How to Design a Phototransistor Circuit

Written by naeem ahmed
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How to Design a Phototransistor Circuit
A phototransistor can be used as a switch that turns on or off with light. (four gold transistors on blue image by Eugene Tokarev from

A phototransistor is a semiconductor device that allows current to flow between its collector and emitter whenever light shines on its surface. Its principle of operation is similar to a regular transistor, which requires electrical current at the base to close the connection between its collector and emitter. Because of its sensitivity to light, a phototransistor can be used as a switch that turns on or off with light.

Skill level:

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Things you need

  • Datasheet of the phototransistor

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  1. 1

    Familiarise yourself with the specifications of the phototransistor, such as maximum current, voltage ratings and switching frequency.

  2. 2

    Identify your needs. For instance, determine if you want to turn something on or off when light shines on the phototransistor.

  3. 3

    Design a circuit on paper that turns off whenever light shines on the phototransistor. In this basic circuit, the emitter of the phototransistor should be connected to ground, and the collector connected to the supply voltage (usually 5V) through a resistor (such as 4.7kOhm, 1/4W). Any device that needs to be turned off should have one of its terminals connected to the collector of the phototransistor, with the other end attached to the ground.

  4. 4

    Understand how it works. You might have a light bulb that works at the supply voltage you have chosen, connected between the collector and the ground. When there is no light, all the current will flow through the light bulb, and the bulb will illuminate. As soon as light shines on the phototransistor, its collector-emitter junction starts conducting, and all the current flows to the ground instead. Consequently, the light bulb is not powered. For this type of circuit, the transistor should be installed such that its face does not see the light from the bulb.

Tips and warnings

  • Many variants of the circuit described here can be designed using multiple transistors and other components to increase speed and efficiency. For example, another transistor can be added between the phototransistor's collector and the 4.7kOhm resistor, such that its emitter is connected to the collector of phototransistor and the collector to the resistor. The base can be supplied with a small voltage, such as 2.5V, which can be accomplished through the same power supply using a voltage divider. A voltage divider has two resistors connected together with one end connected to the supply voltage and other end to the ground. The base of the phototransistor is connected to the point connecting the two resistors.
  • Do not use high AC voltage in this circuit as it will burn the transistor; this can induce life-threatening shocks.

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