This circuit will automatically switch on lights when it gets dark, work in 3 to 6 volts range battery operation that suitable use. It can be used for any purpose involving one or more lamp to be switched on and off depending on light intensity. Trim the R2 to setup the sensitivity of the photo resistor R1 until the desired switching threshold is reached. It will require some experimenting, but it should not be difficult.
R2————————————————–22K 1/2W trimmer cermet
R3————————————————–1K 1/4W resistor
R4————————————————–2K7 1/4W resistor
R5————————————————–330R 1/4W resistor
R6————————————————–1KR5 1/4W resistor
D1————————————————–1N4148 75V 150mA diode
Q1————————————————–BC547 45V 200mA NPN transistor
Q2————————————————–BD438 45V 4A PNP transistor
SW1————————————————SPST toggle or slider switch
B1—————————————————6V or 3 V batteries
In the circuit the maximum current and voltage delivered to the lamp are limited mainly by R6, therefore the ohm law must be used to calculate the best voltage and current values of the bulbs.
At 6V supply, bulbs with 6Volt 500mA supply can be used, but for a total current 1A, 4.5V bulbs must be chosen, as the voltage drop across the R6 will become 1.5V. In this case R6 must be 2W type. At 3V supply, R6 value can be lower to 0.5-1 ohm. For example, supply voltage 3V and R6 1 ohm, total current will be 600mA, choose 2.2V bulbs as voltage drop caused by R6 will be 0.6V. the R5 value must be changed to 100 ohm.
source : www.redcircuits.net