Ordering some of the parts

I noted that the full-spectrum LED's I'm looking at from LSDiodes.com have the following voltages: 2.05 volts for red, 3.2 volts for green, and 3.6 volts for blue; all at 35 mA. Thus, to be driven at full brightness from 5 volts, I'll need an 84 ohm resistor (the closest standard value is 82 ohms) a 51 ohm resistor, and a 40 ohm resistor (closest being 39 ohms.) The 84 ohm resistor will drop the most amount of power at 0.1 watts, so pretty much any size is safe. While I'm at it, driving one of these LED's with all elements on at full brightness is 310 mW, so if I'm to limit the lighting to 6 watts or so, I can only use about 20 LED's at a time … of course, I can just program it so it's not trying to turn on all the LED's white at once.

I put in an order at Mouser.com for the resistors (I had to swap the 82-ohm resistors for 75-ohm since they were out of stock) a bunch of 10 µF capacitors (I already have 48 so I bought 50 more, expecting to wire in no more than 32 LED's) and a bunch of 2N3906 PNP general purpose transistors that I'll use with 74LS138 3-8 multiplexer chips to drive the cathodes of the LED's.

Complicated capacitor LED driver model test

I decided to do the "LED experiment:" how fast can I charge a capacitor with a silicon 2N4123 transistor current follower — something like 10 µF — and how long will it for its charge to dissipate through the LED? In other words, how many LED's can I continuously refresh using this technique? My 20 mA LED that drops 2.2 volts will need about a 340 ohm resistor. It takes about 240 µs to charge a 5 µF capacitor to nearly full voltage — about 500 µs to squeeze in the last few millivolts. Dissipation takes only 650 µs to fall a full volt.

After a few iterations, I settled on a Darlington follower which drops the charge-time to 20 µs. Given a 1 µF capacitor, the rise time is below 10 µs and the decay runs for about 215 µs. If I move up to a 5 µF capacitor, the rise time stays below 10 µs but the decay jumps to 650 µs. If I then move up to a 10µs capacitor, the decay of 1 volt loss jumps to 1.4 ms.

Comparing to the prior pulse-width modulated test, I found the duty cycle needs to be about the same for the same brightness: 28 µs of the 2.1 ms cycle time or 1.3%, so I can theoretically drive 75 LED's in a perfect world.