This circuit lights the backyard of your home at 10 pm and stays on till morning. It helps to provide light in the back yard to avoid nocturnal animals and even prevents an attempt of theft. It is fully automatic and uses an LDR as light sensitive switch.
The circuit has transformerless power supply and a timer based switching circuit. Capacitor C1, diodes D1 through D4 and capacitor C2 forms the power supply section. C1 is an X rated AC capacitor. High volt AC is reduced to a safer level by C1 through the rectance property. Diodes D1 through D4 form a full wave rectifier to convert AC to DC and capacitor C2 removes ripples from the DC. Resistor R1 is a bleeder resistor to remove the stored current from C1 when the circuit is switched off. Resistor R2 reduces the inrush current into the circuit.
LDR is used in the circuit to trigger IC1 at Sunset. The resistance of LDR is very high around 10 Meg ohms in dark which reduces to 100 ohms or less in bright Sunlight. CD 4060 IC is a good choice for long time delay applications. It is a binary counter IC1 with an internal oscillator and 10 current sourcing outputs. During daytime, resistance of LDR will be low so that the reset pin 12 of IC1 remains high which inhibits the working of IC1. At sunset, the resistance of LDR increases and the reset pin 12 of IC will be grounded and it starts oscillating with the components, C3, R4 and R5. Flashing Red LED indicates the activation of IC1. With the given values of C3, R4 and R5, the pin 1 becomes high after 4 hours. That is around 10 pm.
When the output pin1 becomes high, Triac fires to complete the lamp circuit. At the same time, Diode D1 forward biases and stops the oscillation of IC1. IC1 then remains latched till morning giving high output from pin 1. In the morning LDR again gets light and IC1 resets and remains off till sunset. Enclose the circuit in shock proof case with LDR exposed from it. Keep the unit in a place where sunlight is available. Keep the lamp away from the unit to avoid light from it falling on the LDR. VR adjusts the sensitivity of LDR at the particular light level. Enclosing the LDR in a black tube will makes the adjustments easy at the particular light level. This circuit cannot be used to light CFL or Fluorescent lamps since Triac will not give full supply voltage. This may harm the electronic circuit in these devices.
Caution: This circuit is directly connected to mains and there is no galvanic isolation. Do not test or trouble shoot when it is connected to mains. Adjust VR only with a sleeved screwdriver. Do no touch any parts when the circuit is powered.