Electronic – Infra Red LEDs – Voltage Advice

I must confess I never tried this. But the series resistor has an important role: it is there to limit the current through the LED. If there's no resistor, the current may be limited in the end to a value which is too high for the LED or for the driver transistor. Theoretically you should graphically add the U-I characteristics of the diodes and the LED, and see on the the resulting characteristic what's the current for your Vcc value. But the main problem is that this current cannot be reliable predicted, since the U-I catalog characteristics of the diodes and LED give you an typical curve, and this curve will also shift with the temperature.

So while it may work, I wouldn't count on it working in every instance. But you may have some help from an unexpected place: the IC driving your LEDs. Sometimes the digital outputs have internal resistors or other ways to limit the output current, in order to avoid overloading them. So check the spec sheet for your attiny2313.

According to the data sheet, the 3 RGB LEDs are electrically separate. This means that you can connect them in series, using a higher voltage with fewer resistors and transistors. Nick Alexeyev's answer then applies. Assuming a 36 volt power supply, and strings of 8 for green and blue, 16 for red, and 24 for IR, total is 18 channels. I would not go with Nick's suggestion of 48v/12x strings for green and blue, since there isn't enough excess voltage for the limiting resistors to operate reliably, particularly with the Vf variations given in the data sheet. I'd expect that you'd need to measure the voltage drop of each string and tailor the limit resistor values accordingly.

What I think you've missed is power. Assuming 20mA for each LED, total power is 3.6 watts each for green and blue, 2.3 watts for red, and 1.5 watts for IR. Total power is 11 watts in the LEDs. I have no idea how you're going to heatsink this. Well, I do, but it involves using a beryllium oxide substrate for your LED PC board, bonded either to a pretty hefty heat sink, or maybe a TEC cooler. You want the LEDs to run as cool as possible for better lifetime. But trying to do it with FR4 is asking for early death of your LEDs. Similarly, you would also need to calculate the dissipation in your limiting resistors, although for the values I've given I'd expect total dissipation in the 4-5 watt range, and this can be handled with forced air cooling. And with the cooling requirements indicating a certain amount of increased size, I don't think you really need to worry about minimizing the driver board size, although at 18 channels you shouldn't have much difficulty.