2dix-The Student Choice
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To study the operation of Photo Transistor.

AIM-  To study the operation of Photo Transistor.


  • Infrared LED (Tx)
  • Photo Diode (Rx)
  • Transistor (TR 3904)
  • Power Supply (10V variable & 10Vor 5V fixed)
  • Digital MultiMater (DMM)
  • Resisters (1kΩ, 47kΩ, 470kΩ)


A phototransistor is a device that converts light energy into electric energy. Phototransistors are similar to photoresistors but produce both current and voltage, while photoresistors only produce current. This is because a phototransistor is made of a bipolar semiconductor and focuses the energy that is passed through it. Photons (light particles) activate phototransistors and are used in virtually all electronic devices that depend on light in some way.


How a Phototransistor Works

A phototransistor is a bipolar device that is completely made of silicon or another semi-conductive material and is dependent on light energy. Phototransistors are generally encased in an opaque or clear container in order to enhance light as it travels through it and allow the light to reach the phototransistor’s sensitive parts. A phototransistor generally has an exposed base that amplifies the light that it comes in contact with. This causes a relatively high current to pass through the phototransistor. As the current spreads from the base to the emitter, the current is concentrated and converted into voltage


Phototransistor structure-

A photo transistor can be made to respond to light radiations and work as a sensor. A transistor is like a valve that regulates the amount of electric current that passes through two of its three terminals. The third terminal controls just how much current passes through the other two.

Depending on the type of transistor, the current flow can be controlled by voltage, current, or in thecase of the phototransistor, by light.The drawing below shows the schematic and part drawing of the phototransistor in your Robotics Shield Kit. The brightness of the light shining on the phototransistor’s base (B) terminal determines how much current it will allow to pass into its collector (C) terminal, and out through its emitter (E) terminal. Brighter light results in more current; less-bright light results in less current.

Early photo transistors used germanium or silicon throughout the device giving a homo-junction structure. The more modern phototransistors use type III-V materials such as gallium arsenide and the like. Heterostructures that use different materials either side of the p-n junction are also popular because they provide a high conversion efficiency. These are generally fabricated using epitaxial growth of materials that have matching lattice structures. These photo transistors generally use a mesa structure. Sometimes a Schottky (metal semiconductor) junction can be used for the collector within a phototransistor, although this practice is less common these days because other structures offer better levels of performance.

Fig6.1f  Device of Photo Transisto

Circuit Diagram-


Fig6.2f Circuit dig of Photo transistor




Phototransistors are used for a wide variety of applications. In fact, phototransistors can be used in any electronic device that senses light. For example, phototransistors are often used in smoke detectors, infrared receivers, and CD players. Phototransistors can also be used in astronomy, night vision, and laser range-finding.

Phototransistors are often used as discrete devices, but they will be found combined with other components in some applications.




  • There should not be any loose connection.
  • Meter readings should not  be  exceeded beyond their readings
  • Switch off  the circuit when not in use.



From the graph the relation between LED’s voltage & current is observed.

      It is -------------------------- (Linear/ Nonlinear).


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