I have a board with 12V CMOS logic whose inputs are pulled up via a resistor. It was designed to interface from a board via open collector opto-coupler. Now I have a request to drive it from 3V3 logic. Any suggestions? (I have the one time option of modifying the board, but it must remain backward compatible)
You could use something like this
You don't mention operating speed so no idea if its suitable for your particular job but it does the conversion. The inverter and transistor can be whatever's used/available on your board already. If you've no 3.3 V inverters, you can get single gate ICs freely.
If you don't have/want a 3.3 V supply, you can use a second transistor inverter. (Let me know if so and I'll mod' answer.)