Phototransistor Symbol, Construction, Circuit, Connection


The phototransistor is an electronic semiconductor device that can sense light and allow the flow of current through it. The phototransistor is an electronic switching and current amplifying device. The current amplifying property of the phototransistor differs from the photodiode because the photodiode also works almost the same but it can not amplify current.

There are two types of phototransistor, BJT phototransistor, and FET phototransistor. Phototransistor works on the same principle as the normal transistor. Just the difference is, in a normal transistor, we need to apply a voltage to the base terminal to activate but in the phototransistor, no voltage needs to apply its base. When light falls upon its base it got activated and starts to conduct current. 


Phototransistor Symbol


Here, you can see the symbol of phototransistor below.

phototransistor symbol, symbol of phototransistor



You can see the symbol of phototransistor is almost the same as the normal transistor with two arrows pointing towards the base of the transistor. These arrows indicate the light source.




Phototransistor Terminals


Here you can see the terminals identification of a Phototransistor.

phototransistor terminals, terminals of phototransistor



Phototransistor has two terminals - Collector and emitter. And it does not have any base terminal. You can see the small and flat terminal is the emitter terminal and the big terminal is the collector. When it is connecting to an electronic circuit, its collector terminal should be connected to the positive and the emitter terminal should be connected to the positive.




Phototransistor Construction


The construction of the phototransistor is almost the same as the normal transistor. It also three-layer and two junction devices. According to construction and layer, the phototransistor divided into two major types - 1. NPN 2. PNP


One of the major differences between the construction of a phototransistor and the normal transistor is phototransistor has a larger base and collector area than a normal transistor.

Phototransistor Construction, internal construction of phototransistor



The base region of the phototransistor is exposed with a transparent material. So the light can easily enter and reach the base. Old phototransistors are designed and constructed with single semiconductor materials such as Silicon or Germanium but the modern phototransistors are constructed by gallium or arsenide to obtain higher efficiency. 

When there is no light that falls upon the base of the phototransistor then also a low current flow due to the thermally generated electron-hole pairs. In absence of the light, the current flow through the phototransistor is known as dark current.

As the base and collector area of the phototransistor is larger so the base-collector junction is very sensitive to light. And its working very much depends upon the intensity of light. The increase in the level of light causes to increase in the flow of current. 




Phototransistor Circuit Diagram


Here a circuit diagram of phototransistor with output load and Vcc power supply is given below.

Phototransistor Circuit Diagram, Circuit of Phototransistor



You can see it is a light-operated relay using a phototransistor circuit. When light falls upon the phototransistor, it starts to conduct current, so current will flow through the phototransistor and also base to emitter of the transistor T2. So the transistor T2 also starts to conduct the current and as it is connected to the relay, the relay will be turned on. Here the variable resistor is used to control the flow of current through the phototransistor as well as transistor T2. So the main function of this circuit is when the phototransistor detects light, the relay will be got turned on.




Phototransistor Connection Diagram


Here you can see the connection diagram of phototransistor for automatic light control during the darkness.

Phototransistor Connection Diagram, Connection of Phototransistor



The main function of this connection is when the darkness will. occur, the light or bulb will automatically be turned on. Let's know how the circuit works. Its circuit almost the same as the light control relay circuit using phototransistor as we have seen in the before section. Just the difference is here the resistor is connected before the phototransistor. So when there is light available the phototransistor will conduct current means the current will flow through the resistor and the phototransistor, so no current will flow through the transistor T2(BC547).

When darkness occurs, the phototransistor will stop conducting current. So the current will flow through the resistor and the base to the emitter terminal of the transistor BC547. So the transistor will be turned on and it will cause to flow current through the relay coil. Once the relay coil got energized, it will make its NO contact into NC. So the light or bulb will be glow.


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Phototransistor Symbol, Construction, Circuit, Connection Phototransistor Symbol, Construction, Circuit, Connection Reviewed by Author on 6/09/2021 Rating: 5
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