Another way how to drive an open collector/drain logic 3.3V or 5V signal line (active LOW) with MOSFET in a circuit powered by 3.3V or 5V. can detect the 5V presence in 3.3V powered circuit as well. The resistor between MOSFET gate and common ground discharge the gate when voltage is disconnected. The switch below LED emulate activity of some other device connected to the same signal line.
Sometimes this circuit may be cheaper or components - more available instead of op-amp or Schmitt trigger. The MOSFET should be small N-channel signal MOSFET like BSS138 or BSS170.
