Photocell wiring diagram

Light: on

Switch between darkness and daylight. The cell conducts in the dark, so the lamp comes on at dusk and drops out at dawn — the opposite of a hand switch. The copper highlight is the live path, not a wire colour.

A photocell is a switch operated by daylight: it conducts in the dark and opens in the light. Wired in the hot to a fixture, it brings the load on at dusk and drops it at dawn, all by itself. Flip the diagram between dark and daylight to see the lamp follow the sky.

How this circuit works

Electrically it is a single switch in the hot line — no different in shape from an ordinary single-pole. What is different is who throws it. Instead of a finger, it is the ambient light level. When the light falls below the cell's threshold, its contact closes, the hot reaches the lamp, and the fixture comes on. When daylight returns, the contact opens and the lamp goes out.

The inversion is the thing to hold onto: darkness turns it on. That is the opposite of every hand switch you have used, and it makes complete sense once you remember the job is to light a place when the sun stops doing it for you.

Questions people ask

How does a photocell switch a light?

It contains a light-sensitive element that changes the state of a switching contact. In darkness the contact closes and the load comes on; in daylight it opens and the load goes off. So it is a switch that the sky operates instead of your hand.

Why does it turn the light on in the dark rather than the light?

Because its whole purpose is dusk-to-dawn lighting — it should light an area when the natural light fails. So the sensing is inverted compared with a room switch: darkness makes it conduct, daylight makes it open. Toggle the diagram between dark and daylight and watch the lamp follow.

Why do photocells have a short delay?

To ignore brief changes — a passing cloud, a car headlight, or the light it is controlling shining back at it. A little lag stops it chattering on and off at dusk and keeps it from seeing its own lamp and switching itself off.

Occupancy sensor wiring — switched by movement instead of light.
Single-pole switch wiring — the same circuit, a hand on the switch.
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