The statistics for the Super Bright LEDs used are as follows:
| Color | Wavelength | Vf (volts) | I (mA) | Intensity (mcd) |
|---|---|---|---|---|
| Red | 630 nm | 2.0 | 20 mA | 3,000-6,000 |
| Yellow | 588 nm | 2.0 | 20 mA | 2,000-3,000 |
| Green | 525 nm | 3.5 | 20 mA | 6,000 |
| Blue | 470 nm | 3.6 | 20 mA | 2,000 |
I arranged the LEDs so that no current limiting resistor is required. Looking at the schematic, a regulated 12 volts being applied to the circuit via the 7812 Voltage Regulator. The voltage drop across the collector-emitter of each TIP 120 Transistor is approximately 1.5 volts. So the voltage available to the LED module is about 10.5 volts.
Using this 10.5 voltage supply, one can place five red or yellow LEDs in series across this voltage. This would put a voltage drop of 2.1 volts across each individual LED. This voltage drop is well within the 1.7 - 2.4 voltage range quoted in the LED data sheets.
Three blue LEDs in series will place (10.5V / 3 LED = 3.5V drop) a 3.5 voltage drop across each LED. Again, within acceptable ranges for both the blue and green LED's according to the data sheets.
LED Matrix
I positioned the LEDs, using the stated forward voltage, as the voltage drops across the diode to create each LED lighting module. I tried to create a diverse and even lighting as possible. This is what I came up with (see figure 5, below). The 1st LED matrix is what I used in the experiment. The 2nd LED Matrix is the one you should use.
I cut a portion of a radio-shack prototyping printed circuit board (RS# 276-170) to mount the LEDs.