Automatic Street Light Circuit 1

Warning: Please be careful when experimenting or work with this circuit as the circuit contains both AC (mains: 230V) and DC voltage.

The first time I used this circuit was about seven years ago; then it was given as an assignment in one of my electronic classes during my second year in university. The circuit worked perfectly as designed and I decided to share it with you on this site. The circuit automatically turns street or security light on in the night and off when the day breaks.

Automatic Street Light CircuitAutomatic Street Light Circuit

Circuit description

A step down transformer of 300mA is ok for this circuit. The transformer steps the supply voltage down to 12V and the voltage is rectified by the bridge rectifier (this can be replaced with four 1N4001 diodes) to DC. The filter capacitor C filters out the ripples in the rectified voltage to 15V DC. R1 and photocell form a voltage divider circuit across the DC voltage. In the day according to Jameco's catalogue, the resistance of the photocell is 4K while in the night it is 300K. During the day, the voltage across the photocell is just about 0.58V far below the zener voltage of ZD, hence the transistor T is in off state- just like an opened switch. In the night, resistance of the photocell will be around 300K and the voltage across it is about 11.25V, well above the zener voltage. This turns on the transistor, relay is energized and the normally open contact of the relay is closed to complete the AC circuit; current flows through the bulb and it lights. Resistor R2 is connected in series with the relay coil to limit the current that will flow through it as the DC voltage in this circuit is above 12V which is the rated voltage of the relay. Diode D is connected as shown in the circuit to prevent the transistor from any damage that could arise from induced emf generated as the magnetic flux in the coil collapses when the supply to the relay coil is cut off. The diode is forward biased by the emf and conducts to dissipate the stored energy in the coil.