LDR

This post is about LDR. An electronic component used to build light sensors. If you build with or without microcontrollers, you can build light sensors with this component.

How it works?

LDR is the abbreviation for Light Dependent Resistor, nothing more than a resistor whose resistance varies with variation of light intensity received. When more light is received, the resistance is lower and vice versa. The LDR’s characteristic curve is a negative exponential.

The equation of LDR’s characteristic curve.

R_{f}=A\cdot E^{-\alpha }

Where E is luminous energy in Lux, R_{f} is the resistance; A and \alpha are constants given by the material.

Material

Usually, LDRs are made by cadmium sulphide (CdS) because it is the closest to human sensitivity in the visible light spectrum.

Other materials are cadmium selenite (CdSe), lead sulphide (PbS), and lead selenite (PbSe). The material must liberate a few free electrons in the dark and liberate more with light incidence. The lead sulphide (PbS) can respond to infrared light.

Construction

The sensitive light material of LDR stays in zig-zag to have the desired resistance and it is deposited on a ceramic substrate.

Applications

This component can be used in any application that requires a control by light or a luminous intensity sensor for a robot or mechanism. This is the LDR’s representation of schematics; the left one is the more commonly used.

A light sensor with LDR can be built in the forms below.

In the left circuit, the output tension (in Out) increases when there is light incidence in the LDR, and in the circuit on the right, the opposite, tension in “Out” decreases with light incidence. You can link the output of these circuits to the analog input in some microcontroller. Or you can link the output to the base of a transistor to turn on LEDs, relays, motors, etc. In the circuit below, you can replace the LED and R2 with a relay or a motor.

Another application is to get an oscillator like the 555 astable and replace a resistor with an LDR; oscillators always have resistors because the oscillation frequency depends on them. You can create a light-controlled oscillator.