This circuit lights the Premises of House around 6 pm and turns off in the morning. Its working is fully automatic and uses the light sensing property of LDR. Unlike other LDR based circuits, it will not cause lamp flickering during the light transition so that CFL and fluorescent lamps can be used. It also exploits the Schmitt trigger action of timer IC 555 for a clean switching.
LDR and timer IC 555 are used in the circuit for automatic switching. Light Dependent Resistor offers very high resistance around 10 Meg ohms in dark but in light it has only 100 ohms or less resistance. So it it is an ideal component to switch on lamps based on the presence or absence of sun light. Here it is used to trigger the timer IC 555 which is designed as a Schmitt trigger.
The popular Timer IC 555 has two internal comparators. These are Threshold comparator and Trigger comparator. The Set and Reset action of these comparators can be used for On/ Off actions. Here the IC 555 functions as a Bistable with Schmitt trigger action. The upper comparator (Threshold comparator) of IC 555 trips at 2/3 of the supply voltage and the lower comparator(Trigger comparator) trips at 1/3 of the supply voltage. In the circuit, the inputs (pin6 and pin2) of both the comparators are shorted and connected to the junction of LDR and the Preset VR1.
In day light, LDR passes more current and the current into the upper comparator (pin6) is above 2/3 Vcc. This resets the internal Flip-Flop of IC. At the same time, the current into the lower comparator (pin2) is more than 1/3 Vcc. Both these condition causes low output from IC1.
When the light falling on the LDR decreases, its resistance increases, and the current flowing to the upper and lower comparators of IC1 decreases. The input voltage of upper comparator decreases below 2/3 Vcc and that of lower comparator below 1/3 Vcc. This causes, high output from pin 3 of IC1. This triggers T1 and the relay connected to its collector turns on. The lamp gets AC through the Common and NO (Normally Open) contacts of the relay. Only the Phase line is connected through the relay contacts. The neutral line goes as such.
VR1 adjusts the sensitivity of LDR at the particular light level at which the lamp turns on. Capacitor C2 maintains the base voltage of T1 for clean switching action and also avoids relay clicking. Diode D1 removes back e.m.f when T1 switches off.
Note: LDR should be placed away from the Lamp light but it should get day light. Use 6 Volt PCB relay. Wiring connections are given in the diagrams.
Caution: Take extreme care while connecting AC line to the relay contacts. Do not touch the relay contacts when the circuit is connected to mains. Provide sleevings to relay contacts to avoid accidental shorting.