WiremanWireman · Motion sensor or photocell problems: troubleshooting for electricians · 2023 NEC ·
Motion sensor or photocell problems
Lights on all day, never on at night, cycling, or a motion sensor that won’t trigger or won’t shut off.
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How it works
A photocell turns its load on when the light level falls and off when it rises. It has to see sky, not its own lamp or a nearby light, and most have a delay so headlights or lightning don’t switch them. Many photocells are built to fail in the on position, so a light that burns all day is often a failed cell. A motion sensor (usually passive infrared) reacts to heat moving across its zones. It has settings for time delay, sensitivity and an ambient-light or dusk limit that keeps it from triggering in daylight. Wall sensors without a neutral power themselves through the load.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What controls the light?
- A photocell (dusk to dawn)What does the photocell-controlled light do?
- Stays on during the dayTest: Check what the photocell sees
- It was shaded or dirty; now it turns offThe photocell is seeing the wrong light
- Clear view of the sky, still onA failed photocell
- Never comes on at nightTest: Cover the photocell
- Yes, it comes onThe photocell is seeing the wrong light
- No, it stays offTest: Measure the photocell input and outputExpect: Line voltage on black. With the eye covered and the delay passed, line voltage on red too.
- No voltage on the line leadNo power reaching the control
- Line in, nothing outA failed photocell
- Voltage out, lamp still darkThe fixture, not the control
- Cycles on and off at nightLight reflecting back into the photocell, or a lamp cycling
- Stays on during the dayTest: Check what the photocell sees
- A motion or occupancy sensorWhat does the motion-controlled light do?
- Never turns onTest: Walk-test the sensor
- Works in test modeSensor settings
- No response at allTest: Measure the sensor’s supply and outputExpect: Line voltage at the line lead, and line voltage at the load lead while triggered.
- No line voltage at the sensorNo power reaching the control
- Line in, nothing out when triggeredA failed sensor
- Output is there, light still darkThe fixture, not the control
- Stays on or turns on by itselfFalse triggers or a long time delay
- Turns off while people are thereCoverage or time delay too short
- Never turns onTest: Walk-test the sensor
- A photocell (dusk to dawn)What does the photocell-controlled light do?