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Light-Dependent Resistors (LDRs)

Authors:
Geoffrey Hunter
Geoffrey Hunter
Published On:
Mar 3, 2021
Last Updated:
Jun 8, 2025

A light-dependent resistor (LDR) (a.k.a. photoresistor, photocell, photoconductor) is a basic electronic component which can be used to measure light levels. It contains a resistor whose resistance changes when exposed to light. They are one of the cheapest and simplest light-detection components you can buy.

Construction

LDRs are made with semiconductor-based materials to allow them to be sensitive to light. A once popular semiconductor for LDRs was cadmium sulphide (CdS), however it’s use is becoming restricted due to the environmental problems that Cadmium can cause.

The semiconductor has a crystal lattice which, in the absence of light, greatly restricts the movement of electrons across it’s structure. This gives the component a large resistance in the dark. When light strikes the lattice, some of the photon energy is transferred to the electrons, which allows them to break free and conduct. This causes the resistance of component to decrease.

Even though an LDR contains semiconductor material, it is not considered an active device (such as a diode or transistor) as it does not contain a PN junction.

Resistance Changes

LDRs typically have a resistance in the hundreds of kΩk\Omega or MΩM\Omega in the dark, dropping to a few hundred Ω\Omega under bright light1.

Applications

The popular Teletronix LA-2A Leveling Amplifier was an audio compressor produced in the 1960’s which used an electroluminescent panel combined with an LDR to control the gain of the amplifier. As the audio signal got stronger, it illuminated the LDR more, causing it’s resistance to decrease and reducing the gain of the amplifier.2 There were actually two LDRs pointed at the panel, one used for the audio amplifier and the other for the visual meter.3

A photo of the Teletronix LA-2A Leveling Amplifier which used LDRs as part of the gain control circuit.4

Further Reading

A phototransistor is a transistor-based component which is used to detect light levels. It is similar to a standard NPN BJT, but uses light to drive the base rather than an electrical signal. Most phototransistors are sensitive in the infrared region, whilst most LDRs are sensitive to visible light. One benefit of a LDR is that they are non-polarized, so they can work with voltages/currents in either direction.

Footnotes

  1. Nikhil Agnihotri (2021, Jul 12). What are the different types of light sensors?. Engineers Garage. Retrieved 2024-09-30, from https://www.engineersgarage.com/light-sensors-ldr-photodiode-phototransistor-solar-cell/. ↩

  2. Wikipedia (2025, Feb 7). LA-2A Leveling Amplifier [wiki]. Retrieved 2025-06-08, from https://en.wikipedia.org/wiki/LA-2A_Leveling_Amplifier. ↩

  3. Gearspace.com (2019, Mar 5). The Forums > So Much Gear, So Little Time > OPTO Compressor a la LA2A [forum post]. Retrieved 2025-06-08, from https://gearspace.com/board/so-much-gear-so-little-time/1251760-opto-compressor-la-la2a-3.html. ↩

  4. Universal Audio. Home > Analog Hardware > Teletronix LA-2A Classic Leveling Amplifier [product page]. Retrieved 2025-06-11, from https://www.uaudio.com/products/teletronix-la-2a-classic-leveling-amplifier. ↩