Home > Circuit Bricks > Light Sensor - LDR
UPDATED:.21:08 21 October 2013


  • As the light shining on the LDR changes, so does its resistance.
  • In bright light it has a low resistance, in darkness a high resistance.
Like most sensors, an LDR must be used as part of a potential divider, this converts a change in resistance into a change in voltage.

There are 2 ways to connect an LDR in a potential divider. These are shown below and work exactly opposite from one another.

Supply voltage from 3 to 15v.


Circuit Diagrams
An output voltage proportional to how much light is on the sensor.

VR1 can be used to adjust the amount of voltage for any given amount of light.

The output current is very low and anything connected to it must have a high input resistance.

It is common to connect the output to a comparator or transistor driver

Light Detector

Darkness Detector

With the LDR at the top of the potential divider, the brighter the light on the LDR, the higher the output voltage. With the LDR at the bottom of the potential divider, the darker the light on the LDR, the higher the output voltage.

There are 2 or 3 common types of LDR and the main difference is size. For general light sensing any of them are suitable.

A common type is ORP12.



If you want to frequently adjust the output voltage level, use a chassis mount potentiometer. These are commonly used as volume controls and for light dimmers etc.

Otherwise use a preset which you can adjust with a screwdriver and is mounted on the PCB.

For bright sensing use 10K
For dim lighting use 47K - 100K

  • If possible mount the LDR on the circuit board on the PCB, if not you will need to provide 2 pads on the circuit board to connect wires to for the LDR.
Written by Phil Townshend - 2008
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