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What an LDR Is
LDR stands for Light Dependent Resistor (also called a photoresistor or photocell). It is a semiconductor component whose electrical resistance decreases as light intensity increases. In bright daylight: LDR resistance is very low (1–10 kΩ) - the control circuit sees this as 'day' and keeps the light OFF. At dusk when ambient light drops below the threshold: LDR resistance rises sharply (100 kΩ+) - the circuit interprets this as 'night' and switches the light ON. The LDR is typically a small disc-shaped component, 5–10mm diameter, with a textured surface that looks like a maze or spiral - this is the light-sensitive semiconductor film.
LDR vs Photodiode vs Photocell - The Differences
LDR (Light Dependent Resistor): simplest and cheapest. Slow response (seconds). Used in basic solar street light controllers and simple timers. Adequate for dusk-to-dawn switching where fast response is not needed. Photodiode: faster, more sensitive, more accurate lux threshold. Used in higher-quality controllers and standalone photocell modules. Photocell (NEMA socket type): a standardised module combining a photodiode or phototransistor with control circuitry, packaged for easy mounting on any luminaire with a NEMA socket. Interchangeable between luminaires. PIR (Passive Infrared): detects moving warm objects (people, vehicles) - not for dusk-to-dawn switching, but for motion-activated brightening. Often combined with an LDR in smart controllers.
Why LDR-Based Lights Sometimes Misbehave
Light stays on all day: the LDR is shaded (sees darkness permanently) - remove obstruction. Or the LDR has failed (resistance always high) - replace controller. Light flickers at dusk: the ambient light at dusk is right at the switching threshold, causing the LDR to toggle back and forth as clouds move. This is normal for 10–15 minutes. If it continues all night: the threshold is set too high - adjust the sensitivity trimpot on the controller, or replace with a higher-quality controller. Light switches off in bright moonlight: very cheap controllers use LDRs with extremely high switching thresholds - full moon (0.1 lux) is too close to the threshold. Better controllers use 5–10 lux threshold, well above moonlight levels.
