What Is a Photoresistor (LDR)?
A photoresistor (LDR – Light Dependent Resistor) is a resistor whose resistance changes with the light falling on it. It's a simple, cheap light sensor.
How Does It Work?
When light hits it, its semiconductor material becomes more conductive; that is, as light increases, resistance drops. In the dark its resistance is very high, in bright light it's low.
The Light–Resistance Relationship
| Environment | LDR resistance |
|---|---|
| Dark | Very high (MΩ) |
| Dim | Medium (kΩ) |
| Bright | Low (hundreds of Ω) |
How Is Light Measured?
An LDR is usually used with a fixed resistor in a voltage divider; the voltage at the middle terminal changes with light intensity and a microcontroller reads it.
Where Is It Used?
Night lights, automatic street lighting, camera light meters and light-threshold circuits.
Response Time
An LDR's resistance changes not instantly but with a short delay when the light changes; adapting to darkness (recovery) is especially slow. So where fast light pulses must be detected, a photodiode is more suitable.
Common Mistakes
- Expecting a fast response: an LDR is slow; for fast light changes a photodiode is better.
- Forgetting color/wavelength: an LDR is more sensitive to certain wavelengths.
- Not isolating ambient light: stray light corrupts the reading.
Frequently Asked Questions
LDR or photodiode?+
Why is its resistance high in the dark?+
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