There was a post on the Workshop88 mailing list asking for someone to construct a small manual light box for use by the timer at Toastmasters meetings. The specification was, a switched Green LED, switched Yellow LED, switched Red LED, and a fourth switch to blink all the LEDs at a 1 hz rate. The actual timing is done by a human with a stopwatch, the box just signals the speaker. The Toastmasters’ existing setup uses 110 volt incandescent lamps and is not very portable.
Much discussion ensued on the mailing list about what could be done with a Raspberry PI or Arduino, LCD screen, etc. etc. but in the end, I agreed to design and build something simple according to the original manual spec. Ultra bright LEDs could be used with a 555 timer to do the blinking. A standard MN1604 9 volt battery would easily power the LEDs for 8 – 12 hours. And I could use the standard Arduino style enclosure: and Altoids tin.
I’ve built two of these boxes. The photos on this page are from the second – but first came a prototype on a solderless breadboard.
On the breadboard the timer circuit was checked for reasonableness and LED illumination tested. I found with a nine volt battery I could put two LEDs in series, and with a 200 ohm limiting resistor the Red LEDs drew 15 milliamps, the Yellow 20 Ma, and a pair of Greens 12 Ma. The Red and Green were very bright, Yellow not so much. Later I checked the spec sheet on the Yellow LEDs and found they would take 50 milliamps. I lowered the Yellow limiting resistor to 130 ohms which brought the Yellow current up to 35 Ma and then all three pairs were similarly bright.
These Ultra Bright LEDs have a clear plastic envelope with a lens formed in the end which directs most of the light straight up. I sawed the tip of each LED off at a 45 degree angle to remove the lens. This directs more light to the front and reflects much more to the rear.
Here is the final schematic. This is slightly revised from the first model, the timing person wanted a separate power switch, and also wanted the blink cycle to start with LEDs on rather than LEDs off.
The original breadboard had all three of the grounds at the bottom of the 555 chip connected together, a switch from there to power negative activated the 555. In that configuration, the 10 microfarad capacitor started out in a discharged state which resulted in LEDs off. Splitting that capacitor off and hard grounding it causes it to start in a charged state which turns the LEDs on. Switching the 555 pin 1 to ground starts the blinker.
I constructed the 555 module on a bit of perf board with copper pads on one side. A six hole by seven hole piece holds all the parts. This is the layout sketch I used.
Here is an assembled module. It measures 3/4″ by 5/8″.
With the blinker built and tested, I turned to physical construction of the box. Everything should fit in the lid. I made an aluminum template for drilling the ten holes. In this photo, the four switches have been mounted and there are small holes drilled above the nut for the anti-rotation washer.
The LEDs don’t have any formal mounting hardware. To get maximum exposure, they are just inserted in the holes until they bottom out on the shoulder. Then a narrow strip of FR4 perf board is threaded over the leads. This photo is three strips cut from a wire wrap prototype PC board. I use a crosscut sled on a table saw for this, with a narrow carbide blade centered on the fourth row of holes. Width of these strips is important because they help restrain the battery.
The fiberglass PC board strip is tacked down with bits of bent paper clip soldered to the tin lid. That paper clip in the center had to be moved later because it interfered with the battery. Soldering the LED leads in the perf board creates a very rigid assembly. I added the current limiting resistors between the LED pairs.
Spacing between the switches and the LED retaining strip is critical because the nine volt battery has to fit there, but not rattle around. A large paper clip was straightened out, then bent to capture the battery. A paper clip has just enough spring to hold the weight of the battery. This photo shows the battery clip soldered onto the lid. White arrows point to the four Z shaped wires holding the LED retaining strip in place. Ground leads are soldered to each switch and each has been cabled to it’s assigned LED pair.
Here is a photo with a battery installed in the clip. You have to be careful not to short the terminals on the lid lip.
At this point the lid is ready to receive the 555 timer blink module. It is mounted with two more bits of soldered paper clip, this time bent into an L shape. One clip is soldered into the ground hole of the perf board, the second in a vacant hole. White arrows in this photo point out the two module mounting clips also three of the LED restraining strip clips. Note the two retaining strip clips nearest the rim are bent parallel to the lid edge. This is to give clearance for the box bottom.
The last piece of hardware installed was the power switch. I used a small slide switch as that type will be less likely to accidentally turn on in somebody’s pocket. Two eighth inch holes were drilled, then squared up with a small file. I soldered the ears of the switch to the inside of the box.
Finally the wires for power were added, everything tested, and all loose wires laced up with waxed dental floss. A bit of foam tape was added to the bottom to help make sure the battery clip doesn’t come loose. There’s a small paper clip loop soldered next to the power switch to take up strain on the wires there.
Here is the finished box with all three LED strings lit. Note this is not a normal condition, only one color at a time should be on, mostly the Green which only draws 12 Ma (but is so bright it hurts your eyes). Blinking will draw about 60 Ma half the time. Duracell’s data sheet shows a useful life greater than 8 hours with a 50 milliamp draw so I expect this application to last considerably more than that.
This has been an interesting project that was well received by the users. There is a short video on my Dropbox account.
Added: May 27, 2015
I needed a way to get consistent bevel cuts on the modified LED lenses using my bench grinder which is far quicker than the Dremel tool with cutoff wheel method. The solution as any woodworker would know, is to make a jig. In this case though I was able to re-purpose a fixture I made years ago to hold hand plane irons while touching up the bevel. It’s just two pieces of 3/8″ x 3/4″ steel bar stock with half a dozen holes drilled. One piece is tapped, screws through the other then clamp the plane iron in the jig and it can be held tightly to the edge of the grinder’s tool rest.
Adapting this fixture to hold LEDs only required adding a washer slightly thicker than the LED wire leads to one end. I had to grind the underside of the threaded bar to get clearance for the grinding wheel. Two LEDs can be slid into the beveled end and clamped there by the screws. A single long screw through one of the threaded holes at the washer end provides a stop that can be held against the grinder tool rest.
I can tweak the grind angle by adjusting the tool rest, and a few passes across the grinding stone produces the consistent bevel I wnt. Just have to be careful not to grind away too much. I put a dab of nail polish on the ground surface to clean up the scratches.