3 April 2013


LDR is a light dependent resistor. LDR’s or Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically. LDR is used to measure the light intensity. It is an input device, that senses the light and gives the value to microcontroller.

Two cadmium sulphide (cds) photoconductive cells with spectral responses similar to that of the human eyes. The cell resistance falls with increasing light intensity. Applications include smoke detection, automatic lighting control, and batch counting and burglar alarm systems.

The animation opposite shows that when the torch is turned on, the resistance of the LDR falls, allowing current to pass through.

This is an example of a light sensor circuit :

When the light level is low the resistance of the LDR is high. This prevents current from flowing to the base of the transistors. Consequently the LED does not light. However, when light shines onto the LDR its resistance falls and current flows into the base of the first transistor and then the second transistor. The LED lights. The preset resistor can be turned up or down to increase or decrease resistance, in this way it can make the circuit more or less sensitive.

There are just two ways of constructing the voltage divider, with the LDR at the top, or with the LDR at the bottom:

There are just two ways of constructing the voltage divider, with the LDR at the top, or with the LDR at the bottom:

You are going to investigate the behaviour of these two circuits. You will also find out how to choose a sensible value for the fixed resistor in a voltage divider circuit.

Remember the formula for calculating Vout 


The sensitivity of a photo detector is the relationship between the light falling on the device and the resulting output signal. In the case of a photocell, one is dealing with the relationship between the incident light and the corresponding resistance of the cell.



Analog Applications:

  • Camera Exposure Control
  • Auto Slide Focus - dual cell
  • Photocopy Machines - density of toner
  • Colorimetric Test Equipment
  • Densitometer

Digital Applications:

  • Automatic Headlight Dimmer
  • Night Light Control
  • Oil Burner Flame Out
  • Street Light Control
  • Absence / Presence (beam breaker)
  • Position Sensor
Related Projects
Temperature based automatic fan speed Controller
Remote Control Based Home Appliances Control
Season Based Street Light Switching Based On Sensors

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