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How to Use an Infrared Photo Transistor

Transistors are circuit components made from semiconductors. A phototransistor is a transistor that is optimized to be sensitive to light. When it receives enough light, its resistance drops, and when inside a circuit, can allow current to flow. When it is not near a light source, it has a high resistance, and can block the flow of current. For these reasons, transistors are used as light switches.

Infrared phototransistors are sensitive to infrared radiation, or heat. you can find them in burglar alarms and other detection circuits. To use an infrared phototransistor, study its properties and test it in series with an LED.

Things You'll Need

Phototransistor
270k resistor
Red LED
Flashlight
9V battery
Battery holder
Solderless breadboard
Jumper wires

Instructions

- 1
  Connect a battery holder to the solderless breadboard by inserting its red and black leads into two different columns. Do not add the battery at this time.
- 2
  Identify the collector and emitter leads on the phototransistor. This information may be obtained from the device's package, or from a data sheet.
- 3
  Connect the collector lead to the same column the red lead of the battery holder. Insert the emitter lead into a separate column. For a two terminal phototransistor, the shorter lead may be the collector, and the longer one the emitter, so be careful not to place the component in backwards.
- 4
  Place one end of a jumper wire in the emitter column, and insert the wire's other end into a different column. Attach a resistor to this second column.
- 5
  Wire the positive end of the red LED to the resistor's open end. For ordinary LEDs, the positive lead is normally the longer leg. Guide the LED's negative lead into a different column.
- 6
  Connect the negative end of the LED to the same column that the battery holder's black lead is in. If the LED is not close enough, use a jumper wire to do this.
- 7
  Attach the battery to the holder. Shine the flashlight over the phototransistor only. The LED will turn on, and the closer the heat source is, the brighter the LED will shine. The LED will switch off if the heat source is too far away or is completely removed.

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