Light Dependent Switch

On a random day while walking on my street i saw that light in my front yard was still on in the morning because i forget to switch it off, It was important for the lights to be on in the night due to obvious reason so i can not be lazy as well. At that time, i was in my undergrad and was learning about electronics and circuits, so i thought that i could learn about my subjects while solving this problem, so i made this project.

Later with some adjustments, i was also use this switch for street lights in my neighbour, since at that time street lights in my neighbour were supposed to be switched on manual at each night.

1. Light Dependent Resistor (LDR)
2. 5V Dc operable 10A,220V SPDT Mechanical Relay
3. Rectifier (I have used a mobile recharge)
4. BC 547B transistor
5. IN4148 diode
6. 50K potentiometer
7. 1.5K Resistor

Ldr(Light dependent resistor) is a device whose resistance increases with decrease in light intensity i.e its resistance decreases with increase in light sensitivity.

We exploit this property of an LDR to sense the intensity of light in the environment.

An transistor bc547b is used as a switch so that when the resistance increases above a certain level the transistor can act as closed switch. So that relay can be energised.

An freewheel diode is used for protecting circuits from flyback current from relay.

I have used an mobile charger circuit instead of charger circuit because it was cheap.

Potentiometer is used to set the limit of light on which the switch has to be closed.

An resistor is also used for protection of BJT but it can be skipped.

Relay acts as an dc controlled ac switch when the signal from the circuit is given to relay an it is energised and in process it changes its initial position till energisation.

You can refer to the diagram given above for schematics and follow this steps:

1. Assemble the components as given in circuit diagram.
2. Give the power to the circuit through the charger circuit.
3. Connect the terminals from the circuit to the dc input terminal of relay relay.
4. Connect neutral terminal of your lamp to the neutral of the relay and connect naturally open (NO) of relay to the line of the lamp.

• I have not used pot since I had not this idea at that time but it is recommendable.
• Keep the switch and lamp far away from each otherwise hunting will be observed.
• This device is useless for indoor purpose.
• The switching property can be hindered due to external light sources so proper placement of switch is recommended.