Transistor act as a switch: DC Motor control (driver) circuit with animation

In one of the previous article we had already discussed about How transistor acts as switch? Here is another example of bipolar junction transistor BJT switching. In this case we will study a high voltage switching circuit i.e. how a transistor act as a switch for cutting flow of current when a high voltage flows. Today BJT switches are common in many electronic circuits and control systems. The main application of ‘transistor act as a switch’ is in the automation field.This example article explains how a transistor can control a DC motor. Animated working of transistor switch is also given for better understanding.

Circuit diagram of high voltage switching circuit

transistor as switch

Components required

  1. Transistor (BC548)
  2. Resistor (1kΩ)
  3. Relay (6V)


  • The example external circuit contains a DC motor. It gets power from the Vcc via Normally Closed (NC) terminal of relay.
  • Resistor R1 is a current limiting resistor which protects the transistor BC548 from over current.
  • Consider no high voltage is applied to the transistor, then the transistor become OFF and no current will flow through it. Hence the relay remains in OFF state.
  • Power to the DC motor if fed from the Normally Closed (NC) terminal of relay, so the motor will rotate when the relay is in OFF state.
  • Application of high voltage at the base of BC548 causes turning ON of the transistor and the relay coil to energize.
  • Thus the NC terminal is disconnected from the motor, it cuts the flow of current though the DC motor.    See the animation below to make this concept clear.

Animation/ Simulation

(This circuit posted as per the request from one of our reader 
Mr.Ferd Nicodemus) 
Hope you liked this simple explanation of transistor switching in case of high voltages. Let us know if there is something that you could’t clear out. Cheers!

9 thoughts on “Transistor act as a switch: DC Motor control (driver) circuit with animation

  1. Better protect that transistor. Diode across the coil and motor winding. There will be a hugh voltage spike when you shut off the transistor and the winding fields collapse.

    • Hi,
      You are right. But in this post I tried to figure out the theoretical working of transistor as switch. For practical implementation we must use free wheeling diodes.

  2. wowwwwwwwwww..igot it…that was a shit to me before…nd now thnks to u i understood thz…thnx a lot…very helpfull..keep it up

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