For those of you interested in the technical details of Daryll’s Digital Domino, you can download the DDD Eagle Schematic and the DDD Eagle Board. These versions are updated to 1.1 so the additional trace between the IR transistor and the NAND is no longer required, and the silkscreen lists the capacitor as .1uF instead of .22uF. I’ve got a couple more corrections that should go in this, so don’t send these out to a PCB fab quite yet.
How it works
Let’s look at the steps that happen when you setup two dominoes.
Step 1) You press the button on the first domino. This is simple. It connects the IR LED and the RED LED to the battery through a couple of current limiting resistors.
Step 2) The IR transistor on the second domino starts out without allowing any current. When the IR light hits it, voltage flows in to input 2 of the on the first NAND gate.
Step 3) For now we’re going to ignore input 1 on the first NAND gate. We’ll come back to that later. For now, assume that input ends up making output 3 on the first NAND gate high. Which charges the capacitor.
Step 4) If we trace the outputs through the rest of the NAND gates we see the last two NAND gates are actually tied together, and when output 3 is on, the final outputs are off. The LEDs remain off while the transistor is sensing IR light.
Step 5) The IR light is eventually turned off. There’s no more current to the capacitor. At this point the capacitor and the 6.8M resistor make an RC circuit. The capacitor slowly drains, but while it does input 5 on the second NAND stays high. The time it takes is dependent on the size of the capacitor and the size of the resistor. In our case the .1uF capacitor and the 6.8M resistor result in about 1/3 of second.
Step 6) Once the capacitor runs out, input 5 goes low and the LEDs go out.
Step 7) We skipped discussing the NAND gates. We’ve configured the three NAND gates (remember the last two are really combined) with a feedback loop to make a monostable oscillator. That means that is normally stable in one state (in our case off), but that it can change to the on state (oscillate). In our case the RC circuit forces the gates in to an on state for a brief period of time, and then the NAND gates return the output to the off state.
Field of View and Range
The range is somewhere between 6″ and 12″ inches.
Incandescent Light Bulbs
Incandescent bulbs produce a lot of infrared light. If you have a domino that’s less than 5-10 feet from an incandescent light that’s turned on, the transistor will be triggered with the IR from the light. When you turn the light off, the domino will then blink. Because the field of view for the transistor is only 20 degrees (remember it extends up and down, not just side to side) it shouldn’t have a problem with an overhead light, but a desk lamp might set it off.