turn should be traceable to the clock time on the computer hosting the IDE. There is a way to calibrate the PCF8523 but it is not implemented in this sketch. The only time I see changing the clock is if I move to a different time zone.
Short pressing the Function button while “Set Clock” is displayed produces this display:
There is a cursor which can be moved back and forth to the digit that needs changing by short presses on the Function button. It is under the ones digit of seconds in the above photo. Use the Up and Down buttons to make needed changes, then long press Function to finalize and return to the Setup menu.
At this point I have not implemented the hard wire connection to the car battery. It will require some sort of protection circuit and scale the voltage down to the 0-5 volts needed at the ATMega328 analog read pin (A3). I also have to find a way to hide the logger in the back of the car. Thus this is a work in progress.
Input Circuit:
Cars are notorious for large transients on the battery buss. I’ve seen quoted figures from +100 to -75 volts, the worst is seen when the alternator is actively charging and the connection to the battery is suddenly lost. So I spent some time working up a circuit to interface the battery to expensive electronics. It was suggested I use an off the shelf cigarette lighter USB adapter and I did some experiments with one. The problem is the buck converter drops out at about 8.5 volts and also if it stops bucking, you get 12 volts on the output. Also requires a third wire to pick off the battery voltage being measured.
On my experimental breadboard I replaced the auto USB adapter with a good old 7805 regulator. This produced the expected five volts with the input as low as 6.5 volts, so 7805 it is. This is the schematic of the input circuit:
D1 protects against negative transients, R1 and C1 soak up positive transients. I will add an 18 volt zener diode in the future when I find one. D2 and C3 clamp the measurement voltage to 5 volts.
These input components were glued into the box top, ugly but it works. Calibration components were added on the Logger Shield breadboard area.
Testing:
Did much testing today with a variable power supply to see how the logger behaved under brownout conditions. It ramped down good at first, but any input voltage below about 9 did not register. The processor held up down to about 6.5 volts though you could not read the LCD. At 6.0 volts everything stopped.
Thinking it over I realized that the 5 volt regulator was dropping out. That caused the analog reference used for voltage measurements to dip in sync with the dropping input so no wonder it was showing the same voltage every time. The fix was obvious, lower the analog reference voltage to a point where it would stay steady and recalibrate. Only one change to the sketch, adding analogReference(INTERNAL) in setup produced this run:
Which is pretty much what I hoped the device would do. The real fix for this dropout problem will be replacing the five volt regulator with an Adafruit Buck-Boost converter. I thought I had one of those but I can’t find it.
Installation:
I found a pair of copper clips like those on a small battery charger and attached them to the logger power cable. The problematic C-Max battery is in the rear of the car in the area where the spare tire would be if it had one. Rear mounted batterys are usual on a hybrid, which has no 12 volt starter motor and therefore a super low resistance connection is not necessary. But storing the logger where it could access the battery directly might trigger my wife if she saw it, so I picked a location where she would never go. Under the hood.
A C-Max has a pair of heavy metal terminals near the engine. These are designated points where jumper cables or a battery charger can connect to feed or drain power from the low voltage battery. There is a fuse/relay box inches away with about a 6″x10″ flat removable lid. I removed the lid and glued on a chunk of sheet metal cut from an old tape drive enclosure. A big magnet salvaged from a defunct hard drive bolted to the bottom of the logger completed the mounting. It is now stuck on the fuse box lid, happily logging the battery voltage every 30 seconds.
It’s hard to see what’s going on in this photo, but you can see the logger atop the gray metal plate. There is a zip lock baggie protecting the unit from soon to come road salt. Left and below center you can see the positive jumper terminal with the logger wire clipped on. About eight inches up the positive wire, inside the baggie, is an in-line 3 amp fuse holder. The logger negative lead can be seen at the lower left corner clipped to the chassis ground terminal that forms the other half of the jumper cable connection.
First Data Run:
The Logger has been in the C-Max for 8 days over Christmas. The interval was set to 30 seconds, the log file is 600K. During that time we made two short three-five mile trips. Here is the complete capture:
It’s about what I expected from a properly functioning charge system and I don’t think it shows any major phantom drain. This is not a precision instrument but it was calibrated at 12.0 volts against my Simpson 360, so should be close. The important thing is the trend over several days, we typically only go out twice a week.
Following is a blow up of the first peak, on 12/21 You can see it charging at about 15 volts, dropping to 12.5 while waiting in the parking lot of the grocery store.
Buck-Boost Converter:
I thought a better power converter than the 7805 would solve the low voltage range problem. The Adafruit Buck-Boost (#2190) I tried works only to 12 volts. Searching the net for a converter that would work from 2 to 20 volts input did not turn up anything. I finally found and ordered a converter from Pololu (#S10V4F5) which is spec’ed to work to 18 volts. Close enough. It is tiny, even smaller than the 7805 it replaced.
The BBC is a mixed blessing. Switching regulators trade current for voltage, basically tries to transfer power, not voltage or current specifically. The good news is input current went down at 14 volts from 30 Ma sleeping to 10 Ma. The bad news is at the low end, current went up as high as 250 Ma as the boost kicked in to maintain 5 volts. I could not get a good measurement below 3.5 volts as the power supply I have for testing would go into current limiting.
Testing revealed that the low end of the measurement range was severely affected by the series resistor installed to help protect the circuit from transients. The 22 ohm series resistor in fact went up in smoke. I did the math figuring a drop across that protection resistor of one volt would be acceptable, at 300 Ma that comes out to a value of 3.3 ohms. The 22 ohm resistor was replaced by four 10 ohm resistors in parallel, giving a comfortable margin. At the same time the input protection diode was change to a Schottky type which measured half the voltage drop of the 1N4004. These changes can be seen in the above photo. Voltage drop across the protection components does affect calibration significantly at the low end.
Here is the schematic of the final input circuit:
January 2021
Logger has been in the car from December 31 to January 31. There have been no voltage fades. Did have some trouble with Libre Calc manipulating the chart, there are 88,600 thirty second measurements in a 2.2 meg file.
Future:
I’m considering the project done for now and putting it back in the car. I am however experimenting with an Adafruit Metro M0 I bought when I thought there was no hope of the sketch working in an UNO’s 2K of RAM. Most of the code compiles but the M0 does not have EEPROM so changes are in order. There is enough memory on the Metro to add code for an ESP32 coprocessor so an email voltage alert might be possible. Would probably be a whole new build to make room. Maybe next winter.
While this device is intended to monitor a 12 volt battery in my car, it could be modified easily to measure and log anything that can be scaled to the 0-5 volt range of the Arduino (0-1.1 volts with the current analogReference(INTERNAL)). It would require changing the input scaling resistors and implementing a separate power supply. I can do this by adding jacks to the enclosure. Adding code would be a problem If I keep the ATMega328 processor. It’s now using pretty much every last byte of memory.
Two other projects come to mind, three years ago I started building a temperature controller for a modified toaster oven. The objective was to do surface mount PC board soldering, also to retemper hardened steel tool cutters. Both of these applications require specific heat and cool cycles and the logger will be ideal for graphing the temperature profile. Another back burner project I have is monitoring water level in the house sump pump. I have constructed a capacitance based level sensor and could use the logger to watch levels change as the pump cycles, sometimes every 15 seconds in the spring rains.
Revision History:
- Dec 8 2020 Initial software release Beta V07.2 dropbox link
- Dec 14 2020 Added information on the protection and calibration circuit
Changed analog reference. Updated dropbox file link - Dec 15 2020 changed prot ckt R2 to 33k to make cal less touchy. Better voltage graph.
- Dec 31 2020 Rebuilt input circuit with Buck-Boost converter
Analyzing a Sector
In March I saw an article from the Lost Art Press weblog advertising a free plan on the First Light Works web site, for making a paper Sector. Just to pass time during the quarantine. I’d heard of the Sector but never looked into it as a woodworking tool so I download and printed the design and instructions. I think I’m hooked, but how does it work? Down the rabbit hole I go.
A Sector is a very old tool to graphically lay out proportions with dividers. Based on the principles of similar triangles, it’s mainly used for scaling drawings but can be directly applied to a project if necessary. The tool opens to a wide V where you can, on the Line of Lines, see that the distance between the “6” markings is half the distance between the “12” markings, and the “4” marking separation is a third of the “12” separation. That much is intuitive, useful for scaling a drawing up by say, a third. Quick – how tall is a 7 1/2” drawer front if scaled up by a third? Set your dividers to the height of the front, open the sector so the divider just spans the “9” markings, then open your dividers to span the “12”. No math involved, and mathematicians were scarce in the 17th century.
There is also a scale labeled “Line of Circles”. It has markings for “Radius”, Diameter”, and “Circumference”. These are also intuitive proportions. Diameter = 2 X Radius and Circumference = Pi X Diameter. Linear relationships.
Similar models of the Sector are freely available on Jim Tolpin’s By Hand And Eye web site. He will sell you an 11 inch assembled model. Also Brenden Gaffney’s web site has information and a link to a useful YouTube video on the subject. Brenden appears to have an obsession with ancient measuring techniques, you can buy a ruler calibrated in Cubits from him. Brendon also teaches occasional classes at Lost Art Press on making your own Sector. The description file that accompanies the Tolpin, Gaffney, or FirstLightWorks models give several examples of a Sector in use. Gaffney’s YouTube video above, is also filled with information.
Galileo is often credited with the invention of the Sector in the 1690s, but there are versions known from at least a century earlier. Galileo perfected and expanded the idea of a proportioning tool as an aid to military operations and his design includes scales dealing with solid objects, like cannonballs. He made a hundred copies in brass of his design, which he called a “Proportional Compass”, then wrote a document on using the tool, but printed 80 copies only for those who bought one of his instruments. There were no pictures in the tutorial so it was useless unless you had the purchased tool in hand. That’s how you copyrighted things in the 17th century.
The front of Galileo’s compass carries four pairs of scales:
– arithmetic lines, 245 mm in length, divided into 260 equal parts, used for a variety of proportional calculations;
– geometric lines, for solving the following problem: given a regular polygon, find the side of another polygon with the same number of sides, but with area n times the area of the first polygon; these lines can also be used to extract of the square root of a number;
– stereometric lines, for finding the solid with a volume n times that of a given solid;
– metal lines, used, as Galileo says, to give “proportions and differences in weight between the materials indicated on them”—in other words, to determine the specific weights of metals.
On the back are engraved:
– polygraphic lines, for finding, from a side of given length, the circle circumscribed around a regular polygon with any number of sides;
– tetragonic lines, for finding the side of a square, pentagon, hexagon or other polygon with the same area as a circle of given radius, and vice versa;
– adjunct lines, “added” to the tetragonic lines for finding the square of the same area as a circular segment of given chord and radius.
The FirstLightWorks and Tolpin Sectors have a third scale: “Line of Polygons”. You draw a circle, transfer the radius to the Sector Polygon scale at the “6” division, then with the dividers pick off the correct side length to inscribe a Polygon with 4 to 12 sides inside the circle. This is very useful, but not as intuitive as the Line or Circle scales.
IF I can find a suitable hinge, I’d like to make my own copy of the Tolpin Sector. The “Line of Circles” and “Line of Lines” are linear and easy to lay out but “Line of Polygons” is decidedly not-linear. There are web sites that calculate the length of a polygon side (some giving wrong answers) but Google could not find a description that showed how the Polygon line worked. After many pages of crossed out equations and a less than satisfactory prototype, I found a geometric hint that led to a solution to the problem: How far from the pivot point is the mark for the various polygons?
