Automatic Street Light Controller

Introduction:

This is a simple circuit used for operation for switching ON and OFF street light automatically. It uses a light sensor. During day time lights automatically switches OFF. After the dusk when natural light falls to a certain level then sensor circuit gets activated and switches ON. The sensitiveness of the sensor can also be adjusted. A Relay (electromagnetic switch) used for operating high voltage loads that will make easy to turn ON/OFF any electrical appliances that are connected through relay.

Workimng Principle :

This circuit uses a popular timer I.C 555. I.C 555 is connected as comparator with pin-6 connected with positive rail, the output goes high(1) when the trigger pin 2 is at lower then 1/3rd level of the supply voltage. Conversely the output goes low (0) when it is above 1/3rd level. So small change in the voltage of pin-2 is enough to change the level of output (pin-3) from 1 to 0 and 0 to 1. The output has only two states high and low and can not remain in any intermediate stage. It is powered by a 6V battery for portable use. The circuit is economic in power consumption. Pin 4, 6 and 8 is connected to the positive supply and pin 1 is grounded. To detect the present of an object we have used LDR and a source of light.

Here the IC 555 functions as a Bi-stable with Schmitt trigger action. The upper comparator (Threshold comparator) of IC 555 trips at 2/3 of the supply voltage and the lower comparator (Trigger comparator) trips at 1/3 of the supply voltage. In the circuit, the inputs (pin6 and pin2) of both the comparators are shorted and connected to the junction of LDR and the Preset VR1.

In day light, LDR passes more current and the current into the upper comparator (pin6) is above 2/3 Vcc. This resets the internal Flip-Flop of IC. At the same time, the current into the lower comparator (pin2) is more than 1/3 Vcc. Both these condition causes low output from IC1.

When the light falling on the LDR decreases, its resistance increases, and the current flowing to the upper and lower comparators of IC1 decreases. The input voltage of upper comparator decreases below 2/3 Vcc and that of lower comparator below 1/3 Vcc. This causes, high output from pin 3 of IC1. This triggers T1 and the relay connected to its collector turns on. The lamp gets AC through the Common and NO (Normally Open) contacts of the relay. Only the Phase line is connected through the relay contacts. The neutral line goes as such.

VR1 adjusts the sensitivity of LDR at the particular light level at which the lamp turns on. Capacitor C2 maintains the base voltage of T1 for clean switching action and also avoids relay clicking. Diode D1 removes back e.m.f when T1 switches off.

IC 555 Internal Circuit

Note: LDR should be placed away from the Lamp light but it should get day light. Use 6 Volt PCB relay. Wiring connections are given in the diagrams.

PCB Relay and Wiring connection

Caution: Take extreme care while connecting AC line to the relay contacts. Do not touch the relay contacts when the circuit is connected to mains. Provide sleeves to relay contacts to avoid accidental shorting.

Component :

1. 9v Battery
2. On Off Switch
3. LDR (Light Depending Resistance)
4. NE555 Timer IC
5. BC547 NPN Transistor
6. LEDs.
7. Variable Resistance
8. PCB.
9. Relay.
10. 230V AC 25W Bulb.