Jun 172015

Light detector circuit using transistors and relay

This light detector circuit activates a lighting device through a relay when the intensity of the daylight is no longer sufficient.

It can be used to illuminate the main entrance of the house, a showcase that must be kept lit at night, an unsafe area at night, etc..

How the Light detector circuit work?

The light sensor element is a LDR (Light Dependent Resistor), whose resistance depends on the intensity of the incident light. The more light, the less resistance in the LDR and vice versa. To adjust the level of light, with which the relay connects the artificial lighting, the potentiometer P is used

Light Detector Circuit using Transistors

  • When the LDR is illuminated, its resistance drops, the transistor T1, enters the cutoff region, T2 enters the saturation region, decreasing the base current of transistor T3. T3 and T4 enter the cutoff region. This causes the relay is not activated and the artificial lighting is off. Note that T4 is a PNP transistor.
  • When the LDR is in the dark, its resistance is high, the transistor T1 enters saturation region, T2 enters the cutoff region, allowing the transistor T3 being in the saturation region. T4 goes into the saturation region and the relay is on. The relay uses a diode in parallel to avoid burning the transistor T4, when the relay goes off.

The circuit uses a 9VDC power supply, but it can be powered with 12VDC. (You have to change the relay with a 12 volts one).

List of components for the Light Detector Circuit

  • 3  10K resistors (R1, R3, R6)
  • 1  33K resistor (R2)
  • 1  470 ohms resistor (R4)
  • 1  100K resistor (R5)
  • 1  2.2K resistor (R7)
  • 1 470K potentiometer
  • 1  100nF capacitor (C1)
  • 1  1N4004 diode (D1)
  • 3  2N2222 NPN transistors (T1, T2, T3)
  • 1  2N2905 PNP transistor (T4)
  • 1  LDR (Light Dependent Resistor)
  • 1  9 or 12 volts relay (depends on the source voltage)

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