Tuesday, July 7, 2015

Dark Activated Relay

This dark activated switch can trigger a relay to operate an AC lamp at Night. The lamp remains on till morning and then turns off. This eliminates the need of switching the Porch lamp or Backyard lamp daily and also helps to light the premises of the house when the occupants are out of station.

The circuit utilizes the light sensing property of LDR to activate the circuit. The LDR (Light Dependent Resistor) has very high resistance as high as 10 Meg ohms in dark which reduces to a few Ohms in bright light. Resistor R6 (10k) and the LDR forms a potential divider that gives a variable voltage to the base of Q2 Transistor. The Transistor Q2 is connected to base of Q1 through R1.

During day time LDR gets sunlight and it conducts. This increases the voltage at base of Q2. So Q2 gets in conduction mode and reduces the base voltage of Q1 (2N2222). Thus relay remains off during day time.

When the light level decreases at sunset, LDR cease to conduct and the voltage level at base of Q2 Transistor decreases so it turns off transistor Q2. This will increases voltage at base of Q1 conducts. This activates the relay and the AC load connected to the NO (Normally Open) contacts of the relay turns on. This condition remains until LDR illuminates in the morning and then the lamp turns off. Diode D1 protects Q1 from back e.m.f when RL1 switches off.

Step 1: Circuit Diagram
Day night switch

Step 2: Components Required
1. LDR
2. BC548
3. 2N2222
4. 10K, 4.7K, 330Ohm Resistors
5. 1N4007 Diode
6. 12V Relay


  1. if I'm looking to connect this circuit to a 230v mains lamp, how do I go about it?

  2. Very simple just connect the relay as a switch