First off the nerd kit is great! I've already finished my first goal of charlieplexing LEDs to make a snow falling effect we are going to use for our community theater's nutcracker this year.

With that said goal #2 for me is making something that I can detect sales in my vending machines for a lot lower cost than current commercial offerings. Basically you have a motor matrix (similar to what you would use to multiplex leds) so I'll need to detect a positive voltage across 6 possible wires and negative voltage across 12 possible wires.

My best idea right now is to wire optocouplers in series with the motor power lines and using that to get my reading. Now I don't know the voltage or the current that the motors draw necessarily since this will be used in multiple types of machines so the optocouplers seem to be a good option.

Now my question - I don't want to spend $4.00 X 16 to get optocouplers that handle the high current. I want to limit the current going to the optocoupler so I can use the $0.40 - $0.50 optocouplers. This is the first real electrical circuit I've designed to read anything.

So I'm thinking that I can put a higher value resister in series with the optocoupler to limit the current to this, but to connect the optocoupler input and output with a low value resister bypassing the optocoupler so that most of the power will continue on the line to power the motor but enough power will go through the optocoupler to be detected without frying it.

Is this going to work or is there a better way?

Thanks