Skipping over all the details, the solution is 
This is the formula from the University of Regina web site for the side length of an nth degree polygon inscribed in a circle with radius R. The formula was developed from the familiar “Law of Cosines” 
1.0
The farthest mark on the Polygon line is n = 4, a square. At this point the side length formula reduces to 
2.0 
or
2.1 
For all the other Line of Polygon markings, the general formula 1.0 above applies. The c side of those similar triangles will be half of eq. 1.0
3.0
Dividing that opposite side by x, the hypotenuse which is the quantity we are trying to derive gives again the sine of the angle C’
3.0
Now we can equate the two sine formulas.
4.0
Cancelling like terms gives:
4.0
and re-arranging finally shows:
4.1
Which gives the distance x from the Sector pivot to any nth degree polygon mark, for a given length of scale l.
So of course I made a spread sheet. What ancient tool is complete without it’s own spread sheet? Input the length of your Sector’s scale and the sheet gives distance from the pivot for each graduation. You can also change the number of graduations on the Line of Lines scale.
You can download a copy of the spread sheet from Dropbox
https://www.dropbox.com/s/8q7elf674e1omqe/Sector.zip?dl=0
More Boxes – Triangles
I’ve mastered the creation of Rectangular sliding lid boxes, and Hexagonal sliding lid boxes. What’s next? How about three sided boxes? Can’t do a sliding lid because there would be no parallel sides for the lid to slide in. I did a few lift off lid boxes last year and that might work for a triangular configuration.
I realized a triangular miter corner box would need miters cut to a 30 degree angle. This is a problem because the table saw blade will only tilt to 45 degrees. Some discussion on the Dupage Woodworkers forum uncovered two methods – tilt the blade 30 degrees from vertical and clamp the work piece vertically, the other method tilts the blade to 45 and shims up the work so the cut is at 30 degrees. The former method is used in making flag cases where the miters are at 22.5 degrees but I wanted to precisely control the width of the box sides which means using a stop of some kind and don’t see a way to implement a stop with the work sticking straight up.
So I chose the shim method. I have a cross cut sled built specifically for mitering 45 degree rectangular box corners, that could be modified for this purpose. I sawed a 16 degree ramp from a 2×8 and attached it to the sled. I quickly found my homemade flip stop T Tracked across the back fence wasn’t going to work. I used the cutoff from making the ramp to add a bit of T Track at the same angle as the ramp. The flip stop has to drop slightly below the ramp surface because the work is cut face down and the sharp arris would slide under the end. There’s a shallow rabbit to allow that to happen.
The ramp was made to 16 degrees to provide a small amount of blade tilt range so the cut can be tuned. It works well and later a DeStaCo style clamp was added to save fingers.
Calculations for sizing the components are not complicated, basically dividing the problem into a series of 30-60–90 triangles. I assumed the construction would be from a single board, though it is certainly possible to use a different board for the lids than for the sides.
It’s very important for the stock to have parallel edges and a uniform thickness. The critical measurements are the board width which determines the lid size, and the board thickness which along with width determines the length of each side. I made a spread sheet. A 1/8″ allowance for saw kerf works well for me, but I use a Diablo 7 1/4″ thin kerf blade that makes a cut only .066 wide. Your Mileage May Vary.
To make the sled work, you cut the side pieces square, then bevel them later. Dimension the side blanks 1/8″ longer than the sheet specifies to give a bit of leeway in the crosscut jig. I used my miter gauge with an aux fence and stop added to make these cuts.
Once the first side blank is dimensioned, it can be used to set the flip stop for the rest of the sides. Keep in mind you might be making many of these boxes so the stop setting can be used over and over.
The next set of cuts with the blade straight up is for making two lid blanks. I trust the 60 degree click stops on my miter gauge, and do this free hand after drawing an initial line on the board.
Five prepared pieces result ready for miters and rabbits.
The next few steps cut the thirty degree miters on each side piece. I use a Wixey angle gauge to set the blade angle which as discussed above, works with the ramp to make the correct bevel. I found after considerable trial and error with my first box that a blade angle of 45.6 degrees is best for my sled. A tenth of a degree make a noticeable difference in how the box sides fit together so accuracy is important. I use the small square to ensure the Wixey is perpendicular to the table.
An initial bevel is cut on all side pieces with the flip stop raised. Here you can see the DeStaCo clamp I added in use.
Now mark one of the side pieces for the position of the second bevel. (5.086″ here from the spread sheet) Position the side piece on the sled, drop the flip stop, and sneak up to the mark by making thin cuts while moving the stop. Check the overall length.
It’s a good idea (though not necessary) to keep the side pieces in order so the grain flows nicely around the box. Lay the three sides down, align them, and use a marker to print dots on adjacent bevels. Those dots will come in real handy when you have glue applied to the bevels and have to quickly assemble the box. Also, this is a good point to decide and mark which edge will be the top.
With all six bevels cut, you can dry fit the box and see how your angles worked out. It doesn’t get any better than this.
All the remaining milling is done with a 1/8″ wide blade with flat teeth. I use one of the side blades from my Freud Stacked Dado set. A wide ATB blade will work but not look as good on the finished box. Note here that it’s best to make the bevels first then do the grooves which minimizes tear out of the grooves.
The fat blade is swapped into the saw, a zero clearance insert installed and blade height set to 1/8″. The grooves for the lid plates in this project are assumed to be 1/8″ and that is built into the spread sheet. Practically though, I make the lid plate grooves a few thousandths deeper that 1/8″ to allow for wood movement.
Bring the saw fence up to 1/8″ from the blade and we are ready to cut grooves for the top and bottom lid plates in all three side pieces.
Note that one groove uses the top edge as reference, the second groove uses the bottom edge as reference. This is why it is important the stock has a uniform width.
A similar blade set up is used to make the rabbits around the edge of each lid blank. The lid is held vertically against the fence, so the height of the blade sets the depth of the tenon, the distance between blade and fence sets the thickness. I always make two passes as the wood has a tendency to ride up over the blade.
Initially set the fence a little too far from the blade. This will allow sneaking up on the final tenon thickness. You want the lid to float, it should fit easily in the groove but without a lot of play. Once a good fit is found, run all the lid edges. Short grain edges first, long grain edge last will minimize tearout.
Make a small mark on one of the sides to indicate how far down the inside surface of the lid comes.
When the lids are fully rabbited, you can do a dry fit of all five components. Check that the miters can fully come together. If they don’t, it’s probably because the lid plate is a bit too big or the rabbit not deep enough. Fix with a shoulder plane or tweak the rabbit set up on the table saw.
The inside surface of the lid plate should have a small amount of clearance around the edge in case the wood expands. You can check this by dry fitting the lids one at a time or just set the lid in it’s groove. There should be a small space between the inside corner of the lid and the start of the groove.
Now comes the interesting part, cutting the two grooves that will nest together after the box is glued up and cut open. This is an upside down detail of what it will look like. The lid plate fits in the groove at the bottom, the lip of the sawn off top is formed by the middle groove and the lip on the sides is formed by the top groove. The bottoms of those two grooves must meet precisely. Between middle and top grooves is a small bit of wood where the assembled box will be sawn open.
Measure from the top edge of a side to the previous mark that indicates how far down the inside of the lid comes.
Add about .020 to that measurement and set the saw fence to that dimension away from the blade. This will be the position of the groove that forms the lip on the sawn off lid.
Paying attention to which edge you previously selected as the top edge, cut the groove in all three pieces.
Now to position the final groove that will form the lip on the side pieces, set the calipers to the bottom edge of the lid lip groove plus the width of the blade you will be using to cut the box open plus .020″. Set the saw fence that distance from the blade.
Carefully measure the distance from the bottom of the lid lip groove to the outside surface of the side piece. Set the saw blade height to that dimension. If all goes well, the inside of the next groove will coincide perfectly with the inside of the lid lip. It may be best to make this dimension a few thousandths short which will allow for fine tuning of the lid fit later with a shoulder plane.
Cut the final groove. Note this is on the OUTSIDE of the box, the side pieces are face down on the saw table.
The stack of side plates are now completed ready for a glue up. You can do a final dry fit to check everything out.
The entire process outlined above including taking photos took about 3 hours. I did have all the jigs made and debugged but that’s an indication of how long it would take to make these in quantity.
Dry fitting or gluing up something with many parts requires a third hand. This is the fixture I use to assemble triangular boxes. It is a bit of 2×4 with a deep 60 degree channel cut in the center.
It is very effective holding the side plates together for a dry fit, and will be invaluable during the glue up. There’s a small amount of room at the apex of the V notch so glue won’t accumulate.
I also use this V block later against one jaw of the bench vise while planing top and bottom edges true.
I decided to pre-finish the lid plates. It’s always a problem wiping down pooled oil at the edge of the lid and the 60 degree corners of these boxes makes that even worse. I made a quick rack from scraps to support the parts while the Watco oil cures for a day or two.
I’m not going to cover my finishing process here, maybe that will be the subject of a future article. I’ll just point out the secret weapon – A small block covered with 1/16 sheet of wet/dry sandpaper. Wet the piece with oil and sand it in. Wipe it off not immediately but later when the oil has begun to congeal. Time the wiping right and you will have a baby smooth surface.
Using a series of jigs to construct parts helps when you are producing more than one item. You do one setup, run that on all the boxes, then move to the next setup. This run made twelve boxes.
Everything for the 12 boxes is kitted up here, so on to gluing. I use Old Brown, liquid hide glue. I use OBG for it’s long open time, easy squeeze out cleanup with warm water, does not interfere with my Watco oil finish, and most important, if you get it on your shirt it will come out in the wash. It needs to be 120-140F degrees to work well and the work temperature should be above 60F. An electric tea pot preheats my bottle of OBG, then I squirt enough to do one box into a ceramic dish that’s warmed by a modified coffee cup warmer. It’s much more controllable to apply with a cut down acid brush than squirting it on from the bottle.
Glue ups should always be organized and rehearsed, the matching dots are critical. This was my working area. Red Rosin Paper protecting my beautiful plywood bench top. Small blocks of scrap keep the glue surfaces off the paper. Rubber bands and bicycle inner tube sections do the clamping.
This is the only (poor) photo I got of the actual glue application. Paint a good bead across the bevel near the outside edge then draw the glue up to the inner edge. Make sure the area around the grooves is covered as the top will be weak if the glue is thin there. Try not to get it in the grooves themselves but it can be cleaned out later. Paint all six bevels.
Set two of the sides in the V block, paying attention to the dots. I’m usually putting the no dot faces in there but any pair will do.
Insert both lid plates, choosing the best one to be the top. I always grain align the top and bottom plates but that’s not necessary. This photo shows the top plate seated in it’s grooves.
Set the final side piece in place and slip a rubber band around the middle. I rock the box back in the V groove to do this. The rubber band will hold the box together while stretching stronger bands around the ends. Pinch the outside corners together all around, checking that the outside edges of the miters meet correctly.
I glued up all 12 boxes in about 2 hours including fussing with the Camera. Then set them all outside to warm in the sun.
When the glue has set and the clamps removed, I true up both top and bottom edges. It usually takes only one or two strokes with a hand plane. This is a number 3, always use a long enough plane to rest the heel on an adjacent edge.
With a flat and true bottom edge to use as a reference, we can now saw the box top free from the body. Seat the box bottom firmly against the fence and tweak the fence so the saw blade leaves a tiny slice of wood above the groove. In the paragraph above on grooving, we allowed an extra 0.020″ width so leave about half of that.
Proceed with the three cuts needed to free the lid. Do NOT press against the lid on the last pass, let the lid fall free else the lid will kick back and be damaged. Don’t Ask Me How I Know This.
