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.

Thanks!!

—A

 

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.

 

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?

 

How can I test for which lead of the phototransistor is collector and which is emitter?

Do you have a schematic showing how the phototransistor is connected? What do you mean by "test"?

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

Picture of the assembly?