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LDR based Relay Switch

This circuit removes the connection with the switch we are connected with. Let us take an example as bathroom switch; there is more chances to get shock while it is having water content. This situation can be overcome by using this simple circuit.
The LDR used here to detect/sense dark or shadow and it makes turn ON/OFF the relay depending upon the shadow. Here we can swipe our hands or objects above the LDR sensor, to turn ON/OFF the output devices.

Circuit Diagram

Components Required

  • 1. Decade Counter IC CD4017
  • 2. Dual operational amplifier IC LM358
  • 3. 9V Relay module
  • 4. LDR (Light Dependent Resistor)
  • 5. Variable Resistor 10KΩ
  • 6. Resistor 10KΩ-2
  • >7. Resistor 100KΩ
  • 8. Resistor 330Ω
  • 9. Capacitor 33uF / 16V
  • 10. Diode 1N4007
  • 11. LED green
  • 12. Transistor BC547
  • 13. Battery 9V or (Use 9V DC power supply)
  • 14. 230V AC bulb

Construction & Working

This circuit has two sections; sensor-operational amplifier and decade counter to drive the relay. LDR is connected with Variable resistor VR1 and the variable terminal connected with Op-Amp’s inverting input. By changing the value of Variable resistor, we can adjust the sensitivity level of LDR.
The non-inverting input of the Op-Amp LM358 is balanced with resistors R1 and R2. Pin 1 is the output of amplifier 1 and fed into clock input of Decade counter. Capacitor C1 and resistor R3 stabilize the output from operational amplifier.
CD4017 IC counts the clock signals and gives ten different outputs from Q0 to Q9. Here output relay connected with Q0 through switching transistor BC547 and a LED1 becomes ON. The bulb is connected in between normally-open and common pin. When shadow detects by LDR, the relay gets power supply and it leads to turn ON the bulb and this state continues until the next shadow detection.

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