Another way to handle the kick back problem is to adjust the blade low so it leaves 1/16″ of wood for support. Then finish the cut with a fine hand saw.
Because we allowed a little extra at the cut site, when the lid is separated there will be a thin strip of wood remaining on one or both pieces. Remove this with a hand plane, keeping the heel of the plane on an adjacent surface as reference.
Note the use of a second spacer block to keep the relatively narrow lid assembly under control.
Test fit the lid on the body. It is unlikely that it will fit perfectly. I purposely adjust the depth of the grooves so the lids are always too tight. That allows tuning with a shoulder plane at this point. Take thin shavings from the lip on all three sides of the body until the lid fits perfectly. I’m using a Miller Falls 85 here, it’s equivalent to a Stanley 78.
I’m not covering details of applying an oil finish here. I did two experiments with this run of boxes.
First I applied finish to the top and bottom lid plates before gluing up the box. That was because I could see wiping the oil down in those 60 degree points was going to be a pain. This mostly worked but I still got Watco on the lids so had to wipe them down anyway.
Second, I oiled the inside of the boxes separately with Formbys Tung Oil. It’s hard to wipe down the inside of the box with those sharp corners, and the Formbys sets up almost too fast. The inside came out OK but I notice in a few places, the Tung Oil got on the outside of the box and showed light spots later after applying the Watco Natural. Won’t be doing that again.
This is a family picture of the twelve mass produced boxes and the three prototypes. They came out well though I made a few mistakes along the way, resulting in some boxes being shorter than others. This is a novel design but too impractical to make a large quantity. Ted Baldwin, one of the Box Gurus on YouTube says you should never make something with corners so sharp you can’t get your finger in there to clean it out.
Rust Removal Using Electrolysis
Why would you do this?
If you are a person who only buys new stuff at the big orange store you probably won’t need to remove rust. Myself, I am retired and have more time than money. In other words, “Cheapskate”. I watch for garage and estate sales with old tools I can use. My garage is full of restored tools, most tuned to be as good or better than when they were new. “They don’t make them like they used to” is certainly true for woodworking hand tools.
This is my current favorite hand plane. Found at a flea market, all exposed surfaces had a good coat of rust and to further insult, someone had varnished over the rust. It now works beautifully.
Stanley 5 1/4 Derusted and Tuned
Rust removal is not limited to tools. There is an active Facebook group for people using electrolysis in large tanks to clean flea market cast iron cookware. Old car parts need it. I even saw someone had de-rusted a whole car. Near my home there used to be a restaurant “Key Wester”, in the lobby there was a Spanish cannon retrieved from the waters off Florida that had been cleaned with electrolysis. The sign said it was in the tank for months.
There are degrees of rust. I avoid tools with areas that look like scabs. Those will clean (with difficulty) but leave a pit in the surface that may or may not affect the function of the tool. An even layer of surface rust, even if heavy, will usually clean off to a usable surface.
Electrolysis won’t damage underlying iron but doesn’t actually remove the rust. It just changes the red oxide to a black form that is easily scrubbed off, so there’s still elbow work involved. Electrolysis in itself won’t remove paint or plating. But if the paint is loose, it may come off in the scrubbing. I have not had a problem with the Japanning used on hand planes coming off, though any coating that is rusted underneath may separate regardless of the cleaning method used.
Components
These are the things you need to get started.
A waterproof container large enough to submerge the subject. Best to use plastic. A metal container will possibly trigger a short circuit though I have seen people using steel tubs with good results. I usually use either a six gallon bucket or a three gallon bucket. Some of the cast iron cookware people use barrels.
Small Tank in Action
Objects that won’t fit in a bucket can be handled by making a tank to suit. I have de-rusted several full size hand saw blades in a homemade tray consisting of a plywood bottom and scrap molding for the sides. I lined the tray with plastic sheeting and laid electrodes flat in the bottom.
Saw Cleaning Tray From Scraps
A flat tray like this will only do one side of the iron. You have to turn it over every half hour or so. Also I found the plastic grid insulators left a pattern on the saw blade. To reduce that, just move the blade a bit every 15 minutes.
Hand Saw Blade Under Electrolysis
De-rusting and scrubbing can be a dirty mess so it’s best to do it outside the house. I usually move to the driveway and do scrubbing on an old plastic sign. It’s a good use for those placards left up after election day.
An anode (or anodes) at the inside edge of the tank. Anodes are connected to the positive terminal of the power supply. Steel plates are good, I have seen cut up coffee cans, discarded hacksaw blades, even rebar. Remove paint or other coating facing the inside of the tank or they won’t work. Some people warn against using stainless steel because of possible heavy metal contamination of the electrolyte. However the cookware people on Facebook recommend ONLY using stainless, and I use stainless plate salvaged from electronic equipment, bent to clip over the sides of the buckets.
Avoid aluminum and galvanized steel. Aluminum will disappear and Zinc contaminates the electrolyte.
It’s a good idea to add a layer of porous electrical insulation. Too often one of the objects falls into the tank and can short circuit the power supply. I use plastic grid cut from a milk crate over my larger anodes. Occasionally the plates need to be cleaned, so make any insulation removable. Multiple anode electrodes must be electrically bonded together.
Small Electrodes, Large Electrodes
A source of Direct Current. This can be from 3 to 24 volts, an automotive battery charger is typically used. A small one with 6-15 amp capacity is fine, lots of these show up at garage sales. Try to get one with a meter so you can see if you are drawing too much or too little current, and bonus points if the charger has a six volt setting. Switching down from 12 to 6 volts is an easy way to bring the current into range if you mix the electrolyte too strong.
Battery Chargers
Some newer chargers have short circuit protection built in and will not start if they don’t see at least some voltage on the leads. The charger above on the right has an “Activate” position that will source current no matter what into a dead battery. Also watch out for really old chargers that have Selenium rectifiers. They work but are inefficient and if you short one long enough to blow out the rectifier you can’t believe how bad it smells. DAMHIKT.
There are people using cast off PC power supplies for electrolysis but that’s way beyond the scope of this article.
Electrolyte to make the solution electrically conductive. The classic electrolyte is Arm and Hammer Washing Soda (Sodium Carbonate) dissolved in plain water. Many supermarkets carry it. Baking soda (Sodium Bicarbonate) works but is more expensive. People also use Borax but I have not tried it. All these are non-toxic, you can pour it down the drain or on the lawn when you’re finished.
Sodium Carbonate
The starting recipe is one tablespoon per gallon of water. That’s about a handful in five gallons. Washing Soda doesn’t dissolve easily in cold water, if you heat the first gallon it will dissolve better. I fill the tank half way then pour in water heated in an old coffee pot. Stir until the Carbonate dissolves then fill the tank the rest of the way. Undissolved powder left in the bottom of the tank will eventually go into solution because the mixture warms from the heating effect of the current which gradually increases the power draw, possibly overloading the charger long after you’ve started it up.
Scrubbing tools. Again, electrolysis doesn’t remove the rust, it changes it to a form that is easily scrubbed off. Most chemical de-rusting methods are the same in this regard. My favorite scrubbing tool is a brass bristled brush. Stiff enough to remove black oxide but not so stiff that it scratches cast iron. Brass won’t round over edges that shouldn’t be rounded over, like the mouth of a plane. Rotary steel wire brushes will do that. Brass brushes used to be common in the barbecue section of hardware stores, but I only see steel these days. Another source is suede brushes from a shoe repair shop.
Stiff plastic brushes may also work, and Scotch Brite scrubbing pads are used often.
Scrubbing Station
Keep a bucket of clean water next to the scrubbing area. When a piece of iron is scrubbed clean it will flash rust again very quickly in the air. The easiest way to avoid re-rusting is to store the cleaned parts submerged in fresh water. That may be counter intuitive but it keeps oxygen away from the metal until you are ready to dry the piece.
Scrubbing with a brass brush when the part is still wet with carbonate solution will transfer a small amount of brass to the surface of the iron giving the part a goldish cast. I like it, I think it looks antiquey. If you don’t want color, just rinse the part good in clean water before scrubbing. I scrubbed the saw plate in the above picture with a copper brillo pad and, surprise, the saw has a copper tint now.
Set up the electrolysis tank. So far we have a bucket full of Carbonate solution with anode electrodes around the inside surface. The iron components to be de-rusted must be carefully hung in the middle of that solution so they don’t touch the anodes. I use a board with fat copper wire threaded through numerous holes, this forms the cathode and is connected to the negative terminal of the charger. The board has screws near the ends spaced so it can be wedged onto the sides of the bucket, providing some security against parts accidentally moving and causing a short circuit. Battery clips soldered to the Copper buss support smaller items while larger parts like a plane body get dangled from their own wire wrapped around the cathode buss. A large part may be drawing several amps so you need at least #18 wire. Remember, positive to the outside anodes, negative to the tool.
Cathode Hangers, Large Tank & Small Tank
Another caveat, the process is essentially line of sight from the anode to the part. If you have one part shadowed by another part, it won’t get cleaned well, so you have to arrange dangling objects carefully.
Once the parts are in the tank make one last visual check for potential short circuits. You won’t be able to see through the soup when the process is running. If you’re sure every thing is separated, start the charger and read the current. A good initial amount would be 2/3 rated current as it will increase a bit as the solution warms up. If the current is not high enough, add more carbonate. You can just stir some in, but a better way is to make a hot, concentrated solution in the coffee pot then pour some of that into the tank. If the current is too high you can either lower the voltage or add water to the tank to dilute the carbonate.
Six Gallon Tank
Small Tank in Action
It will take from one to six hours in the tank to get most parts back to an clean state. I usually remove parts every hour and scrub them a bit just to see how the reaction is going. Unlike some chemical methods, it doesn’t hurt to leave the parts in longer than necessary.
After the final scrubbing, one at a time pull the parts out of the rinse pail and thoroughly dry them with towels, heat, and compressed air if there are any holes. Apply oil or paste wax to the bare iron to inhibit future rusting.
Other ways to de-rust an iron object.
Search “electrolysis rust removal” on Google or YouTube. Many chemical methods use a mild acid. Vinegar is popular, it’s usually 6% acetic acid and sometimes salt is added. Naval Jelly works quickly, it contains phosphoric acid. Christopher Schwarz mixes up a citric acid solution, you can get citric acid powder from Amazon. Acid techniques will eat away the underlying metal if the part is left in the soup too long. Electrolysis uses a basic solution which is more iron friendly.
Evapo-Rust is the chemical mentioned most often. Nobody knows what’s in it, the MSDS says “Proprietary non-hazardous chelating agent”. It supposedly will not attack bare iron if you leave it too long.
Mechanical methods remove rust quickly but run the risk of also removing or scratching the iron. Years ago I restored my very rusted garage sale table saw with Wet/Dry sandpaper lubricated with WD40. Scraping the rust with a razor blade is not perfect but is good for something like an old saw where you want to keep the etch intact. A rotary wire brush in a grinder works quickly but will erode a cast iron part. If you don’t care about flat surfaces and crisp edges go for it.
This is an estate sale plane I’m using as a before/after electrolysis demonstrator. The parts were laid on their side in a shallow pan half submerged with a sheet anode in the bottom. I masked the still rusty side with duct tape before scrubbing.
I thought the plane was a generic piece of crap when I bought it but after revealing the Zenith logo stamp and some research, I find it is a very nice tool – Sargent OEMed to Marshall Wells Hardware Co. It’s identical to a Sargent 14C which was only made from 1910 to 1918 and well worth restoring.
Zenith Electrolysis Example
Zenith Cleaned Side
Zenith Before and After Bottom
Method for Sharpening Hollow Plane Blades
I recently acquired two wooden beading planes, a 1/4 inch and a 1/2 inch. A beaded edge on a wooden project is decorative and more resistant to damage than a plain square edge. Planes that do beading are not particularly rare, this one came from the MWTCA tool show at Garfield Farm August 4, 2019.
