I am building a simple circuit to sense if three different inputs have +12Vdc on them. I am using the circuit here: http://arduino.cc/playground/uploads/Learning/Level_shifting_4_arduino.pdf.
I have to sense three different sources of +12Vdc. I am going to use three 4N33s. In the circuit at http://arduino.cc/playground/uploads/Learning/Level_shifting_4_arduino.pdf can I use one R3 across each of the 4N33 (i.e. Arduino +5Vdc -> R3 (4k7) -> Paralleled to pin 5 of OK1, OK2 and OK3. Pin 5 of OK1, OK2 and OK3 to digitalpin 1, 2 and 3 respectively)?
I think R3 is only biasing the input of the digitalpin to high, then when current flows across the opto-coupler, you get a emitter/collector flow thus dropping then input low. Is this how it works or does R3 serve some other purpose (bias for the transistor??)
Cheers for your help.