Light Dependent Resistor

Find out more about LDRs and their use in renewable energy projects

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.

Light dependent resistor (LDR, photocell, photoconductor)

A typical light dependent resistor is pictured above together with (on the right hand side) its circuit diagram symbol. Different LDR's have different specifications, however the LDR's we sell (1) in the REUK Shop (2) are fairly standard and have a resistance in total darkness of 1 MOhm, and a resistance of a couple of kOhm in bright light (10-20kOhm @ 10 lux, 2-4kOhm @ 100 lux).

Uses for Light Dependent Resistors

Light dependent resistors are a vital component in any electric circuit (3) which is to be turned on and off automatically according to the level of ambient light - for example, solar powered garden lights, and night security lighting.
An LDR can even be used in a simple remote control circuit using the backlight of a mobile phone to turn on a device - call the mobile from anywhere in the world, it lights up the LDR, and lighting (or a garden sprinkler (4)) can be turned on remotely!

Light Dependent Resistor Circuits

There are two basic circuits using light dependent resistors - the first is activated by darkness, the second is activated by light. The two circuits are very similar and just require an LDR, some standard resistors (5), a variable resistor (6) (aka potentiometer), and any small signal transistor (7)

In the circuit diagram above, the LED (8) lights up whenever the LDR is in darkness. The 10K variable resistor is used to fine-tune the level of darkness required before the LED lights up. The 10K standard resistor can be changed as required to achieve the desired effect, although any replacement must be at least 1K to protect the transistor from being damaged by excessive current.

By swapping the LDR over with the 10K and 10K variable resistors (as shown above), the circuit will be activated instead by light. Whenever sufficient light falls on the LDR (manually fine-tuned using the 10K variable resistor), the LED will light up.

Using an LDR in the Real World

The circuits shown above are not practically useful. In a real world circuit, the LED (and resistor) between the positive voltage input (Vin) and the collector (C) of the transistor would be replaced with the device to be powered.

Relay used in Light dependent resistor circuit

Typically a relay (9) is used - particularly when the low voltage light detecting circuit is used to switch on (or off) a 240V mains powered device. A diagram of that part of the circuit is shown above. When darkness falls (if the LDR circuit is configured that way around), the relay is triggered and the 240V device - for example a security light - switches on.

NEW For details of a more advanced light/dark sensor circuit click here to read our new article LM741 Light/Dark Sensor Circuit (10). We have these circuits available for sale.

Web Link References

(1) http://www.reuk.co.uk/buy-MINI-LDR.htm
(2) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(3) http://www.reuk.co.uk/electric-circuit.htm
(4) http://www.reuk.co.uk/Solar-Powered-Irrigation.htm
(5) http://www.reuk.co.uk/Resistor-Colour-Codes.htm
(6) http://www.reuk.co.uk/buy-VARIABLE-RESISTORS.htm
(7) http://www.reuk.co.uk/What-is-a-Transistor.htm
(8) http://www.reuk.co.uk/buy-5MM-RED-LED.htm
(9) http://www.reuk.co.uk/Relays-and-Renewable-Energy.htm
(10) http://www.reuk.co.uk/LM741-Light-Dark-Sensor-Circuit.htm