Quarter Inch Beading Plane
This is what a beading plane does. The bottom beads are single, the top two have been cut in from each of the two sides. The small notch that forms the inside of the round is called the quirk.
Quarter Inch Beads Cut
A Stanley 45 came with seven beading cutters, 1/8″ to 1/2″. Because of the design of the skates, a Stanley 45 bead has a fat quirk. Wooden bead planes make a more desirable, delicate profile. Here are the blades from my two beading planes. See how the profile lines up with the bottom right or the top left bead in the above picture. The small flat part on the left edge cuts the quirk and it, plus the concave part, must match the profile of the plane’s bed precisely. The quirk is extra important because it is the first thing that touches the wood and guides the plane through the rest of the cut. Thus it receives heavy wear and most beading planes have hard boxwood reinforcement there.
Half Inch and Quarter Inch Beading Cutters
Shaping the Cutter
The half inch beading plane had a seriously abused cutter. The quirk part had been improperly sharpened so the first order of business was to grind that back to a proper square edge on the bench grinder. Inserting the cutter back in the plane with the quirk edge aligned showed the concave part was now about an eighth of an inch too high.
Shaping the concave edge was done by scribing the bed profile on the back of the blade, then using a rotary die grinder. I have also used a Dremel tool on Stanley 45 cutters with a similar process. It’s important the tools not move, so the grinder was secured in a jig I made years ago for a different purpose.
Grinder Support Jig
Some fussing with shims was required before the grinding point was parallel to the workbench.
Checking that Grinding Point is Parallel to Workbench
Now that the grinder is true to the bench, I can make a 30 degree block to hold the cutter at the right bevel angle. This 30-60-90 block was made with dozens of cutoff corners from making hexagonal box lids. I used to use it as a bicycle wheel chock. It just needed a 3/8″ spacer to bring the ramp up to the grinding point. The small bit of cherry under the left edge registers against my workbench to hold the block true.
Note that not all cutters are made to 30 degrees. Stanley 45 beaders are 35 degrees and you have to make a ramp to match that or you will spend a long time grinding.
Thirty Degree Cutter Support Block
A photo of the ramp from the top. Black stains are from grinding swarf. The pine spacer has been planed off to match the ramp surface.
Thirty Degree Cutter Support Block
All the pieces are together and I could reshape the concave part of the half inch cutter. I noted cheap grinding points are not straight or very round so spent some time with a diamond grinding wheel dresser to clean it up. It took about an hour to remove an 1/8″ of hardened edge. Have to be real careful around the quirk, and don’t burn the steel.
Shaping the Curved Bevel
Sharpen and Hone the Cutter
Shaping is not sharpening. You need finer grits to create a cutting edge. Flattening the back is the same exercise as for any other plane blade or chisel and I honed the quirk by setting the blade up in a jig. These narrow blades are not happy in the cheap side clamp honing jigs. I found the best way to clamp one up was to lay the blade flat on the rods that connect the two sides of the jig. Check after every adjustment that the cutter is still tight against the rods. I honed the quirk first by using the jig just at the edge of my stones. Then did the flat on the other side, then the concave edge.
Bead Cutter Locked in Side Clamp Honing Jig
Here’s how to set the side clamp jig for the proper angle. Lock the protractor at 30 degrees. Hold as in this photo, when the edge of the protractor is flat against the back of the cutter and the beam of the protractor just touches the roller, the jig is at 30 degrees. Slide the cutter back and forth until this happens then tighten the clamp screw. This technique will work for any plane blade or chisel.
You have to do this for each of the three edges on the beading blade, one setting will NOT work for all.
Adjusting the Guide for Proper Angle
Now to the actual honing of the concave surface. Clamp a dowel a bit smaller than the curved opening in the bench vise. Then hold a quarter sheet of sandpaper around the dowel with one hand while sliding the blade back with the other hand. Repeat, repeat, repeat. I have sandpaper from 50 to 1500 grit available, mostly wet/dry lubed with WD40 and will work up through most of the grits. Remove the burr from the back on a 4000 grit waterstone after every grit change.
Just keep that roller on the dowel and it works well. Be careful not to remove much material from the inside of the quirk, else the plane will bind badly.
Sanding the Curved Bevel with Honing Guide
Smaller dowels will flex in the bench vise so I made three wooden bridges for additional support.
Collection of Dowels for Various Hollow Blade Sizes
Here I am using one of the bridges to support a dowel rod.
Using a Support Stick for Thinner Dowels
It’s a real joy to use a plane that’s properly tuned and beading planes are no exception. I’m looking forward to my next batch of boxes having beaded edges.
Tin Can Projects
Altoids tins. Aren’t they wonderful? The Standard Arduino Enclosure. I have four of them sitting in front of me on the desk mostly because I haven’t found an appropriate spot in this crowded room to store them. So I decided to gather them all and take a family portrait. There are at least six years worth of winter projects, most of them are documented elsewhere on this web log.
Clockwise from 12:00:
Morse keyer 2016:
The most elaborate evolution of my romance with the KC4IFB Iambic Keyer software. Uses a Teensy 3.2, has seven memories, LCD display of code sent, class D audio amplifier, software to monitor the two 18650 batteries, real time clock, and code practice.
Toastmasters Timing Light:
This project doesn’t contain an Arduino. Just a 555 timer and some switched LEDs. This was a commission from 2015.
Morse Keyer 2017:
Made to be small enough that I could listen to code practice while walking in the neighborhood. A 32U4 Adafruit Feather is inside.
Morse Keyer 2014:
This was the first and smallest of the KC4IFB keyers. It has an ATTiny85 inside, a 2032 battery and little else. Technically not an Arduino but programmable through the Arduino IDE. A Tiny85 sells for less than a dollar. It’s amazing what you can do for cheap these days.
Capacitance Based Water Level Sensor:
The current project, not written up yet. It will send water level measurements from my sump pump well to a remote alarm and display unit. Nothing inside but a Pro Mini Arduino. The PVC pipe is the sensor.
At 6:00
Capacity and Resistance Measurement Instrument:
2019 project, battery powered Pro Mini. Uses the same capacity measuring code that is inside the Water Level Sensor. I added programming to measure resistance as well.
PL Tone Generator
This was my first microcontroller project. There’s a Diavolino from Evil Mad Scientist Laboratories inside. I Downloaded the Arduino IDE and got it working in one weekend. It has a pair of thumbwheel switches used to select from a couple dozen sub audible tone frequencies which are used to trigger repeaters on the two meter ham band. I have never gotten around to installing it in the radio though.
Audio Adapter for Si5351 Signal Generator
Divides an RF signal down to audio frequency and filters that into a sine wave. It will go down to One Hertz easily. Based on an article in QST, there is no Arduino inside, just battery, TTL dividers, and MAX294 filter chips. It has two independant channels and is the most densely packed of all the Altoids tin projects I’ve done.
Si5351 Signal Generator:
A late 2017 project, with the 18650 battery it would not quite fit in an Altoids tin. That box was intended to hold a gift credit card. Has a Teensy LC inside for control and has three independent outputs, each can generate frequencies from 100 Khz to 160 Mhz.
At Center
Morse Keyer 2015
This was my final hack of the KC4IFB iambic code with memories, sine wave audio, sending decode, buffered PS2 keyboard, and code practice implemented. A Pro Mini inside but no battery.
I’ve had lots of fun doing these projects. Learned a lot – and the most outstanding thing I’ve learned is that the time required to complete a project is inversely proportional to the size of the box you’re putting it in. Thanks to all the people whose programs I pilfered to construct my own. That’s how Open Source works.
Arduino Based Capacitance Meter
Thirty five years ago I made a capacitance measuring meter from an article in 73 magazine. It’s built into a metal recipe card box about 3″ by 5″ and uses a timer chip similar to a 555 in different frequency ranges to apply AC to the capacitor under test. A microamp meter measures the current passed. Simple but each range has a separate pot for calibration. There’s two 9 volt batteries that seem to always be dead when you need the meter. It’s so old the Sharpie markings have faded out.
The 35 Year Old Capacitance Meter
Some time ago I built a prototype water level sensor that uses the fact that water has a very high dielectric constant. Immersing a capacitor in water should result in a significant increase in capacitance. I’ve been looking for a reliable capacitance measuring circuit ever since.
I found an article on the Circuit Basics weblog that analyzed and tested three different Arduino techniques. They found the most promising method was a sketch from the Pic Tutorials web site in the UK. Test results indicated a range from a few picofarads to 1000 microfarads. Best of all, it uses no external parts! Just two wires connected to the Arduino A0 and A2 pins. So this created a diversion from the water level project – building a stand alone C meter.
I tested the sketch on a Diavolino and on a 3.3 volt Pro Mini and it worked well. I adapted the code to output to a 16×2 LCD then started the build in an SAE (Standard Arduino Enclosure, Altoids tin) with selecting a pair of banana jacks from the junk box for the measurement connection. I spaced the jacks so I could use a standard two pin banana plug if needed, then discovered I didn’t have quite enough room to fit in the LCD. I wish somebody would make a 3.3 volt, 3/4 size 16×2 LCD. So.. I ordered a 128×64 OLED display from Adafruit. It cost twice as much as an LCD but would fit easily and could display more information.
Locating the Adafruit OLED on the Altoids lid
With the small display size, there would also be room for a couple of slide switches in the lid. While waiting for the OLED to arrive I parted out a small phone charger pack. Most of these contain a single 18650 cell and a small PC board with a charger and boost converter. I’ve used them before, they are sometimes on sale for as little as $2. The main problem is creating a hole for the USB jack and firmly attaching the PC board to the box. The holes are a drill and file exercise. Here you can see the board is tack soldered at the top of the micro connector and at the side of the USB A jack.
Soldering Salvaged Charger Board Into the Altoids Tin
I brought out the boosted five volt leads but since the final build is all 3.3 volt parts, did not use them. The 18650 itself is fastened by a soldered tin strap. I also squirted in a bit of RTV sealant to make sure there would be no movement. Both plus and minus tabs are insulated with Kapton tape as I did not want the SAE to be grounded in this design.
Soldered Tin Strap Restrains the Battery
The OLED is held in the lid by four screws and the Sparkfun Pro Mini is mounted directly on the back of the OLED by a five pin header soldered into digital 2, 3, 4, 5 and 6. These make the Data, Clock, D/C, Reset and Chip Select connections. Two wires complete the OLED power and ground – orange and green in this photo.
Showing How Pro Mini is Attached to the OLED
This is a top view of the arduino/OLED assembly. You can see power and ground wires on the right and leads to the A0 and A2 pins on the left. These are all that are required for the sketch to measure capacitance!
The Pro Mini/OLED Sandwich
First trial with the measurement sketch. Very happy with the result.
First Test with 18650 Battery
Now to put it into the SAE. A rectangular opening was cut in the lid with a Dremel cutoff wheel, filed to fit. A thin plastic layer fits across that opening to protect the OLED, and four #2 screws attach the assembly. Two DPDT slide switches fit between the Pro Mini and the measurement jacks, one switches the power leads between charger and the Arduino, the other switches the measurement jacks between capacitance pins and future resistance measuring pins. The black heat shrunk object is a 3 amp fuse.
Box Interior
Here the device is powered up from it’s internal battery and measuring a capacitor marked 4n7. Close enough for me. I have a few 1% capacitors and did better calibration later on in the build.
