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Automatic street light Using NPN Transistor BC547


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Objective: To design an automatic street light control system that turns on the street lights when it gets dark and turns them off when it becomes daylight.


  1. Light Sensor (LDR): A Light Dependent Resistor (LDR) is used to detect the ambient light conditions. Its resistance decreases as the light intensity increases.
  2. NPN Transistor (BC547): The BC547 transistor is used as a switch to control the power supply to the street lights.
  3. Relay: A relay is an electromechanical switch that is used to control high-voltage devices like street lights. It is activated by a small current from the transistor.
  4. Resistors and Capacitors: These are used to set up the biasing and timing circuits for the transistor.

Working Principle:

  1. During daylight, the LDR has low resistance, preventing the base current to the transistor.
  2. As it gets dark, the LDR resistance increases, allowing a small current to flow into the base of the transistor.
  3. The transistor then amplifies this small current and allows a larger current to flow through its collector-emitter path.
  4. The larger current activates the relay, which, in turn, closes the circuit for the street lights, turning them on.
  5. When daylight returns, the LDR resistance decreases, reducing the base current, and subsequently turning off the transistor and the relay, turning off the street lights.

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