Photoresistors (LDRs) — Resistance That Changes With Light
A photoresistor, also called a light-dependent resistor or LDR, changes resistance according to the amount of light falling on it. It is a simple way to turn light level into an electrical resistance.
Less light → higher resistance
A light-sensitive voltage divider
Just as with a thermistor, an LDR is often paired with a fixed resistor as a voltage divider. The output can operate a comparator, transistor, relay driver, or analog-to-digital converter.
Common uses
- Night lights and dusk-to-dawn controls.
- Automatic display brightness.
- Simple beam-break detectors.
- Camera and light-meter circuits.
- Light-following hobby projects.
- Older optical audio and control circuits.
It is not a fast precision photodetector
Photoresistors are inexpensive and easy to understand, but they are not normally chosen when very high speed, precise linearity, or tightly controlled spectral response is required. Photodiodes and phototransistors are often better for those jobs.
Response time and memory
Some photoresistors respond relatively slowly compared with semiconductor photodiodes, and their resistance can take time to settle after a large change in illumination. That delay can actually be useful in simple lighting controls, but it matters in faster circuits.
Next: Varistors.