June 27, 2009


In real this is how an LDR looks like..

A Light Dependent Resistor (aka LDR, photoconductor, or photocell) is a device which has a resistance which varies according to the amount of light falling on its surface.

Symbol for LDR

Light sensor circuit

Opposite is a simple light/ dark sensor. This can be connected as an input or switch to another circuit. The sensors has three green wires (1, 2 and 3). Wire 2 should always be connected to one of the inputs. If wire 1 is also connected then the sensor acts as a dark sensor. If wires 2 and 3 are connected to the inputs then sensor operates as a light sensor.

light sensor circuit click for next stage

Functions of LDR...

An LDR and a normal resistor wired in series across a voltage, say 5V DC, can be used to develop a signal. Depending on which is tied to the 5V and which to 0V, the voltage at the point between them, call it the sensor node, will either rise or fall with increasing light. If the LDR is the component tied directly to the 5V, the sensor node will increase in voltage with increasing light. It doesn't really matter which way (rise or fall) because the designer has a choice of several simple circuits to invert the signal as needed for whatever function is to be performed.

This voltage at the sensor node (between the components) will vary gradually with the light level. To use for security light control, it's desired to develop a signal that switches level from one extreme to the other suddenly as light level goes from light to darker and darker. This result could be called a binary signal, a signal with two states (perhaps about 4V and 0.5V), indicating yes/no or 1/0. To develop this, the sensor node voltage can be compared, by an electronic circuit, with some threshold voltage to determine whether the light level is more or less than the threshold. The result of the comparison would give a "DARK? yes/no" determination.

Working of LDR:-


Typically, photocells only operate at low current, so you don't want to use them to directly drive a load, they're more for sensors. However, if you connect a positive voltage source in series with the photocell and then connect the other end to the gate of a transistor, you can use the transistor to drive the load, such as an LED. That way, if you shine a light on the photocell, the LED will turn on. In the sources is a good graphical representation and explanation of this.


Photoconductive cells are used in many different types of circuits and

Analog Applications

· Camera Exposure Control
· Auto Slide Focus - dual cell
· Photocopy Machines - density of toner
· Colorimetric Test Equipment
· Densitometer
· Electronic Scales - dual cell
· Automatic Gain Control – modulated light
· Automated Rear View Mirror

Digital Applications

· Automatic Headlight Dimmer
· Night Light Control
· Oil Burner Flame Out
· Street Light Control
· Absence / Presence (beam breaker)
· Position Sensor


This formula is responsible for calculating the output power of LDR


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Electronics is the study and use of electrical that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves. and semiconductors. The pure study of such devices is considered as a branch of physics, while the design and construction electronic circuits to solve practical problems is called electronic engineering.

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