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.

Trace it yourself

Find the supply on the left. The upper terminal is the hot; the lower one, dropping down to the bottom edge, is the neutral. Finger on the hot.
Follow it to the first terminal of the photocell. Now look at the blade. It is dark, so the cell is conducting — the blade lies on the second terminal. Go through.
Up and over on the switched hot to the top terminal of the light. Cross the lamp: this is the only place the hot side meets the return.
Down from the lamp's bottom terminal and back along the neutral to the supply. Loop closed, lamp on.
Now make it daylight. The blade lifts. Your finger stops at the cell's first terminal — and notice it is still copper up to there. The lamp is dark; the cell is live.

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.

What goes wrong

1. The cell can see its own lamp

Symptom: at dusk the light comes on, goes off, comes on again — and keeps cycling until it is fully dark.

Run the diagram in your head as a loop. Dark: the contact closes, the lamp lights. Now the lamp's light lands on the cell. The cell reads "daylight", opens, the lamp goes out. It is dark again — the contact closes — and round it goes. The delay built into a real cell slows this down but cannot stop it if the cell is aimed at the fixture. The circuit is perfectly wired; the cell is pointed at the wrong sky. Turn it, shade it, or move it, and the cycling stops.

2. The cell put in the neutral

Symptom: it switches perfectly — and the fixture is live all day with the light off.

Slide the blade across to the return side in your head. Opening it still breaks the loop, so the lamp goes out in daylight and nobody notices. But the hot now runs unbroken to the lamp, and every terminal on the fixture sits at line voltage from dawn to dusk. Same lesson as the single-pole: the switching device goes in the hot, whatever operates it.

3. A three-wire cell with its neutral left off

Symptom: the light never changes state — stuck on, or stuck off, whatever the sky does.

The diagram shows the cell as a bare switch, which is honest for the switching part. But many cells are three-wire: the sensing electronics need their own supply, hot to neutral, to work at all. Leave that neutral off and the electronics are dead, so the contact simply rests wherever it rests. Nothing is shorted, nothing trips, and the lamp does whatever the resting contact says. Before you blame the cell, check it has the supply it needs to think with.

Sur le chantier

A photocell is a switch with a delay and an opinion about the sky, which means half the call-backs are not wiring at all — they are aim, and the other half are somebody who did not wait out the delay before deciding it was broken.

Common mistakes

The cell where it can see its own fixture. Perfectly wired, and it hunts on and off all evening. Aim it at open sky away from the lamp, or shade it from the fixture.
The cell landed in the grounded conductor. It switches, so nobody notices — and the fixture sits at line voltage all day with the light off. The switching device goes in the ungrounded conductor whatever operates it.
A three-wire cell with its neutral left off. The sensing electronics need their own supply to think with. Without it the contact simply rests wherever it rests and the lamp never changes.
Taping the lens to test it and not waiting. The built-in delay is there to stop it flickering at a passing headlight. Cover it, then give it the time the sheet says before you call it faulty.
Aiming it at the sun instead of the sky. Direct low sun early and late makes it switch to the sun's position rather than the ambient level.
Tucking it inside a fixture's lens where it bakes. Heat kills them faster than anything else on the pole.

What the inspector looks for

The switching device in the ungrounded conductor.
A weatherproof entry: the right knockout, the gasket seated, the threads made up, and nothing left for water to track down into the fixture.
The bond carried through to the fixture and, on a pole, to the pole.
On a pole or standard, the access and the in-line protection the book asks for on that kind of supply.
Support and the conductor entry for the run feeding it.

Check it before you call it done

Prove the circuit dead at the fixture, not just at the panel.
Land it, make the entry and the gasket up, and dress the conductors so water cannot follow them in.
Energise in daylight. The lamp must be off. If it is on, the cell is either covered or the wiring is the other way round.
Cover the lens completely and wait out the delay. The lamp comes on. Uncover and wait again. It goes off.
Read across the cell in daylight with the lamp off: you should see the full supply across an open switch. That proves it is the cell holding the circuit open and not a dead lamp.
Come back after dark and look at the fixture from where the cell is. If the cell can see this lamp or any lamp near it, move it now.

Questions fréquentes

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.

Voir aussi

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

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