Simple Light Operated Relay Circuit

Simple Light Operated Relay Circuit work like small auto light switch.

It uses special sensor like LDR or photo diode which work like small eye.

When light appears sensor gives signal to the circuit and then circuit turn the relay ON or OFF

This help for turning outside light at night or start sprinkler type in morning.

Circuit Working:

Parts List:

This circuit see light and can turn ON the relay to power lamp or other AC thing when it get dark.

It is good for night lamp with no need of switch.

It uses a LDR to see light.

When light is bright its LDR resistance is low around 100 ohm but in dark the resistance goes very high like 10 mega ohm or more.

VR1 is used to set how sensitive LDR is.

In daytime LDR allow current to connect in other way so gate of T1 does not get base signal and relay stays OFF.

When it gets dark the LDR give more resistance and then the current connects to base of T1 and T1 start the work.

Then relay turns ON and load also turns ON.

VR1 can change when relay turns ON like at the dark level.

The LED flashes and shows when relay is working.

Formulas:

Below is a easy formula to find base resistor R1 for light relay circuit:

Base Resistor R1 Calculation:

To make transistor BC547 work good and safe we need resistor R1 to control base current.

Use this formula to find base current IB:

IB = (VCC – VBE) / R1

where:

• VCC is supply voltage for 12V
• VBE is base emitter voltage of BC547 which is around 0.7V
• IB is base current

Now change formula to find R1:

R1 = (VCC – VBE) / IB

Pick R1 value so IB stay between 5mA to 20mA which is the safe range for BC547.

Note:

This formula help to start making circuit but the real value can change depending on parts and how circuit work.

How to Build:

To build a Simple Light Operated Relay Circuit we need to follow the below mentioned steps:

Gather Parts:

• Take all the required parts needed as shown in the above circuit diagram.
• Use breadboard or PCB for making circuit.
• Be sure board is clean with no dust or dirt.

Wiring the Circuit:

• Put LDR with one resistor in series and this make voltage divider.
• Connect this to base of transistor T1.
• Connect emitter of T1 to ground.
• Collector of T1 connects to one side of relay coil.
• Other side of relay coil connects to +12V power.
• Put one diode across relay coil as it save T1 from back EMF.
• Add LED with resistor from T1 collector and this shows when relay is ON.
• Put preset VR1 with LDR to adjust light sensitivity.
• One side of VR1 to connected to base of T1.

Power Supply:

• Give power to circuit with correct voltage.
• Be sure relay match the power supply.

Testing Circuit:

• Turn ON the power and see if it is working.
• Turn VR1 to adjust when relay turn ON by light level.

Put in Box:

• If all is working good then put circuit inside box.
• Box help keep away dust and water.

Be Careful:

• If using AC mains be very careful.
• Be sure all wiring is safe and no shock happen.
• Always use insulation and proper ground.

Final Test:

• After everything is completed test one last time.
• Check if it is working safely and is good every time.

Note:

• If anyone is not sure how to work with high voltage or electronics then ask help from someone who knows it better.

Conclusion:

To conclude Simple Light Operated Relay Circuit is easy and useful way to control relay using light level.

With light sensor and some small parts this circuit can turn ON or OFF light or other device when light changes.

It is simple, save energy and good for many use.

References:

Light-operated Relay Circuit.

Relay-controlled light circuit