Working with Capacitance Sketch
Measuring resistors with an Arduino is a well developed application. Construct a divider with a known resistor and the part to measure, then use an analog input to measure voltage at the junction. I did a spread sheet analysis of quantization errors with this technique. The calculated value can be off a lot if the measurement is anywhere near the limits of the A/D reading. My code keeps the analog reading near the center by using four resistance ranges in the known part of the divider. 100 Ω, 1000 Ω, 10,000 Ω, and 100,000 Ω. This should give repeatable measurements from 10 ohms to 1 meg ohm.
Working in Resistance Measuring Mode
I also added a 10k/10k divider between power and ground of the Pro Mini. This is connected to A3 to monitor battery voltage. In this photo you can see some of the resistor measuring tree tucked under the left end of the board. The 100 Ω resistor between 11 and 12 is used to sense ground on the measurement jacks because I didn’t have a free contact on the R-C switch to tell the processor which mode it’s in. I look for a hard ground on the negative measurement jack to indicate R mode (thin blue wire). Pin 11 goes high to apply a strong pull up to pin 12 .
Note: V1.1 The sense resistor is revised to 200 Ω and connected to A7 instead of D12. This allows detection of ground in 5 milliseconds instead of 100 with no overload on the digital pins.
Pro Mini with Battery Monitor Divider and Resistance Range Tree
I calibrated with a handful of precision caps and some 3% resistors from Frys.
Calibrating
This is the “Calling It Done” shot. Resistance and capacitance measuring is working, that cap is marked 330 uF but that’s near the end of analog measurement range. The circuit is not very accurate above 200 uF.
Current drain with the display as pictured is about 12.5 milliamps, a full charge on the 18650 should run the device for a week.
Final R/C Meter
Update Jan 29:
Added code to check battery voltage and display an on screen alarm if less than 3.0 volts. That’s complicated because the reference for analogRead by default IS the battery, which doesn’t matter to the resistance or capacitance code because they measure a ratio not an absolute voltage. So… VREF has to be switched to the stable internal 1.1 volt supply and it turns out, that’s not a straight forward process. The Arduino.cc page on analogReference says: “After changing the analog reference, the first few readings from analogRead() may not be accurate.” You have to do an initial dummy analog read to get the change started then delay at least 5 millisec for an internal capacitor to equalize.
The battery measuring voltage divider is changed to 39k on the RAW pin and 10k to the ground pin. That puts the divided voltage in range of the INTERNAL reference.
Update Jan 31:
Banana jacks and alligator clips work great for leaded parts but surface mount, not so much. I made an adapter to make it easier to measure those tiny capacitors and resistors. An old ISA prototype card was sacrificed, (didn’t think I needed one of those these days), the fingers are about the right spacing and they’re gold plated. I cut out a small section and firmly bolted it to a two prong banana test plug.
Surface Mount Adapter
The small bit of epoxied on wooden coffee stirrer makes a fence to help corral the part. You place the surface mount component across two of the fingers and press down with a toothpick to do a reading. One contact finger is skipped at the right end to make a wider spaced dock for larger components like electrolytic caps. In this shot I got lucky, the capacitor stayed contacted after I released the toothpick.
Adapter in Use
Now I have to find enough pill containers to sort the hundreds of surface mount parts I’ve salvaged. That will have to wait until garage sale season.
If you are interested, you can download the sketch and a schematic from Dropbox.
25 Jan 2019 Version 1.0 Initial build.
26 Jan 2019 Version 1.1 Changed R-C mode detect to an analog reading.
Pullup resistor increased to 200 Ω, detect time reduced to 5 Ms.
29 Jan 2019 Version 1.2 Added code to read batt voltage and alarm if < 3V
Notes on the Schwarz Folding Bookstand
In June 2018 Popular Woodworking published an article written by Christopher Schwarz on making a small bookstand. It folds up into a neat package about 7″x3″x2″. The PW article shows the stand folded and unfolded, but doesn’t really show how it works. Several people including me, complained to PW about the lack of detail. PW responded by posting the entire article along with a short video of the bookstand folding and unfolding on their web log. Schwarz covers the construction well in the article. I will not repeat his details here but will write about the methods and tools I used, plus some minor changes in the design.
I had four slabs of walnut that used to be engraved commemorative plaques. They are 10″x14″ and about 11/16″ thick after I planed off all the text. There is a 3/8″ cove on all four edges and keyhole hanger slots cut in the back.
Rescued Commemorative plaque
Could I salvage enough wood from one of these to make a bookstand? Of course! Could I salvage enough to make two bookstands? Probably. Could I stretch it to three? Maybe. The first plaque I cut up did yield three bookstands but I had to glue cutoff scraps together in several places. Nevertheless, it worked and I gained a lot of understanding of what needed to be done. I proceeded to cut up a second slab after thinking through a more detailed cut plan. So if you have a 10×14 slab of walnut maybe you can use this:
Cut Plan for the Plaque
Most of the PW project is based on sticks 7/8″ wide and 3/16″ thick. The article specifies 7″ length, mine have to be a little bit shorter, 6 5/8″ because of the cove. My table saw is currently equipped with a Diablo 7 1/4″ finish blade that makes a very thin kerf, just over a sixteenth. I can cut a 7/8″ strip from the plaque then resaw that into two 3/16″ sticks with a little bit left over, or into a 3/8″ thick piece plus one 3/16″ stick. The back has some kind of finish that I planed off after the board was sliced up.
One section of the my cut plan produced a 3 1/2″ wide slab that I resawed into a 3/8″ and a 3/16″ section. The 3/8″ thick component was cut into six 1 1/8″ ledge parts, while the 3/16″ part made eight of the outside rail/stile sticks. For three bookstands I needed six ledges and 18 rail/stile sticks. Six shorter rail sticks form the foot and prop parts. The center frames consumed six 3/8″x7/8″x1″ blocks and six 3/8″x7/16″x6 5/8″ pieces for the frame stiles. I had to glue plugs and patches into some of the keyhole slots but I made it.
Three Bookstand Kits
Initially, I made the inner frame slightly wide. I cut the two inside stiles oversize then set the final dimension by laying down two of the 7/8″ wide sticks with a thin coffee stirrer in between, marking the glued up frame to that width. The approximately 1/16″ space down the center assures room for the stand to fold completely. Gluing up the center frame was difficult. I could not keep the one inch stiles from sliding around when I applied clamps. The wads of rubber bands you can see in the above photo helped, allowing me to position the four components, then apply larger clamps when the glue began to set up.
Constructing the first group of bookstands pointed out issues with the bottom rails. Schwarz shapes the bottom of the center frame as a half circle with a full 7/8″ radius. This brings the trimmed edge very close to the lower rivet counterbores and I had a couple of those break out while setting a rivet. My second batch of bookstands has a 7/16″ radius on each bottom corner, leaving more meat in that area.
Also I decided that bottom rails don’t need full half circle rounding. They are glued to the flat surface of the ledge, so these rails only need a radius on the top inside corner.
Bookstand Glued Assemblies
While finishing the first batch of stands the glue joint broke between a bottom rail and ledge on three occasions. The problem is if one of the bottom rails is rotated past it’s normal open position, the ledge will contact the inner frame and put a lot of stress on the glue joint. So on the first three I redid the glue and put nails through the bottom rail into the ledge to reinforce that point. My revised design with the bottom rails only half rounded will hopefully reduce or eliminate that weak point.
Danger With Only One Side Unfolded
All the rails and stiles need counterbored holes for the rivets. Accuracy of these holes, centered, and 7/16″ from the end is important to the finished stand folding smoothly. I built the specified fixture but not wanting to spend $20 on the counterbore bit Schwarz had, I dusted off a technique used in previous projects. Some router bits will make a flat bottomed hole. I used the 3/8″ keyhole bit seen in the middle of this photo. A spiral upcut bit would probably work as well. Some of the counterbores came out slightly off center when I used the router bit alone so I think the best procedure is:
- Start all holes with a 1/16″ pilot bit
- Mark all roundovers with a compass. Compass point fits nicely in the pilot hole
- Start all the counterbores about 1/16″ deep with a 3/8″ Forstner bit
- Change to the router bit and flatten the counterbore bottom
- Drill the pilot hole out to 9/64″
The Forstner hole will guide the pilotless router bit. Note that the top rail has holes on both ends, and its counterbores are on opposite sides. You only need to counterbore to about half the 3/16″ thickness to hide the rivets.
Fixture for Drilling Outside Rails and Stiles
Once the round overs are marked, they can be cut out. I used a disc sander for the first batch of bookstands, but the second set of three I pared to the line with a sharp chisel and refined with a sanding block. It was just as fast as the disc sander. This picture shows some of the marked round overs. Note – top and bottom pairs here are for bottom rails and have only one corner marked.
Ends Marked for Rounding
You have to glue the ledge to the bottom rail. Note – the counterbore goes on the back of the rail, and the whole thing has to be kept square. I used leftover 7/8″ bits of wood to support the front of the ledge while fussing the bottom rail flat against the ledge while gradually tightening the clamps. I’m using Old Brown LHG so it will be easier to fix if I mess up. Here are two glue ups in progress.
Ledge and Bottom Rail Glue Up
While the bottom rail glue was setting up, I drilled rivet holes in the center frames. To mark the positions I fitted a 9/64″ transfer punch into one of the rails. Then holding the frame and the rail tight to a square, tapped the punch down. I then used the drill press to make 3/8″ counterbores with a Forstner bit and ran a 9/64″ bit through for the rivets. Note – on the center frame the counterbores are on the front at bottom and on the rear at the top. Second note – If you screw this up you can make a patch from one of the thin scraps using a 3/8″ plug cutter DAMHIKT.
This photo also shows the center foot pieces in which I have pre drilled pockets based on spacing learned from the first batch of stands, 1 inch, two inches and three inches up from the bottom.
Marking for Frame Drilling
Hinges for the foot and prop that support the unfolded bookstands need to be created. I used 4 penny finishing nails instead of the 6 penny Schwarz specified in the article. This gives a little more leeway when drilling through the pivoting part. First the holes have to be laid out. I have a gauge set to exactly half the foot thickness, and scratch the pin locations from the face on both sides of the frame. Drilling halfway from each side reduces the chance of a misalignment. Mark for drilling with an awl in the gauge scratch 3/16″ from the inside edge of the center frame rail.
The prop and foot must be firmly held in position while drilling for the pins. The foot goes on the rear of the frame against the bottom frame rail. The prop is hinged on the front of the frame against the top rail. I put a spacer cut from a playing card between the parts and the frame rails which gives some clearance for the part to swing open.
Tape the whole thing together.
Holding Foot and Prop Prior to Drilling for Hinge Pin
I carefully checked that the drill press table was square to the quill. Then mounted the taped up frame in a vise and pushed a 1/16″ hole halfway through the foot and prop from both sides of the frame.
Drilling for Hinge Pins
Next removed the 1/16″ pilot bit and replace with a 7/64″. Made 1/8″ deep counterbores on one frame stile only. This allows for the finish nail head to be sunk below the surface.
Hinge Pin Counterbore
The final hinge step is to cut the head off one of the 4 penny nails and chuck that up in the drill press. Remove the foot and prop from the center frame and use the cut off nail to ream the hole made by the pilot bit. Also ream the two holes in the counterbored side of the frame.
Cut the taper in the prop stick. I just hogged off the wood with a chisel. Dry fit the foot and prop in the frame but don’t drive the nails in until satisfied with how they unfold. I had to chamfer the foot and prop edges above the pins to get satisfactory unfolding.
With all the parts drilled I could do a dry fit checking for interference between the moving parts. A few spots needed tuning with sandpaper or a block plane.
