Testing generic phototransistor

Thread Starter


Joined Apr 7, 2012
I have a custom optical switch I built ages ago and didn't document fully. How can I test for which lead of the phototransistor is collector and which is emitter?

Would rather not take the whole thing apart to look at the discrete device.




Joined Jun 19, 2012
Use your DMM in diode test mode first, locate the LED terminals by looking for a VF about 1.2 volts, once you have that sorted, give it 20 mA and test the transitor with 5V with a 10k resistor in series. The orientation that gives the biggest voltage swing across the transistor when you pulse the LED on and off is the correct one.


Joined Aug 7, 2020
Phototransistors are always NPN, so either the emitter will be connected to the negative supply, or the collector will be connected to the positive supply. Who mentioned there being an LED?