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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)

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    \$\begingroup\$ Use another opto-coupler fed from 3v3 through an appropriate current limit resistor. Or use an NPN BJT instead. \$\endgroup\$ – Andy aka Jun 8 '17 at 9:09
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    \$\begingroup\$ put up a schematic, 'designed to interface' could hide a multitude of sins! \$\endgroup\$ – Neil_UK Jun 8 '17 at 9:11
  • \$\begingroup\$ @Neil_UK It's just a CMOS gate input pulled up to 12V via a 4K7 resistor \$\endgroup\$ – Dirk Bruere Jun 8 '17 at 9:15
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If you want to amplify a 3.3V logic signal to 12V, this will do that as long as the low-voltage logic can sink a couple of milliamps

schematic

simulate this circuit – Schematic created using CircuitLab

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You could use something like this

schematic

simulate this circuit – Schematic created using CircuitLab

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.)

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    \$\begingroup\$ Thanks. I get the idea - I'll try and see how well I might implement it. Since other parts of the system look like they may be undergoing a redesign new opportunities arise. Backward compatibility is a pain. \$\endgroup\$ – Dirk Bruere Jun 8 '17 at 9:55

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