Successful Dry Fit
Each bookstand will get two layers of Watco Natural before assembly. After the rivets are installed, I will apply one more coat of Watco and finally paste wax.
Three Stands Drying
Rivets. I had no experience with copper rivets prior to this project. Schwarz says they are easy and they were for the most part. I bought 75 Tandy rivets on Amazon, the PW article listed a source for a pound which would make a hundred bookstands but I only need about four for next Christmas. I think it looks better, by the way, if all the rivets face the same direction.
Now I watched my father set rivets in sickle bar mower blades a hundred times but I could never do it right. They are normally swedged with the round end of a ball pein hammer, but because in this project the rivets are recessed, you need a tool. Schwarz used a type of nail set which I’ve never seen to reach into the counterbores. I made a punch tool from the sawed off end of an auger bit by hollowing the flat end slightly with a Dremel grinder. The hollow helps to keep the punch from sliding off.
Homemade Rivet Setting Tool
First you have to drive the burr washer down on the rivet shank. I tried two methods, both worked. The first, as shown in the article, is drilling a 9/64″ hole up the center of a hardwood dowel rod to make a setting tool. The second method uses the drill press quill to force the washer down. In the photo below left, a short piece of tubing supports the head of the rivet. The chuck is adjusted to slide loosely on the #12 rivet shank. It takes quite a bit of force to get the burr started. Note – there are lots of Youtube videos on setting copper rivets.
Set Burr With Drill Press or Dowel
When the burr is firmly seated, I cut off the excess rivet shank above the surface of the wood with a pair of tile nippers left over from a long ago bathroom project. Biting the copper part way from two or three directions distorts the shank less.
Cut Rivet to Length
I flatten the cut off shank flush with the wood surface with a rotary file bit.
Trim Rivet Flush With Surface
The PW article shows the parts being joined lying on a steel plate while the rivet is swedged. I don’t like that because the rivet head sits loosely in a counterbore, and just using a flat plate as an anvil will make the joint loose. I made an anvil from a steel rod that fits inside the counterbore, clamped that in my bench vise with the bottom end resting on one of the big guide rods. Then I support the other end of the assembly at the appropriate height with a wood block clamped in a small vise.
Swedging First Rivet
I tap the concave punch holding it at a slight angle, then move the tool to a different spot. I’m trying not to hit the rivet directly in line with the shank as that may swell the whole shank. This isn’t leather, it will split the wood DAMHIKT. Just gently form the sides until the burr washer is evenly captured and the mushroomed over part is below the surface of the wood.
Swedged Copper Rivets
I fastened first the top rails to the outside stiles. Next attached the top rails to the center frame. Finally attached the bottom rail and ledge to the frame, constantly checking that the parts didn’t interfere when folded and unfolded. It’s much easier to remove a bit of wood before the rivets are set. And the pre-applied finish needs to be completely cured or the parts may stick together – another DAMHIKT.
Swedging Last Rivet
Six rivets done and time to test the unfolded bookstand.
Assembled Bookstand Unfolded
A final coat of oil is optional, but paste wax protects the finish and shines it up. Merry Christmas to my three sisters, hope they don’t see this before December 25.
Update July 11, 2018
One of the stands in this final batch somehow got the foot and the prop reversed i.e. prop was hinged on the rear face of the frame and the foot hinged on the front face. In this condition you can’t unfold the foot to the rear as it is longer than the prop. I redrilled for the hinge pins at the correct position and in the process broke the glue joint on one of the ledges. Maybe OBG isn’t all it is advertised to be.
So I am now nailing the ledges to the bottom stiles. Thats nailing into the edge of a 3/16″ thick bit of hardwood with a very small wire nail. You must pilot drill for it. A cut off brad was not long enough to act as a pilot so I used a wire cut off a stiff paper clip as a drill bit which worked well in the drill press. I sunk only one nail near the stress point by the rivet. A number 2 screw would be better but I couldn’t find any long enough.
Here is a family photo of the final three bookstands.
Three Folding Bookstands
Update November 8, 2018
Here is a stand made by Al Jones using a laser cutter. Perfectly radiused!
Bookstand made by Al Jones – Laser Cut Components
More Sliding Lid Boxes – Hexagons
I had good success last year making simple sliding lid pencil boxes for the Dupage Woodworkers Club. My construction method is documented in this Weblog post. This spring I adapted the method and jigs to produce six sided boxes. The hexagonal box construction is very similar to the earlier rectangular pencil boxes so please refer to that post for details. Here I will describe the few differences.
Obviously there are two more side pieces to deal with. That’s the bad news. The good news is they are all the same length so the spacer is not required. I expected the glue up to be a big problem with the additional surfaces but with slow setting Old Brown liquid hide glue it hasn’t been an issue. There are two handle pieces to cut instead of one, and making the hexagonal lid plates is more complicated.
First, the math. The hexagonal lid plates are made from rectangular blanks. The length of the rectangular blank is the width divided by cosine of 30 degrees. To find the length of the side pieces, take half the lid blank width, add the thickness of the side stock, subtract 1/8″, then divide by the cosine of 30 degrees. Trust me, it works.
I’m using a Diablo 7 1/4″ 40 tooth finishing blade now, it cuts a very narrow kerf. I modified my regular cross cut sled to cut the lid hexagons. There is a batten tacked to the sled to establish the 30 degree angle. Actually it worked better to measure 150 degrees from the fence face on the obtuse side of the batten. This angle is critical. Next I added a movable stop to position the rectangular blank at the correct spot.
Hex Lid Jig Stop Down
The stop has a hinged end, as I quickly found the small triangular cutoffs would catch on the saw blade and be launched into low earth orbit. Raising the stop lets the cutoff fall free.
Hex Lid Jig Stop Raised
The movable stop has to be calibrated to match the lid stock. I draw the hexagon onto one of the blanks then the long side of the rectangle is placed against the batten with the corner touching the stop. The stop is tweaked until the blade cuts on the line. I cut the marked blank half way to see how it’s going, then loosen the stop screws and adjust. Once the stop is calibrated it’s simply rotating a rectangular blank until the four edges are cut off.
I made a you tube video of the jig cutting a hexagon. It’s the best way to see what’s going on.
Here’s enough lids to make sixteen boxes. It goes very quickly.
Completed Hexagonal Lids
Cutting the six side pieces requires a dedicated cross cut sled with the blade set at 30 degrees off vertical (60 degrees from the saw table). I use an adjustable flip stop as described in the sliding lid box post. There is a note at the end of the pencil box post for Doug Stowe’s method that does not require the stop to flip up.
30 Degree Crosscut Sled
To calibrate the stop, make the first bevel by raising the stop and bringing the stock in from the left with face side up. Note if you have a saw with a right tilt blade, these directions will be reversed.
Side Jig First Cut
Measure and mark the side length on the stock then with the stock on the right side, carefully place the mark right at the saw kerf in the sled fence. Adjust the stop to that position and cut the second bevel. Once the stop is calibrated the rest of the sides go quickly. 16 boxes will need 192 cuts. For these boxes I saved time by cutting the lid grooves in the long stock before the stock was sliced into sides.
Side Jig Second Cut
The side pieces are dot marked to maintain grain direction. Designating the two pieces with three dots for handles makes the opening side exactly opposite the starting grain discontinuity. Rabbiting the lid plates and cutting off the handles is similar to the rectangular box procedure.
Gluing the hex box is similar to gluing the pencil boxes but the assembly jig is different. It now has three sides, one adjustable to account for different sized boxes. People with six hands might not need the assembly jig.
Adjustable Hex Box Assembly Jig
This is the jig with a box nestled between the battens. It’s a dry fit with rubber bands. I use stronger bands cut from bicycle inner tubes for the real glue up.
Hex Box Assembly Jig In Action
These are the first couple of boxes made from construction pine during the debugging phase of the jigs. Cupped lid stock is more a problem with these than it was with the narrower pencil boxes.
First Hex Boxes
I made a number of boxes from Cherry. These two were specially done for the Beads of Courage project. Before slicing the sides, I glued on a beveled strip of Cherry at the top and bottom, and inset a small strip of Maple in the top edge. They are about 7″ wide.
Beads of Courage Boxes in Cherry
These are the sixteen boxes made for the club Christmas drive. Menards had glued up, 1x12x48″ Poplar panels on sale for $5, I bought two. With careful measurement and calculations each panel made eight boxes.
Completed Run Sixteen Hexagonal Boxes
Update August 2018:
Five boxes made for the Dupage Woodworkers fall Beads of Courage project. Cherry with strips of Aspen as accent.
Beads of Courage boxes
Update September 2018:
Trying a vertical pencil box design. I like it. Doesn’t take up so much room on your desk. If I make them a little bit longer, will be good to store spaghetti.
Crosscut sled not needed, just tilt the blade and use the fence. Gluing is easier because it’s all long grain but the top and bottom edges are now end grain which makes the lid slot weak. Each one of these used about 11 inches of a 1×6.
Vertical Pencil Box
Sliding Lid Pencil Boxes
Update September 2018:
I had trouble with getting the two lid handle cutoffs to align with each other. Sometimes there is a small step where the two glued on pieces meet. It is caused by a small amount of play in the vertical kerf of the old wooden miter box I use to cut off the handles. The Adria tenon saw is slightly narrower than the slot.
I have come up with a sure fire way to ensure the two handles are cut off the side pieces at an identical distance from the top. Note here, I always center the lid handles on the two sides with three dots so the three dot junction defines the front of the completed box. These two 1/8″ x 1/4″ wood splines fit precisely in the lid groove slots. I use them to index the two side pieces back to back before placing them in the miter box.
Two Hardwood Splines for Aligning Front Pieces
The splines are inserted in the top and bottom lid slots in the two front box pieces. The two pieces are sandwiched back to back.
Fitting Front Pieces Back to Back
Both three dot ends are nearest the saw handle. The sandwiched sides are shoved up against the stop, set about a half inch from the miter box kerf. Sawing down the kerf will now cut off handle pieces of identical height.
Back to Back Side Pieces Ready To Cut
When I remove the cut off handle pieces, I mark the end that came from the three dot edge of the side pieces. This helps align the handles over their original side piece at glue time, which ensures there is not a vertical grain discontinuity.
Marking Three Dot End of Lid Handle Pieces
The cut off handles are tested for fit on the lid plate tenon. I have my Miller Falls 85 rebate plane clamped upside down in the bench vise to do any tuning necessary.
Fitting Handle Pieces to Lid Plate
Update October 2018
Someone on YouTube asked how the dimensions of the rectangular blank used to make the lid hexagons were derived. I made this drawing while working through the math myself.
Analysis of Hexagonal Lid
Here is a circle with a hexagon inscribed. Almost everyone has done this with a compass at some time, draw a circle then step off the hexagon points by marching the compass around the circle. So sides of the hexagon are the same length as the radius and I have drawn one of the side-center equilateral triangles formed. Now I have drawn a rectangle (red) outside the hexagon representing the wooden blank needed to make the lid. What are the dimensions of the blank? The long dimension left to right is equal to twice the radius of the original circle. The short dimension top to bottom can found by looking at the small triangle formed by dropping a vertical line from the top left point of the hexagon. The vertical adjacent side of this triangle will be equal to the radius times cosine 30 degrees, and this is half the short dimension. Thus the total short side of the red rectangle is equal the the diameter of the circle (long dimension of the rectangle) times cosine 30 degrees.
More typically, the short dimension is known first as it is usually the edge to edge dimension of the board forming the blank. The long dimension will then be, short dimension divided by cosine 30 degrees.
