I am trying to read a speed signal from a tractors transmission control unit. Its 3.3v and 1-2khz pulse. I'd like to optocouple it and have a few 4n37 available.

As I really dont want to screw up the vehicle electronics I'd like to verify what I am planning to do.

The 4n37 needs a forward current of 10ma so I'll put a 220 ohm resistor in series there.

On the arduino side can I just use the internal pull up resistor and nothing else to read the opto?

Is the 4n37 fast enough? Turn on/off time is <10us so I am thinking it will be fine.

Anything else I need to watch for?

 

Actually on rethinking this I think optocoupler is overkill as both will be using same power source and so not really isolated. I think I can feed the pulse straight to arduino inputs.

 

Better to use opto, it will help in clamping the ignition noise to a safe level.