Simple Light Operated Relay Circuit work like small auto light switch, as it uses special sensor like LDR or photo diode which work like small eye.
Furthermore, when light appears sensor gives signal to the circuit and then circuit turn the relay ON or OFF; and this help for turning outside light at night or start sprinkler type in morning.
Circuit Working:
Parts List:
| Components | Values | Quantity |
|---|---|---|
| Resistors | 1k 1/4 watt | 1 |
| Preset 10k | 1 | |
| Semiconductors | Transistor BC547 | 1 |
| Diode 1N4007 | 1 | |
| LED any 5mm 20mA | 1 | |
| LDR | 1 | |
| Relay 12V | 1 |
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 and it also uses a LDR to see light.
When the light is bright, the LDR resistance becomes low, around 100 ohms, but in the dark, the resistance becomes very high, such as 10 megaohms or more; also VR1 sets the sensitivity of the LDR.
During, daytime LDR allow current to connect in other way so gate of T1 does not get base signal and relay stays OFF and 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.
Hence, VR1 can change when relay turns ON like at the dark level and 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; so 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:
- First, take all the required parts needed as shown in the above circuit diagram and use breadboard or PCB for making circuit; also be sure board is clean with no dust or dirt.
Wiring the Circuit:
- Next, put LDR with one resistor in series and this make voltage divider and connect this to base of transistor T1.
- Then connect emitter of T1 to ground, collector of T1 connects to one side of relay coil and other side of relay coil connects to +12V power.
- Also, put one diode across relay coil as it save T1 from back EMF.
- Now ad LED with resistor from T1 collector and this shows when relay is ON.
- After that, use preset VR1 with the LDR to adjust the light sensitivity and connect one side of VR1 to the base of T1.
- Give power to circuit with correct voltage and be sure relay match the power supply.
Testing Circuit:
- Turn ON the power and see if it is working and then turn VR1 to adjust when relay turn ON by light level.
Put in Box:
- If all is working good then put circuit inside box and this box will help keep away dust and water.
Be Careful:
- If using AC mains be very careful and be sure all wiring is safe and no shock happen.
- Always use insulation and proper ground.
Final Test:
- After completing everything, test the circuit one last time and check that it works safely and properly 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; also it is simple, save energy and good for many use.