Current limiting the new LED's

I originally bought resistors to use with the RGB LSDiodes.com but I realized the voltages and currents might be different. Indeed, the 3mm red/blue and 3mm green LED's require different values. I calculated the results with either a 4-volt or 5-volt supply (which I'll probably use.)

Color

Voltage

Current

Volts
across
resistor

Desired Resistor

red

2.2 V

20 mA

2.8 V
3.8 V

140 ohms
190 ohms

green

3.3 V

20 mA

1.7 V
2.7 V

85 ohms
135 ohms

blue

3.2 V

20 mA

1.8 V
2.8 V

90 ohms
140 ohms

Anyway, I tested using 150 ohm resistors for blue and green and a 180 ohm resistor on red. This balanced things so each LED was driven at about 30mA with a 7.5V source. The 150 ohm resistors dissipated 0.14 watts and the 180 ohm dissipated 0.17 watts, but with 20 mA, the figures are 0.06W and 0.08W respectively, allowing me to use 1/8W resistors. I also noted that I can use two layers of Scotch tape to diffuse the light nicely. I checked the circuit with 5 volts — green was 18mA, blue was 20mA, and red was 22mA — very close to ideal and easy to fix in software, so I went ahead and bought packs of 150-ohm and 180-ohm resistors.

LED's with bit modulation

I met a guy named Dave who referred me to an idea for driving LED's with a microcontroller using a shift-register and latch with a technique called "bit modulation." It's a derivative of pulse-width modulation except it's much better suited to driving multiple outputs concurrently.

In traditional pulse-width modulation (I'll assume it's all for LED's although it can apply to any averaged output) the LED is driven at full-brightness for some percentage of the time at a relatively high frequency (enough that the human eye's persistence of vision can't detect the blinking.) So, for instance, to create the illusion of 10% brightness, the LED is turned on for 1 unit of time and then off for 9 units of time. Usually the frequency of the wave remains the same and only the duty-cycle changes.

In bit-modulation, the desired modulation is assumed to be a binary value. The least significant bit (2^0) is read and the LED is turned on or off depending on its value for 1 unit (2^0 units) of time. The same is done for bit 1 (2^1) only for 2 units of time (2^1.) This continues for successive bits. The resulting brightness output averages to the value over 2^(n+1) clock cycles — the illusion works as long as the LED is on for [duty-cycle]% of the time and off for 100%-[duty-cycle]% regardless of how many times the LED is turned on or off during a cycle.

The huge advantage here is that a shift-register with a latch can be used to set the values for any number of LED's in a very short amount of time. That is, if you have X LED's, the same bit from each of the X LED's desired duty-cycles can be shifted in to the register, the latch can be triggered, and then the CPU will wait for (2^bit) time units until the next bit is loaded.

Unfortunately, I'm using a system where the red, green, and blue anodes are on three wires and the common-cathode of each LED cluster is brought back on one wire — so for X LED's I need 3+X wires. I dismissed the idea — although really cool — because it just wasn't practical for what I wanted to do.