Grades 3–5 · Systems
How does a streetlight know when evening arrives?
Some streetlights have a small light sensor called a photocell. Light reaching the sensor changes an electrical signal. The signal gives a controller information about how bright the surroundings are.
In the diagramA sensor, a rule and a lampSensorControllerlight readingdark long enough?Rule can ignore brief shadowsSome lights use photocells; others use clocks or networks.
When the measured light stays low enough, the controller switches on the lamp. When daylight returns and the reading becomes high enough, it switches the lamp off. The sensor does not need to see a clock or understand the word evening.
A passing shadow should not make a light switch on and off repeatedly. Controls can require a reading to last for a while, or use different levels for switching on and switching off. That makes the response steadier.
Other streetlights follow a schedule or receive commands through a network. In each case, the lamp is only the part we see shining. Information, a control rule and a power connection make the service work.
About this explanation
Starting knowledge: Understand a sensor noticing light and a switch controlling a lamp.
Sources
- Signify — Outdoor sensing