Two of the problems commonly associated with street lights are false triggering due to slight variation in the intensity of ambient light and no control over switching action. Here is a simple switching circuit for street lights that overcomes these problems.
The circuit is built around popular op-amp IC 741 (IC1), 14-bit ripple counter CD4060 (IC2), SCR BT169, BC557 and other components. IC1 along with LDR1 enables IC2, which drives transistor T1 into conduction. IC2 is also used to trigger SCR1 to switch on the street light. Removal of trigger turns the light ‘off.’
IC CD4060 has an inbuilt oscillator around its pins 9, 10 and 11. Pin 12 is the master reset (MR) control. The oscillator is disabled when pin 12 is high and enabled when pin 12 is low.
In daytime, i.e., when light is falling on LDR1, its resistance decreases and the high output at pin 6 of IC1 cuts off pnp transistor T1 and disables IC2. At this stage, SCR1 remains untriggered to switch off the street light.
At night, i.e., when no light is falling on LDR1, its resistance increases and low output pin 6 of IC1 drives pnp transistor T1 into conduction. This enables IC2 and its internal oscillator starts oscillating.
After a preset time, pin 14 (Q7) of IC2 goes high and SCR BT169 is triggered through resistor R9 and diode D3. This energises RL1 and street light is turned on. This time interval can be varied by connecting the gate of SCR1 to pins 6, 13, etc of IC CD4060 (not shown in Fig. 1). Transistor T2, which is normally conducting, is driven into non-conduction when output pin 3 (Q13) of IC2 goes high, which de-energises relay RL1 to switch off the street light. This time can be adjusted with the varying preset resistor VR2.
Put simply, the street light turns ‘on’ when Q7 of IC2 goes high and ‘off’ when Q13 goes high, provided pin 12 of IC2 remains low.
The circuit works off regulated 12V DC. You can assemble it on any general-purpose PCB and enclose in a suitable cabinet. The mains AC terminal for the street light is connected to the normally-open (N/O) contact of relay RL1, so the street light turns on when the relay energises.