How to use electrically isolated weak signal for Arduino interrupt?

Question

I'm trying to use the signal generator from a DSO Nano v2 to drive an Arduino interrupt for testing purposes. I have measured the output of the signal generator at approximately 13mA @ 2V DC.

I have tried using a 4N28 optcoupler alone but I found that the voltage differential at the output was nowhere near enough to trigger the interrupt (it was ~0.4V), I assume there is not enough input current for the IR LED to saturate the transistor.

How can I make this work? Also, would this solution apply for other comparatively weak signal sources.

Would an opamp be able to boost the output signal?

Answer 1

@Chris it sounds like the circuit is not complete... do you have a pull up like R2?

This is what I expect to be using in this situation. The opto does not exists in CircuitLab but you get the idea. With only a 4N28 with CTR (Current transfer ratio) of 10% assuming R1 is 80 ohmsish and the diode forward drop is 1.2V, 10mA LED current, we should see 1mA sink capacity on the Output to Micro...

NOTE: The grounds will be separate in this type of circuit normally.

I like CircuitLab... just wish generic symbols were available by default...

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

An op amp would be very capable of boosting your signal if configured as a non-inverting amplifier, but even that would be overkill. You can buffer that signal with one resistor and one BJT and it would be sufficient.

However, like I mentioned in the comments, any voltage over 3 V is enough to trigger an Arduino, and there are ways of reducing that voltage to even less if you want. You should be able to connect your signal generator directly to the Arduino.