PNP versus NPN sensors

[apalrd]

Originally Posted by Ether

Could a simple (and quite inexpensive) NPN transistor be made to work?

Originally Posted by Kevin Sevcik

I'm hesitant about that without my Art of Electronics to remind me about transistor fundamentals. I think that's alright, but you should have high values for the resistors. Probably dividing down till the base voltage is less than 3.3V. If I remember correctly, an NPN has a diode from base to collector which would be forward biased if you drive the base with more than 3.3V. Which I think would be bad, or atleast wouldn't work properly.

You can use an NPN BJT with a single series resistor of high enough resistance to limit the base current. I don't think the voltage divider is necessary.

You can also use a FET with a pull-down resistor on the gate. This is what I would pick. The resistor value doesn't matter, as long as the current through it isn't higher than the limit for the PNP in the sensor.