Automatic street light system is very common nowadays as it provides intelligent street lighting mechanism. It provides light automatically during night without any human interference. These energy saving street lights make use of incandescent lamps instead of LEDs.So here I will teach you how to make an electronic circuit for street light automation. The heart of this circuit is a LDR (Light Depended Resistor) which is connected as a potential divider with a 56K resistor. The drop across LDR is used for switching the transistor. Solar led street lighting systems are the advanced versions of this ordinary automatic street light controller.
Circuit diagram of Automatic Street light using LDR
- Rectifier power supply (Step down transformer [1A], Diodes-4, Capacitor-470μF)
- Resistors (56KΩ,100Ω)
- Transistor BC187 x 2
- 6V or 12V Relay
Working of Automatic Street Light circuit
- The bulb should remain OFF during daytime and turn ON automatically during night.
- The unique property of light depended resistor is utilized here. LDR is a variable resistor which has very low resistance in the presence of light and very high resistance in the absence of light.
- In this circuit, we create a potential divider network with an ordinary resistor in one arm and a LDR on the other arm.
- According to Ohm’s law (V=IR), voltage drop across the resistor increases when its resistance increases.
- Here the drop across LDR varies with changes in light intensity. That is voltage drop across the LDR is minimum in the presence of light and maximum in the absence of light.
- One end of the LDR is connected to the base of a BC 187 transistor. At night, the drop is very high (> 0.6V) and it is sufficient to turn on the transistor.
- When the first transistor is ON next one will also turn ON. Thus the relay coil energizes and the bulb will glow.
Read further: How to connect relay: Relays working with animation
- During day time the drop is very small (< 0.6V), so the transistor remains in OFF state.
- The diode Df is the free wheeling diode used for decaying the energy stored in the relay coil when the circuit is switched off.