a light-dependent resistor (LDR)

An LDR is the simplest light sensor there is: a resistor that gets less resistant the brighter the light. In the dark its resistance is huge — hundreds of kilohms or even megohms. Shine light on it and the resistance tumbles to a few hundred ohms. There is no junction, no polarity, nothing to get wrong: it is just a light-controlled resistor you drop straight into a voltage divider.

Inside is a film of light-sensitive semiconductor, classically cadmium sulfide (CdS), printed in a zig-zag between two contacts. Photons free up charge carriers and make the material conduct better, so resistance falls as light rises — a wide swing, often a factor of a thousand from dark to bright. The relationship is far from linear: resistance follows roughly a power law of the light level, so doubling the light does not halve the resistance. To use one, put it in a divider with a fixed resistor and read the tap voltage; pick the fixed resistor near the LDR's resistance at the light level you most care about, so the divider is most sensitive there.

Why this matters: LDRs are cheap, dead-simple, and forgiving, so they fill night-lights, street-lamp switches, camera and phone brightness sensors of older designs, and countless beginner projects. The honest caveats: they are slow (tens of milliseconds to respond, useless for fast light pulses), their response is very nonlinear and varies a lot between parts, they drift with temperature, and the classic CdS type contains cadmium, a toxic heavy metal now restricted in many products. When you need speed, linearity, or precision, reach for a photodiode instead — but for is-it-light-or-dark, the LDR is hard to beat.