Electronic – Need help with simple digital circuit that can change state and reset

Do you need to perform a specific real-time action, or just record the event?

If the former, you will never accomplish this using commodity hardware and consumer-grade interfaces. You will need both a laboratory-grade signal processor ( it will be a card that can access hardware interrupts directly) and a real-time operating system like VxWorks. A user-centric OS won't interrupt a disk write just because the USB bus wants some attention.

If the latter, get yourself a signal recorder and post-process the buffer output at your convenience.

Your specifications lack detail, but let's assume the following:

1. A single threshold voltage, the same for both positive and negative voltages.
   For example, if the threshold is 4V, then anything between +/- 4V input will give a zero, and anything more than +/- 4V would be a one on the digital output.

2. Vin can be +/- 4V to +/- 300V minimum

3. Input is isolated with no ground reference

4. No input power supply needed

You could try a circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

I've used a very similar circuit to provide zero crossing detection on mains voltages. The input threshold should be a little under 4 V with this opto.

The CPC3982 is a depletion mode FET that works as a constant current source for the opto once the input voltage is high enough. R1 should be selected to give the low current opto 500 uA. This limits the power dissipation in the FET to around 150 mW at 300 V input. When the voltage input is low (under 5 V) the FET will be turned on. At about 2-3 V the opto current will be low enough to register a zero on the digital ouptut, though this will depend on the value of R1. You can increase the low voltage threshold by reducing R2.