Can anyone please say why this tutorial for an Arduino-based auto-ranging ohmmeter calls for PNP transistors to switch between resistors for the voltage dividers rather than NPN transistors? If you were to use MOSFETs instead, would you need to use P-channel instead of N-channel? I have some N-channel MOSFETs with low Rds(on) at low gate voltage.
Yep P channel MOSFETs would be used. Note they are keeping a common ground, not your meter's +5 to ground on other equipment. This helps keep the smoke contained.
An Arduino pin already has P-channel mosfets built-in, so Q3,4,5 are not really needed. Just connect those three pullup resistors directly to the Arduino pin, and switch them on/off by setting the pin to INPUT or OUTPUT/HIGH with pinMode. Q1,2 require more current, so use external mosfets there.
Leo..
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