Here’s a simple and low-cost street light switch. This switch automatically turns on the light at sunset and turns it off at sunrise. The automatic function saves electricity besides man-power.
Broadly, the circuit can be divided into power supply and switching sections.
Pressing switch S1 connects mains to power the circuit. Mains is stepped down to 9.1V DC by resistor R1, diode D1 and zener diode ZD1. The output across ZD1 is filtered by capacitors C1 and C2. The output voltage can be increased up to 18V or decreased to 5V by changing the value of zener diode ZD1.
The switching circuit is built around light-dependent resistor LDR1, transistors T1 through T3 and timer IC1. The resistance of LDR1 remains low in daytime and high at night. Timer IC1 is designed to work as an inverter, so a low input at its pin 2 provides a high output at pin 3, and vice versa. The inverter is used to activate triac 1 and turn street bulb B1 on.
During daytime, light falls on LDR1 and transistors T1 and T2 remain cut-off to make pins 4 and 8 of IC1 low. Since transistor T3 is also cut-off, IC1 is not triggered. As a result, output pin 3 of IC1 (connected to the gate of triac 1 via resistor R5 and red LED1) remains low and the street bulb does not glow.
At night, no light falls on LDR1 and transistors T1 and T2 conduct to make pins 4 and 8 of IC1 high. Due to the conduction of transistor T3, trigger pin 2 of IC1 remains low. The high output of IC2 at its pin 3 turns the street bulb ‘on.’
Assemble the circuit, except LDR1, on any general-purpose PCB. Use long wires for LDR1 connections so that it can be mounted at a place where sufficient light falls on it.