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
Promoted By Zergnet


You May Also Like

  • What Is the Purpose of a Transistor?

    Electronic engineers use transistors to control the flow of electricity in circuits. They can operate up to billions of cycles per second,...

  • How to Test Your Car Alternator for Power

    When a car battery isn't providing enough power to start a car, people often assume that the battery is bad. Before buying...

  • How to Know Bad Transistors

    Transistors are semiconductor devices usually used for amplification. The most popular ones are bipolar. Bipolar transistors have either NPN or PNP layers,...

  • How to Make an IR Receiver Circuit

    Infrared, or IR, technology is commonly used in electronic devices. The term infrared refers to electromagnetic waves that travel faster than the...

  • Infrared LED Specifications

    Infrared LEDs are a specific type of light-emitting diode (LED) that produces light in the infrared spectrum. Light in this range is...

Related Searches

Check It Out

How to Build and Grow a Salad Garden On Your Balcony

Is DIY in your DNA? Become part of our maker community.
Submit Your Work!