LDR (light-dependent resistor)

A resistor whose resistance falls as light increases.

Schematic letter: LDR
Unit / key rating: Ohm (Ω)

Pinout

2 leads: No polarity.

Real photos

What is it for?

An LDR's resistance is very high in the dark (hundreds of kΩ to MΩ) and drops in light (a few kΩ). That's how street lamps and night lights switch on by themselves.

It's usually wired as a voltage divider with a fixed resistor, and the midpoint voltage is read by a processor or comparator.

Things to watch out for

⚠ LDRs respond slowly (tens of milliseconds); use a phototransistor or photodiode for fast light detection.

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