LDR

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

How it works?

LDR is the abbreviation from Light Dependent Resistor, nothing more than a resistor which change it resistance with variation of light intensity which receive. When more light receive, lower the resistance 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 from human sensitivity in the visible light spectrum.

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

Construction

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

Applications

This component can be used to any application which require a control by light or a luminous intensity sensor to a robot or mechanism. This is the LDR’s representation to schematics, the left one is the more used.

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

In the left circuit the output tension (in Out) increase when there is light incidence in the LDR and to the circuit in the right is the opposite, tension in “Out” decrease 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, relay, motors, etc. In the circuit below you can replace the led and R2 for a relay or a motor.

Other application is to get an oscilator like the 555 astable and replace a resistor to a LDR, oscilators always have resistors because the oscillation frequency depends on them. You can create a light controlled oscillator.