Automatic Light Control Circuit diagram using LDR

These circuits can be used for different automatic light sensing projects including automatic night lamp, automatic street light controller, dark sensor circuits, etc.

Automatic night lamp circuit using IC 555

An automatic night light circuit controls the switching of light by sensing the surrounding light intensity. The circuit ON the LED light, when the incident light intensity falls below a particular limit. And OFF the light when the incident light is above the limit.

In the circuit, the threshold and trigger pin of the NE555 has connected with a voltage divider network. It can obtain a trigger voltage which is half of the voltage at threshold pin. In a 555 IC, when the threshold pin has a voltage above 2/3Vcc, the output switches to a low state. And output switches to a high state when the voltage at trigger pin falls below 1/3Vcc.

automatic night lamp, LDR night light

The LDR (Light dependent resistor) has a negative co efficient of resistance with the light intensity. So, when the intensity of the incident light is high, the voltage drop across the LDR decreases. Thus, the voltage across the threshold pin reaches the threshold value and the Light OFF. Similarly, with the decrease in light intensity, the voltage across the threshold pin and the trigger pin decreases and the Light ON.

The light sensitivity of the circuit can be adjusted by the varying the potentiometer R1.

To operate external lamp with the same circuit, interface a relay with the output pin3 of the 555 IC. So it can control 230V AC lights or similar externally powered lights with the same circuit. For relay interface, refer the circuit diagram of the automatic power cut off circuit.  The relay can be connected similarly as shown in that circuit.

Components required

IC – NE555

Resistor – R1 – pot 407k, R2,R3 – 82K, R4, R5, R6- 220Ω


Capacitor – C1 – 0.01uf

Diode – D1 – D9 – White LED

Automatic Light Switch using LDR and Transistor

This circuit adjusts the output light with respect to the ambient light intensity. That is as the incident light from the surroundings reduces, it automatically increases the brightness of the LED. Similarly, inverse as the light increases. Hence this circuit can consider as a simple light level regulator. Because it show’s a gradual variation in light, not a steady ON – OFF switching.

In the circuit, the sensing part of the circuit is a photo resistor or LDR, (Light Dependent Resistor) which have a negative coefficient of resistance with respect to the intensity of light. That is the value of resistance varies inversely proportional to the light intensity.

In the above circuit, the LDR is connected across the Base-Emitter terminal of the transistor. So, with the variation in the intensity of light, the base current inversely changes in accordance to it. Thereby adjusts the collector current or current through the LED.

Components required

Transistor – Q1- BC547

Resistor – R1 – 39k, R2 – LDR -5mm

LED -White

Supply – 6v battery

Automatic ON OFF Light using Op-amp

Here unlike from a transistor circuit, the output has only steady ON state or OFF state for varying ambient light intensity. The LED ON and OFF when the amount of incident light rise or fall with respect to a threshold intensity value. Whereas the transistor circuits lights LED  gradually.

automatic light, LDR automatic light using opampIn the circuit the op-amp act as a comparator. In a comparator circuit, the output will be at either positive saturation or at negative saturation. The comparator output will be high (positive saturation) when the input voltage of non-inverting terminal is greater than the inverting terminal. And low output (negative saturation) state for a higher inverting voltage. When both terminals kept open, the output will be positive saturation voltage (due to offset voltage of practical op-amp ICs).

As the light falls on the LDR, its resistance decreases and a voltage drop created across the resistor R1. In the circuit the non-inverting terminal is open. And thus for a small rise in the inverting terminal voltage, the output switch to negative saturation and the LED will reverse biased. In full darkness, the resistance of LDR will be high and the voltage drop across is maximised. Whenever the full voltage drops across the LDR, the op-amp switches to positive saturation and ON the LED. The circuit has a high sensitivity to light, a small variation in LDR resistance can switch the circuit OFF.

The sensitivity of the circuit can be varied by adjusting the resistance R1. Because variation in the value of R1 can alter the threshold point to a small extent. For a wide adjustment, a reference voltage value is required to be added to the non-inverting terminal. A potentiometer or voltage divider circuit can be used to obtain a variable reference voltage.

Components required

Op-amp IC – LM741

Resistor – R1 – 39k

LDR -5mm

LED -white 6V

Supply – 6v battery

For here circuits, the LDR should properly be placed to sense the incoming light.

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