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12V DARK SWITCH WITH 741 OP AMP Schematic Circuit Diagram

It works as a dark key with the LDR that you see in the diagram. The circuit consists of four resistors, one opamp, two potentiometers, one LDR, one switching transistor and one lamp. In the circuit, the value of the resistor R1 is directly proportional to the internal resistance of the LDR and operates as a voltage divider between 6-12V.

A 12V Dark Switch with a 741 Operational Amplifier (Op Amp) is a circuit that acts as a light-sensitive switch. It controls the lamp based on the intensity of light falling on the Light-Dependent Resistor (LDR). Here’s a description of the components and their functions in the circuit:


  1. Light-Dependent Resistor (LDR): The LDR’s resistance changes based on the intensity of light falling on it. In this circuit, it acts as a sensor for detecting light levels.
  2. Operational Amplifier (741 Op Amp): The 741 Op Amp is configured as a voltage comparator. It compares the voltage at its non-inverting input (which is set by the LDR and potentiometers) with a reference voltage at its inverting input.
  3. Resistors (R1, R2, R3, R4): These resistors are used to set the reference voltage levels and control the sensitivity of the circuit.
  4. Potentiometers (Pot1, Pot2): Potentiometers are adjustable resistors used to fine-tune the voltage levels at different points in the circuit.
  5. Switching Transistor: The transistor acts as a switch, controlling the lamp based on the output from the operational amplifier.
  6. Lamp: The lamp is the output device. It is turned on or off based on the light levels detected by the LDR.

Working Principle:

The LDR, in combination with R1, forms a voltage divider connected to the 6-12V power supply. As the light intensity changes, the resistance of the LDR changes, altering the voltage at the non-inverting input of the op amp. Potentiometer Pot1 is used to set a threshold voltage (reference voltage) at the inverting input of the op amp. Potentiometer Pot2 is used to adjust the sensitivity of the circuit.

12V DARK SWITCH WITH 741 OP AMP Schematic Circuit Diagram 1

The sensitivity of the circuit depends on the internal resistance of LDR1 and is adjusted using the RV1 potentiometer. RV1 acts as a voltage divider, continuously comparing the resistance values of LDR1 and R1 with the voltage divider value U1. When RV1 is set at 6V and LDR1 is in dark, a positive voltage is applied to the non-inverting input of U1. This triggers Q1, illuminating L1.

As L1 emits light, LDR1’s internal resistance decreases, pulling the opamp output to 0V and turning off T1 and L1. The presence of R4 ensures a constant voltage on the opamp output, preventing Q1 from turning off completely. RV2 adjusts the gain: at high resistance, the opamp functions as a comparator, while at low resistance, it works as an amplifier.

741 Op Amp Pinout


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