When setting up the crosscut sled jig to cut the hexagon from a properly sized blank, the long side (bottom) of the rectangle is slid down the angled batten until the top right corner of the blank contacts the flip stop. The flip stop is adjusted until the saw cuts exactly to the right hand hexagon point which is the exact center of the blank’s short side. The easiest way to do this setup is to actually draw the hexagon on the first blank, then adjust the stop until the saw cuts right down the line. Initially move the stop in farther then necessary and make a partial test cut halfway down the line. Then move the stop out a bit at a time until the blade is cutting exactly on the line and the cut finishes at the midpoint of the short side. Beware of sawdust accumulating on the angled batten which will throw off the calibration.
Update May 2019
After making too many mistakes doing the box calculations, I finally built a spreadsheet for the hex designs. It is a second tab on my previous sheet for rectangular sliding lid boxes. There are three options:
- Size box based on available lid plate stock.
Use this if lid stock width or length is your limiting factor. - Size box based on available side stock.
Use this if you have a finite length of stock for the sides and want to make the largest box you can. - Size box based on desired side width.
Use this option if you want a box with a defined side width.
Note side height is not a factor in the calculations. The box can be as tall as you want.
You can download the spread sheet in both xls and ods format from
https://www.dropbox.com/s/h0ckxvwsr0komyz/SlidingLidBoxCalculator.zip?dl=0
Small Router Table
Our local woodworkers club makes wooden toys every year as Christmas presents for disadvantaged children. Most of these are band sawn shapes from 2 inch stock. They are cut out, drilled, sanded, and then we do a round over on all exposed edges. I made small router tables as dedicated tools for the round over step. This post will outline my construction though I’m using photos of the finished product.
Small Router Tables
Small trim routers are perfectly adequate for a 1/8″ or 3/16″ round over. I have a Dewalt and a Porter Cable so made two tables. I selected 3/8″ plywood for the table top as I had scraps on hand salvaged from (should be obvious) cable reels. Quarter inch ply might flex too much and a half inch thick table might require extending the router bit uncomfortably far. For this dedicated application, a 12 -14″ width and an 8″ depth is fine.
The first step is locating screw holes to mount the router. I removed the base plate and used that to mark the location of the four holes. Remember the router will be upside down so the mounting plate here is bottom side up. The Porter Cable router has an asymmetrical hole pattern so this is important.
Lay Out Mounting Holes
Both my routers have round head mounting screws for the base plate so to match, the screw holes need to be counterbored to sink the screw heads below the table surface. Do this first with a forstner bit, then drill through with a smaller bit to fit the screw threads. Don’t counterbore so deep that the screw attachment is weakened.
Mounting Holes Counterbored
Also at this time mark and drill a pilot hole for the router bit to come through the table. Just make a hole big enough for the bearing to come through, probably 1/2″ or 5/8″. The hole will be opened up to clear the cutting edges later on.
Locate Hole for Bit
I added a 3/4 inch bit of scrap to the bottom of the table to provide a boss for extra leg stiffness. These are attached with countersunk screws from the table top. The legs are set into holes drilled with a 10-15 degree splay angle. Splay is not absolutely necessary but makes the table more rigid when it’s clamped down. Leg dowels should be at least 1/2 inch thick and long enough that you don’t have to bend over to allow your glasses to focus on the bit. 10-12″ is good.
Support Boss for Legs
I used a piece of 3/4 stock to make the table feet. Notice the legs are offset from center to provide a larger area for clamps. I located the foot holes by assembling the legs into top, then marking where the splayed legs touched the feet.
Table Foot
This photo shows the splayed legs assembled and glued. The feet need to be parallel to the table. I ensured this by clamping the feet to the workbench while glue was applied. I then set a heavy weight on the table top while the glue was setting.
Table Legs Splayed
Getting a consistent splay angle for the legs is not difficult. I made this tapered jig for the drill press. The exact angle is not critical as long as all eight holes are drilled the same. 10-12 degrees is good. A clearly marked center line is important.
Tapered Jig to Guide Splay
Pick a spot at the center of the table bottom and draw sight lines to where each leg hole will be drilled. The legs will lean out exactly on those lines.
Splay Sight Lines
Photos here are from my completed table so I marked another bit of scrap to better show how the sight lines are laid out on the two bosses.
Lay Out Leg Boss
Here you see the tapered jig clamped to the drill press table. Align the sight line on the boss with the center line on the jig and drill.
Taper Splay Jig Aligns Hole
Then align and drill for the other leg.
Right Side Hole
The feet are drilled similarly. Here I used the upper boss to mark the sight line angle on the foot after finding where the holes go by inserting the leg dowels into the top boss. Note when you are drilling the splay direction along the sight line is opposite for the feet. On the top the legs lean out. On the bottom they lean in.
Once the feet are drilled, you can clamp and glue the whole thing together.
Duplicating Angle on Feet
When the glue is set up, I assembled the router to the table. The router bearing should clear the hole at the top but not yet the rest of the roundover bit.
Pilot Hole for Router Bit
With the table clamped down in working position, I start the router and slowly raise the bit. The bit’s cutters open the hole to make a perfect zero clearance opening.
Bit Raised Through Table Top
This photo shows the finished table clamped to my work bench. I sanded and sealed the top with a couple coats of danish oil, then applied paste wax. The toys slide over the bit really smooth. You have to route a bit of scrap a few times to get the bit height set exactly right.
Table Clamped to Bench
Just slide the blank into the bit until it touches the bearing then run it to the left to do the roundover. Yo need to experiment some as moving too fast will result in a ragged edge, moving too slow will burn the wood. Needless to say, keep your fingers away from the bit, and also be aware that sometimes the bit will grab bad grain or a knot and throw the piece off the table.
Routing Roundovers
Audio Adapter for the Si5351 Signal Generator
I had just finished building a three port RF signal generator when the January 2018 QST arrived in the mail. January is the annual Do It Yourself issue and there was an intriguing article by Keith Kunde, K8KK titled “A Low Distortion Digital Audio Oscillator”. The heart of Keith’s circuit is a MAX294 filter chip, a sophisticated 8th order switched capacitor filter capable of rounding off a square wave into a pretty good sine wave. The ‘294 is fed with a square wave at the desired frequency and also a clock signal 100 times the desired frequency. Keith generates his clock with a 555 timer and then feeds a divide by 128 chip to develop the base output frequency. My new Si5351 RF generator could be a crystal controlled clock source and all I would only need the divider and filter circuits to add audio capability. The MAX294 costs about six bucks each.
Goals for the Audio Adapter were:
- 20 to 20000 Hz capability
- Sine or Square wave output
- Line level output, one volt peak to peak into 600 ohms
- Be as portable as the signal generator, meaning battery operation in an Altoids tin
- Two channels
- Direct frequency readout from the signal generator display
That sixth requirement means decimal dividers throughout. If I add one additional decade to the ‘294 chip’s clock divide by 100 requirement, the total division would be 1000. When the signal generator display shows 25 megahertz, the output would be 25 kilohertz. No math necessary.
The MAX294 has an uncommitted on board op amp but it is intended as a pre or post processing filter and has weak output specs. To get line level output I decided to buffer the ‘294 output with an LM358 op amp connected as a voltage follower. Later on, this proved to be the most troublesome part of the project.
Internet searching did not turn up many choices for decade frequency dividers. I decided on the 74hct390 chip which I have used before. It’s a dual decade divider, each side has a divide by two, and a divide by five stage. Cascading these gives a divide by ten with BCD output of the counter state. For this two channel build I would need a total of ten divide by ten stages, achievable with five 74hct390s. This diagram shows the basic filter wiring.
Basic MAX294 Application
The last divider stage is a divide by two so the ‘294 filter sees a symmetrical square wave. 100k and 39k resistors form a voltage divider network to get the TTL level square wave down to an amplitude the ‘294 can handle without clipping.
I decided the project was indeed viable and ordered parts from Digikey.
Software in the signal generator was tweaked to have a lower range limit of 100 Kilohertz. The Si5351 should go all the way down to 10 Khz with appropriate software support but rather than press my luck, I added two additional decade divider stages to the design. A three position switch selects a frequency tap from the chain so the frequency ranges are:
- Divide by 10**3 100 to 25,000 Hz
- Divide by 10**4 10 to 2,500 Hz
- Divide by 10**5 1 to 250 Hz
With the additional ranges, I can still read the LCD display directly but have to mentally move the decimal point. The first requirement is met. Also added is an SPDT switch to connect the op amp input to either the filter input (square wave) or the filter output (sine wave). That meets the second requirement.
Digikey parts finally in hand, I bread boarded the circuit. The dividers are on the left, the op amp is on the right.
Audio Adapter Breadboard
This is the final schematic of the filter circuits. Eagle made the drawing and I did go through the exercise of generating a PC board just to see if there really was room for the parts. Since the project is a one-off, I built everything on perf board, using 30 gauge wire wrapped around IC pins with my standard wire wrap tool – a ballpoint pen refill. It would have been a lot faster if I had a real PC board.
Analog Board Schematic
Analog Board
A 2000 mAh portable USB power pack was sacrificed to obtain an 18650 battery, it’s associated charger, and 5 volt up converter. That would satisfy the portability requirement. I stuffed the battery, charger board, four RCA jacks and three slide switches into an Altoids tin while waiting for Digikey.
I used Eagle to generate a schematic and proposed PC board for the higher level digital divider circuits. This board had to fit between the 18650 battery and the range switches, and I had to nip off the corners to work around the battery charger circuit. This board is mounted on #4 screws soldered to the bottom of the Altoids tin. I built the digital divider board first, it is much simpler than the analog boards.
Digital Board Schematic
Digital Divider Board
I used the analog portion of the bread board to verify the digital dividers were working properly. I found the 74hct390 chips would accept a signal as high as 90 MHz. They will be loafing at 25 MHz. This photo shows the digital board seated in the bottom of the Altoids tin. Only six wires will connect to the analog board in the tin lid, 5 volt power, ground, two divided outputs from the range switches, and two audio output leads. Audio connects to 1000 ufd capacitors nestled on either side of the battery.
Digital Divider Board and Charger Installed
Now came extensive testing on the breadboard. Keith Kunde’s QST article discusses the level and DC offset considerations of the MAX294 filter when used with a single ended supply. I used a pair of 10k resistors to create a half supply voltage virtual ground and bypassed that rail with 22 uF capacitors. I’m using DC coupling between the dividers, the filter chip, and the op amp so had to do some fussing with the voltage divider parameters Keith talks about to get a filter output with no clipping. My final divider has 100k resistor in series with the TTL level divider output, and a 36k resistor to virtual ground. I also found that connecting a VOM in series with the 5 volt supply to measure current seriously upset the DC balance for some reason. It appears the final circuit will draw about 25-30 milliamps from the battery. At that rate, the adapter should run 60 hours on a charge.
But the output waveform from the LM358 buffer amp was horrible! To make a long story short, Google “LM358 Crossover Distortion”. The ‘358 has a class B push pull output stage and when the signal crosses over from one side to the other it takes a short nap. After a good bit of troubleshooting and research, the fix is simple. Add a 1000 ohm resistor from amp output to ground. This forces the output stage into a class A region and I was able to get 2 volts peak to peak out with no glitches and it will drive a 100 ohm load with no clipping or distortion. I believe 2200 ohms to ground will also work and draw less current through the amp.
Also the ‘358 has a slow slew rate which causes the sides of the square wave to lean noticeably. If I knew then what I know now I would have ordered an MC34072 amp which has better specs and no crossover problem, still less than a dollar.
The following photos are the analog board top, bottom and trial fit into the lid. The board is supported in the Altoids tin by the pot mounting nuts at the front and a single L shaped bent paper clip soldered to the lid at the back. I’ve also added an LED in the lid to remind me to turn the power off.
