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A Light-Dependent Resistor (LDR) or photoresistor is a passive component whose resistance decreases as light intensity increases. The GL5528 is a popular choice with a resistance range from ~1 MΩ in darkness to ~1 kΩ in bright light.
| Component | GL5528 LDR |
| Dark resistance | ≥ 1 MΩ |
| 10 lux resistance | 8 – 20 kΩ |
| Spectral peak | 540 nm (green) |
| Response time | 20 ms rise / 30 ms fall |
| Max voltage | 150 V |
| Package | 5 mm photoresistor |
CdS (cadmium sulphide) semiconductor forms more conductive paths when photons of sufficient energy excite electrons into the conduction band. Combined with a fixed pull-down resistor (e.g. 10 kΩ), it forms a voltage divider whose output voltage rises with light intensity.
Non-linear response — logarithmic mapping required for accurate lux values. Slow (20–30 ms) response unsuitable for fast flicker detection. CdS contains cadmium (RoHS restricted). Sensitive to temperature drift. No spectral selectivity.