Light Dependent Resistor

Posted on Feb 4, 2014

A Light Dependent Resistor (aka LDR, photoconductor, photoresistor, or photocell) is a component used commonly in electronics which has a resistance which decreases as the intensity of the light hitting its surface increases and visa versa. A typical light dependent resistor is pictured above together with (on the right hand side) its circuit diag

Light Dependent Resistor
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ram symbol. The vast majority of LDRs are made from cadmium-sulphide (CdS), and they are very cheap, but also not very accurate. They are very good for detecting changes in light levels and determining if it is `dark` or `light`, but without individual calibration they not suitable for accurately measuring light levels. Pictured below is a typical light dependent resistor. It has two wire leads which terminate in the face of the light detector - the two metal dots you see are the ends of those electrodes. The main body of the light detector component is made of ceramic - an excellent insulator. On the face of the ceramic a thin strip of cadmium sulphide is coated in a zig-zag pattern (to maximise the length of the strip while keeping the component small) which is connected at each end to an electrode. The front face is then coated in clear plastic, epoxy, glass, or similar. Cadmium-sulphide is a high resistance semiconductor, and is chosen for light detectors as it gives the same spectral response as the human eye - i. e. it `sees`/responds to the same range of wavelengths of light that we can see. Specifically, cadmium-sulphide is a photoconductive material. That means that photons of light hitting it with sufficient energy will release electrons from their atomic bonds. When a negatively charged electron is freed by a photon of light, it leaves behind a positively charged `hole`. When a voltage is applied across the two...

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