Computer optical isolation interface circuit diagram

This circuit uses an optocoupler to isolate the computer from the peripherals. IC 2 is an optocoupler device; IC1 is a threshold comparator, and the threshold voltage is adjusted by P1; R2 and C1 form a low-pass filter to prevent IC1 from false triggering.

When TTL outputs the " 0" state, it lights up the primary of IC2, so its secondary is turned on, causing IC 1 to output the "1" state; otherwise, it outputs the "0" state.