Skip to content

An Ultimate Guide to Light Sensor Circuit: LDR Circuit

In basic electronic projects, one of the coolest circuits is the light sensor circuit. Its main function is to detect the light in the surroundings, and the result can be detected by the brightness of the LED.

What does this circuit use for?? It is used for knowing the working of Light Dependant Resistors, Negative-Positive-Negative transistors, and the sensitivity of the resistor in the circuit. If we want to know the presence and absence of lights in the environment, these types of circuits can be used. These types of Light sensor circuits are used in commercial and non-commercial areas. These can be used in homes and industries. These can be used to save energy. If the street lights are on due to some negligence, then we can use these circuits to off them. These sensors can also detect X-rays.

It is a simple circuit that can control electrical appliances in homes and on the streets. We can control lights, fans, air coolers, street lights, and a number of other electrical appliances by using this circuit. To maintain load management, we can on and off these appliances by this circuit.

We can also control the street lights which are erected on risky highways. This circuit contains major electrical and electronic components

  1. Light sensors
  2. Darlington Pair
  3. Relay

Below is a brief introduction to these components.

· Light Sensor

Light Sensor Schematic
Light Sensor Schematic

Different types of light sensors like photocells, photodiodes, PV cells, phototubes, PMT, phototransistor, and many more. But LDR is used as a light sensor in this light sensor circuit.

These circuits don’t produce any kind of energy.

But the change in the light changes the resistance of LDR. It can be used in dirty environments. It can be used outdoors and indoors. We can say that this circuit is much preferable to another circuit with many qualities. Light intensity control Light DependentResistor (LDR). These resistors are composed of Calcium Sulfide (CA’s), which is used to reveal photoconductivity. CA’s is a high-resistance semiconductor material.

LDR exhibits a very high resistance of a few mega ohms during nighttime when the light illuminated on the LDR decreases. LDR exhibits very few resistances of 100ohms during day time when the light illuminated on the LDR increases. Hence the resistance decreases with the high intensity of the light illuminated on the LDR, and the resistance increases with the low intensity of the light illuminated on the LDR. We can say that the relation between resistance and the light is inversely proportional to each other.

· Darlington Pair:

It’s an electronic component made with the combination of two Bipolar Junction Transistors connected in such a way that it allows a very high amount of current gain. This transistor is used in this light sensor circuit. As compared to the general transistor, this Darlington transistor has a very high current gain.

The input delivered to the load through the Darlington pair is calculated as the sum of the transistor’s input current and gain. We are aware that if the base voltage is more than 0.7 volts, the transistor will turn on, but for a Darlington pair, two transistors must be turned on. Therefore, the base voltage must be 1.4 volts.


In order to activate the load or to link the load to both the light sensor circuit and the AC mains, a relay is essential in the light sensor circuit.

The coil that typically makes up a relay is energized whenever it receives enough supply (the needed quantity of supply depends on the relay’s rating).

Practical Applications of Light Sensor Circuits:


The practical applications of Light Sensor Circuits are:

  1. It is used to design various embedded systems based on sensors such as security alarms and fire alarms etc.
  2. It controls street light systems automatically.
  3. It can manage a light system on a risky and dirty highway.

· Sunset to Sunrise Lightning Switch:

This switch is specially designed to control the light illuminated on the LDR light sensor automatically, as we know that the LDR resistance changes with the change in the light intensity illuminated on the LDR. The 555 timers are linked in biostable mode and receive the LDR output. The trigger of a load through a TRIAC is managed by the 555 timer’s output. As a result, this light sensor circuit automatically turns on the load at sunset in the evening and turns it off at sunrise in the morning.

· LDR Based on light Intensity Control for Street Lights

We use HID Lamps in a proposed system to turn on and off the street lights because the energy consumption of these lamps is high. But this HID Lamps system has some drawbacks, so in order to overcome these drawbacks; we use LDR because this system has LEDs to meet up the needs.

This project consumes low power as compared to HID Lamps because the light-depending resistor is installed in it, which reduces the light according to the daylight.

A streetlight is made up of a bunch of LEDs that can be controlled by this LDR system as the microcontroller has been installed in it, which is comprised of programmable instructions that control the light.

As traffic on the roadways tends to drop in the late hours of the night, the light intensity is kept high during peak hours. It then gradually declines till dawn. Finally, the street lights go out fully in the morning and come back on at 6 o’clock in the evening.




                Get Fast Quote Now