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transistor as a switch

Transistor as a Switch Project with circuit diagram

Solid state materials can be grouped in to 3 classes :- Insulators, Conductors and Semiconductors. Semiconductor material is neither conductor nor insulator. It has conductivity between conductors and insulators and is sensitive to temperature, illumination, magnetic field and minute amount of impurity atoms. This type of sensitive material for controlling or switching current is very much required in modern age. Today electronics  means present of semiconductor devices.

Semiconductor Devices are the foundation of electronics industry. For Learning electronics it is necessary to have basic knowledge of Semiconductor device. A common semiconductor device is a Transistor.

  Aim of this project is to determine characteristic of a NPN transistor by varying base voltage and detect Cut-off Region and Saturation Region.

We have used a potentio-meter for varying Base voltage. After assembling the circuit use a digital multi-meter and scale mark the voltage. You can also attach a multi-meter permanently.

A low power indicator such as LED is used for detection of conductance of transistor. LED is connected to power supply through collector of transistor so that when transistor is conducted then circuit will be complete and led glows.

After making the project. Place the battery and start rotating knob of POT. You will see LED begins glowing reaching 0.5V and at 0.7V it will be at full bright. Thus you can mark cut off region (fully off) and Saturation Voltage (full dark). Most of the transistor will conduct from 0.5 to 0.7 Volt.

This type of circuit is commonly used in voltage stabilizer for high voltage cut off. A relay is placed at LED connection.


Parts Used

NPN Transistor BC548

Resistance 1K and 220 Ohm

10K POT with knob

LED and Battery Container


Transistor voltage (NPN )

1. As a switch: OFF = 0 to 0.3V, ON=0.7 V (Cutoff and Saturation Region )

2. As an Amplifier: 0.5 to 0.7V (Active Region)

How do transistor works ? Watch this video.

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