Book 13, Question 108
Why Do Streetlights Switch on by Themselves at Dusk?
A streetlight switches on by itself because it has a small light sensor! In daylight, the sensor keeps the lamp off. When it gets dark, the lamp turns on.
More fun facts
- In Chandler, Arizona, city rules say every LED streetlight is controlled by its own photocell.
- The photocells there are factory set to turn on at one foot candle of light and turn off at two foot candles.
- So a light switches on when daylight fades to one foot candle, but stays on in the morning until the light reaches two.
- The photocells are installed facing north.
- For older sodium lamps, a photocell is made to fail so the lamp stays on; for LED lamps, it fails so the lamp stays off.
- Electronic photocells include surge protection to guard them against power surges.
Sources: chandleraz.gov
Questions kids ask
Why do some streetlights turn on during a dark storm?
Their light sensor senses that it is dim enough, even if it is daytime.
Does every streetlight have its own sensor?
Many do. Some cities give each light its own photocell, while others control lights in groups.