How to Test Photo Transistors


In order to repair electronic equipment that senses light or uses it for signaling, you may have to test its photo transistors. These are similar to common silicon transistors, except that their bodies have a clear window and the silicon inside is fine-tuned to respond to light. In addition, most photo transistors don't have the base lead that ordinary ones have. To test a photo transistor, set up a simple circuit to turn a light-emitting diode (LED) on and off. The circuit's connections vary slightly for an NPN or a PNP photo transistor.

Things You'll Need

  • Photo transistor
  • Photo transistor data sheet
  • Solderless breadboard
  • 560-ohm 1/4-watt resistor
  • 5 mm LED
  • 60-watt incandescent lamp
  • 9-volt battery clip
  • 9-volt battery
  • Read the photo transistor's data sheet and determine if the device is an NPN (negative-positive-negative) or a PNP (positive-negative-positive) type.

  • Connect one lead of the 560-ohm resistor to the transistor's collector. Connect the other lead to an unused breadboard column. If the transistor is an NPN type, connect the LED's cathode lead to the unused breadboard column. Otherwise, connect the LED's anode lead. Connect the LED's other lead to a free column. If the transistor is an NPN, insert the battery clip's red wire into this column. Otherwise, insert the black wire.

  • Connect the battery clip's remaining wire to the transistor's emitter.

  • Snap a 9-volt battery into the clip. Cover the transistor's window with your thumb to block out any light. The LED should remain off.

  • Bring the lamp to within two feet of the transistor. Make sure that the light can reach the transistor. Remove your thumb from the transistor window and turn the lamp on. The LED should light up. If it does not, the photo transistor is likely defective.


  • Photo Credit transistor image by Alex from
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