Simple Light Operated Relay Circuit

A light operated relay circuit is like a tiny automatic light switch.

It uses a special sensor like an LDR or photodiode that acts like a tiny eye.

When it gets light out, the sensor sends a signal to the circuit, which then flips a switch relay on or off.

This can be used for things like turning on porch lights at night or starting a sprinkler system in the morning.

Circuit Working:

Parts List:

This circuit, which is sensitive to light can activate a relay to turn on lamps or other AC loads when it detects darkness.

It is ideal for use as a driver for switch less night lamps.

The circuit uses an LDR as the light sensor.

The resistance of the LDR is low about 100 ohms in bright light, but increases to 10 meg or more in darkness.

The preset VR1 adjusts the sensitivity of the LDR.

During the daytime, the LDR conducts preventing the gate of T1 from receiving base bias which keeps the relay de energized.

As the light intensity decreases the LDR offers more resistance allowing more current to pass to the base of T1, causing it to conduct.

The relay then switches on the load.

VR1 can be adjusted to trigger the relay at a specific light level.

An LED indicates when the relay is activated.

Formulas:

Below are the formula for the transistor base resistor R1 for above circuit diagram simple light operated relay circuit:

Base Resistor Calculation R1:

In order to maintain appropriate switching operation and keep the base current of the BC547 transistor at a safe level, the base resistor R1 is essential.

The following formula can be used to get the base current I_B:

I_B​ = V_CC​−V_BE​​ / R1

where,

• V_CC is the supply voltage is +12V.
• V_BE is the base emitter voltage of the BC547 transistor is commonly measured at 0.7V.
• I_B is the base current.

The formula may then be rearranged to find the value of R1:

R1 = V_CC​−V_BE​​ / I_B​

Select a value for R1 that will guarantee that I_B is within the operational range of the BC547 transistor, which is typically between 5 and 20 mA.

Note:

The circuit design process may be initiated using these formulae.

Depending on the intended operating circumstances and the characteristics of individual components, adjustments can be required.

How to Build:

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

Gather Components:

• Collect all the components mentioned earlier.
• Use a breadboard or a PCB to build the circuit.
• Ensure the board is clean and free of any debris.
• Connect the LDR in series with a resistor to create a voltage divider.
• Connect this to the base of transistor T1.
• Connect the emitter of T1 to ground and the collector to one side of the relay coil.
• Connect the other side of the relay coil to the positive supply voltage.
• Connect a diode across the relay coil to protect T1 from back EMF.
• Connect an LED in series with a current-limiting resistor from the collector of T1 to indicate when the relay is activated.
• Connect a preset VR1 across the LDR to adjust sensitivity.
• Connect one end of VR1 to the base of T1.

Power Supply:

• Connect the circuit to a suitable power supply.
• Ensure the voltage matches the relays specifications.

Test the Circuit:

• Power on the circuit and test its functionality.
• Adjust VR1 to trigger the relay at the desired light level.

Enclosure:

• Once the circuit is working correctly consider placing it in an enclosure to protect it from dust and moisture.

Safety Precautions:

• Always work with caution when dealing with mains voltage.
• Ensure the circuit is properly insulated and grounded to prevent electric shocks.

Final Testing:

• After assembling the circuit perform final tests to ensure it operates safely and reliably.

Note:

• If you are not comfortable working with mains voltage or electronics in general consider seeking help from someone with more experience.

Conclusion:

To conclude, a light operated relay circuit is a simple yet effective way to control the switching of a relay based on the ambient light level.

By using a light sensor and a few other components, such circuits can automatically turn on or off lights or other devices in response to changes in light intensity offering a convenient and energy efficient solution for various applications.

References:

Light-operated Relay Circuit.

Relay-controlled light circuit