Opamp 741 Based Mini Project: Trip the Relay Using Photoresistor(LDR)

by ADMIN · November 26, 2016

Opamp 741 Based Mini Project: Trip the Relay Using Photoresistor(LDR)

The basics of the Light Depended Resistor (LDR) or photoresistor is discussed separately in another post. In summary,
In the presence of brightness, the LDR has low resistance.
In the absence of brightness, it has very high resistance.

Figure shows a photoresistor(LDR) ie, cad cell as the sensor and an operational amplifier as the control unit.

LDR based mini Project

Construction of the Circuit:

This circuit is constructed, to energize the relay coil (Trip the relay circuit) when the photoresistor is in the presence of light.

Note about Cad Cell:
Cad cell or LDR has a resistance of about 50 ohms in direct sunlight.
Also it has more than 1000 ohms in the darkness.

Resistor R1 and the photoresistor form a voltage divider circuit. It is connected to the inverting input of the OpAmp 741.
A potentiometer is used to preset a positive voltage at the non inverting input of the opamp.
This control adjusts the sensitivity of the circuit.
Resistor R4 limits the base current to the BJT.
The transistor is used to control the current needed to operate the relay coil.
Opamp 741 does not have enough current rating to control the relay.
Diode D1 is used as a kick-back diode.
The LED is mounted on the outside of the setup and used to indicate when the relay coil is energized.
Resistor R3 limits the current to the LED.

Operation of the Circuit:

Note about Opamp:
When Opamp is in open loop configuration,
If the inverting input is high, then output will be low.
If the non-inverting input is high, then the output will be high.

LDR is in the presence of brightness:

Assume that the potentiometer has been adjusted to provide a 6V at the non-inverting input.
When the cad cell is in the presence of light, it has low resistance.
So, less than 6V is applied to the inverting input.
Since the non-inverting input has higher voltage than the inverting input, the output is high.
Consequently, the LED and transistor are tuned ON.
As the transistor is turned ON, it forms the closed circuit (12V, relay coil, transistor).
Thus the relay coil is energized. ie, the relay tripped. This concept can be applied in real-time scenario.

LDR is in the presence of darkness:

When the cad cell is in the presence of darkness, its resistance increases.
When the resistance is more than 4.7K, a voltage more than 6V is applied to the inverting input.
It makes the output of opamp at a low state.
Consequently, the LED, BJT are in OFF state.

Please use this partial circuit in your real time project.... Please share in the comments section, if face any difficulties while implementing this concept in your project....

Opamp 741 Based Mini Project: Trip the Relay Using Photoresistor(LDR)