Could someone please tell me what resistor to use on the emitter on this page: (the one connected to pin 3 on the diagram half way down)http://letsmakerobots.com/node/19030
It says 39R, is that 39 ohms? That doesn't seem like a normal resistor value to use on an LED to me, but I'm not familiar with IR LEDs.
Well 39R does mean 39 ohms. However the info is incomplete, to say that 39 is the correct value or not. They don't say what the applied voltage is. Back calculating it by adding voltage drop across the resistor (Vr=.05 X 39) + (Vir = 1.7) = 3.65 volts doesn't make much sense either. Also if you read the datasheet for the device, you will see that 50ma of emitter current is the absolute maximum current rating, not the recommend current of 20ma.
So assuming you want to power this emitter with an Arduino output pin, you would calculate the resistor value as R = (5.0v - 1.7v) / .020 = 165 ohms.