This dark activated switch can trigger a relay to operate an AC lamp at Sunset. 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.
Variable resistor VR and the LDR forms a potential divider that gives a variable voltage to the inverting input (pin2) of IC1. The non inverting input (pin3) of IC1 gets a fixed voltage (half supply voltage) from the junction of the potential divider R1 and R2. Feedback Resistor R3 gives some hysteresis so that the relay turns on when the light level falls to a particular value and does not turns off again until the light level rises above this value. This prevents spurious switching by sensing mild changes in the light level. This also prevents relay clicking when the light level gradually falls at sunset.
During day time LDR gets sunlight and it conducts. This reduces the voltage at pin2 of IC1. Since this voltage level is lesser than the voltage at pin3, output of IC1 goes high to make the PNP transistor T1 off. Thus relay remains off during day time.
When the light level decreases at sunset, LDR cease to conduct and the voltage level at pin 2 increases above the voltage level at pin3.Output of IC1 then turns low and T1 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 T1 from back e.m.f when T1 switches off.
VR can be used to adjust the sensitivity of LDR at the particular light level, say at 6 pm to turns on the lamp.