Top of Analog Board
Bottom of Analog Board
Connecting Digital Divider Board to Analog Board
I took a couple of weeks part time to wire and debug the analog circuits but I was finally satisfied and mounted the board in the Altoids tin lid. It JUST clears the battery and the inductor on the battery charger. This photo is as finally assembled. Note a small bit of blue clay on the charger inductor, that’s how I checked the clearance.
Interior of Finished Audio Adapter
These are the money shots of the fully assembled Audio Adapter.
Dual Channel Audio Adaptor
Dual Channel Audio Adaptor
But does it work?
This shot has sine wave selected on one channel and square wave selected on the other.
MAX294 Filter Out and In
A 10 KHz sine wave. As K8KK noted in QST, there is some clock noise on the wave form at 100x the output frequency.
10000 Hz Sine Wave
This is a 1 (ONE!) Hz sine wave. Notice the sweep speed setting on the right. It took some creative manipulation of room lights and the camera shutter speed to get this to show properly. You can definitely see the clock noise on this trace. It looks a lot like the synthesized sine waves I experimented with a couple of years ago.
One Hertz Sine Wave
The next two photos show the waveform when the two channels are combined in an external resistor network. 1500 ohms from either side to a junction, then 1500 ohms to ground to load the signal.
Combined Waveform: 10000 + 10500 Hz
Combined Waveform: 1000 Hz + 1100 Hz
Finally, here is the family connected together to the resistive combining network.
Set Up for Combined Waveform Test
This was an enjoyable but sometimes frustrating project. It’s definitely the densest thing I ever built and would be much simpler if I wasn’t too cheap to order a PC board. I learned that 0603 resistors are not a good choice for hand soldering. I learned that LM358 op amps suck. It is working great now though, and even though I have two other audio oscillators, this is a welcome addition to my test equipment stable.
de WB8NBS/9
Addendum
April 27 2018: I acquired a Brother P-touch labeler. Labels help my feeble memory. The P-touch came with a short reel of 12 mm black on white background – these labels were made in two line mode which fits well on the edge of the lid. I will probably replace them with black on clear sometime as that would better keep the Altoids theme.
PTouch Labels Added
Arduino – Si5351 Powered Signal Generator
Arduino-Si5351 Signal Generator
Device Description
For a long time I wanted a general purpose signal generator. Now Direct Digital Synthesizer hardware is available on a single programmable chip. The Analog Devices AD9851 is used in many ham radio projects, also widely used is the Silicon Labs Si5351. Either of these can be obtained from many sources such as Adafruit, Sparkfun, or on EBay and there’s lots of information on the internet. Even Amazon has them. A few dollars gets a DDS chip that will tune continuously from audio to VHF mounted on a small breakout board. I purchased an Si5351 board from Etherkit because they offer a version with a TCXO.
Silicon Labs makes the Si5351 in several variations. It’s intended use is as a multiple output clock generator with up to eight individually programmed frequencies. Most commonly available breakout boards though, use the A version with three outputs. Digikey has the bare chips for less than $1. Connectors are optional, most boards are set up for SMA female jacks. The signal generator I built brings out all three outputs but I used good old RCA phono jacks. SMA connectors are wonderful but the cables to use them are pricey. My box will be used to check and align receivers so precision impedance control is less important.
AD9851 chips have a real Digital/Analog converter on board, it uses an amplitude lookup table to produce a fair sine wave. The data sheet says:
“The AD9851 uses an innovative and proprietary angle rotation algorithm that mathematically converts the 14-bit truncated value of the 32-bit phase accumulator to the 10-bit quantized amplitude that is passed to the DAC. This unique algorithm uses a much-reduced ROM look-up table and DSP to perform this function.”
which sounds suspiciously like the quarter wave sine synthesis I experimented with a few years ago. I’ll have to revise my WordPress pages about quarter wave DDS techniques to include the phrase “angle rotation algorithm”. The price for pure sine wave complexity is, the device has only a single output. In contrast, the Si5351 outputs a square wave on all it’s ports – it is intended to serve as a programmable frequency clock source but for my purposes I don’t care and the available multiple outputs are attractive.
I breadboarded the Etherkit breakout along with one of Paul Stoffregen’s Teensy-LCs I had on hand for the controller. A Teensy-LC has an ARM Cortex M0 heart clocked at 48 Mhz. An ordinary 16 Mhz Arduino would probably work in this circuit but would have trouble with sweep. I used basic software from N6QW to get started and my oscilloscope soon showed it was working. Then a series of enhancements to the N6QW sketch followed:
- expand to controlling all three outputs
- setting up frequencies using rotary encoder and LCD menus
- save and restore setup in EEPROM
- add output power change to the menus
- add output OFF to the power menu
- add sweep capability with menu control
The Box
A thin metal gift card box was cut up to form an enclosure for the generator. It is about a quarter inch larger than the usual Altoids tin in all three dimensions. I needed the extra volume to fit in a battery and charger removed from a cheap phone power pack. These booster packs usually contain a single 18650 cell and it just fits.
Arduino-Si5351 Signal Generator Interior
The components are, top to bottom, blue 16×2 LCD board supporting the Teensy-LC. The entire unit can be 5 volt powered either from the Teensy USB jack or from the battery charger, I added a fat diode to isolate the two sources. Both the Teensy and the Si5351 board are using their on board voltage regulators. The LCD is five volt, but the Teensy-LC does not have 5 volt tolerant inputs. There are 2.2k resistors between the LCD data pins and the Teensy to smooth over that discrepancy, these would not be necessary if the LCD was a 3.3 volt unit. The pot below the Teensy adjusts the LCD contrast.
At left below the LCD is the Adafruit (Bourns) rotary encoder. It just clears the battery. Right of the encoder is a strip of perf board with the three output select pushbuttons mounted. The LCD, encoder and buttons are Adafruit parts.
At the top of the open box bottom is the salvaged 18650 cell. The measured drain of the whole circuit is less than 100 milliamps so the battery should last half a day at least. The gooey glob at the center is a 2.5 amp fuse. To the right of the battery is the on/off switch and isolation diode. The salvaged Lithium charge/boost circuit is the green board at bottom right. It has it’s own separate USB jack because the USB data leads are not accessible. To program the unit you have to plug into the Teensy USB jack.
Finally the blue board in the bottom is the Etherkit breakout. The actual Si5351 chip is at the left end, next to the TCXO. There are three RCA output jacks mounted in the left side of the box, the fourth jack next to the handle is for syncing the scope when sweeping.
Operation
On power up the unit reads saved frequency settings from EEPROM. The display then shows frequency, current port selected, and the output power setting.
Click one of the three buttons to change the port selected to display on the LCD.
Rotating the encoder knob will change the frequency digit under the flashing cursor. Press click the encoder knob to change the digit under the cursor, you can set the cursor to change digits from 1 Hz to 10 MHz.
Hold one of the port select buttons down and turn the encoder knob to change output power for that port. The chip has choices of 8 milliamps, 6 mA, 4 mA, 2 mA, and OFF. Silicon Labs’ spec for driver impedance is 50-85 ohms.
If any of the above settings are changed, the software waits ten seconds, then copies the current settings into EEPROM. In the event the unit gets confused, it can be restored to last saved settings by cycling the power switch. Also it can be returned to default settings by holding down all three port select buttons, while powering up.
Arduino-Si5351 Signal Generator Frequency/Power Setting
Sweep Function
Sweep is accessed from a separate menu. Press down the encoder knob for more than two seconds (a long press) and release to enter sweep parameters for the currently selected port. A short click of the encoder knob will advance the menu through the sweep choices, currently +/- 0, 1000, 5000, 15000, 50000, or 150000 Hz. The unit sweeps from frequency minus that amount to frequency plus that amount so the total width of sweep is twice the setting. Sweeping is done by reprogramming frequencies in 20 steps between the limits.
A second long press of the encoder knob will return to the frequency menu. The letters “sw” appear on the LCD to indicate that that port is set up to sweep. All three ports can be set up but only the port currently selected in the display will be sweeping at any given time.
A pulse is available at a phono jack on top of the box to trigger an oscilloscope at start of each sweep iteration.
Sweep parameters are not currently saved in EEPROM.
Arduino-Si5351 Signal Generator Sweep Setup Screen
The Software
I am using the Si5351 library from Etherkit. I added a function to completely disable the output drivers to save battery when a port is set to “OFF” so I will include the library in the zip file.
Most of the sketch, implementing multiple port and the menus, was straight forward. The one thing that gave me fits was the quadrature nature of the rotary encoder. This is a mechanical switch and it glitches. Not a problem if you are say, setting an analog output from 0 to 1023, you probably wouldn’t notice but if you need to change a digit by exactly one, glitches are intolerable. Conventional debouncing a switch usually just waits a certain amount of milliseconds after a transition is detected, read again and proceed. There are debounce libraries for doing this. I had no luck. I finally found an post by Oleg Mazurov about treating the quadrature pulses as a Gray code. It helped a lot and I posted about my findings in the Adafruit Feather forum.
The Adafruit rotary encoder outputs four state transitions on each of it’s 24 rotary detent positions. To get one increment per detent, I tried skipping four state reads with a software counter but it was still glitching if the knob was inadvertently touched. What finally worked was changing the Gray code decode table to disallow all but one positive change and one negative change. The allowed changes remaining are in the middle of the detent so glitches near the resting position are unlikely.
Sweep is accomplished by reprogramming a port frequency on the fly. Tests indicate this is slow and the 20 step sweep takes about 65 milliseconds which limits the sweep rate to about 15 per second. I found a very good way to time something is to write a 1 to some output pin at the beginning, then write a 0 at the end. It’s easy to see exactly how much time the intervening code took by measuring the pulse on an oscilloscope. The send_frequency function takes 3.5 milliseconds. Looking at the Si5351 library, the set_freq code is long and complex. It might be possible to speed this up by using the set_freq_manual call but I’m not smart enough to do that. Porting this code to a 16 Mhz Arduino might make the sweep unusably slow.
Addendum April 4, 2018: Amplitude Modulation Experiment
I thought I might try generating an AM modulated signal by summing three generator outputs. A 1.1 MHz signal modulated at 1 KHz would have a 1.100 MHz carrier, a 1.101 MHz upper sideband at half the carrier amplitude, and a 1.099 MHz sideband at half the carrier amplitude. I breadboarded a resistive combiner as follows:
Resistive Combiner
The Si5351 was set to equal drive power on all three outputs. I examined the output on the oscilloscope, channel 1 connected to AM Out, channel 2 to Audio Out with sync taken from Channel 2. The result is not encouraging. It does output an amplitude modulated signal but the waveforms are bizarre. I see a blocky square wave changing at a 1000 Hz rate but there is some phase problem I can’t control. This is the best I could capture:
AM Modulated Signal
Connecting an audio amp and speaker does show a 1 KHz tone if the phases happen to line up just right. I found that setting any one of the generators to be one Hz off frequency results in a rolling pattern with about a one per second beat note in the speaker.
This method of faking Amplitude Modulation is certainly not precise or controllable. Setting one of the outputs off frequency by a few Hz does give a useful warbling tone in an AM receiver.
Files
A zip archive is available including sketch source, the Si5351 library, and a schematic
Jan 4, 2018 V1.1 including the sweep function
Jan 5, 2018 V1.11 Sweep bug would not change freq while in sweep mode. Cleaned up schematic.
https://www.dropbox.com/s/hk7nc063ipjisr7/SignalGenV1.11.zip?dl=0
https://www.dropbox.com/s/hk7nc063ipjisr7/SignalGenV1.11.zip?dl=0
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