Motion sensor and LED bulb not working properly: causes

When a motion sensor that worked for years with incandescent bulbs misbehaves after an LED swap, the cause is usually the sensor wiring and the type of load.

A motion sensor on a stairwell or in a hallway has worked quietly with an incandescent bulb for years. Then the bulb is swapped for an LED and odd things begin: the bulb glows faintly when it should be off, blinks now and then, or never switches off at all. In most of these cases the bulb is not faulty; the sensor's electrical requirements simply do not match how an LED behaves.

Glowing and blinking when off

This is the most common complaint, and the cause lies in how the sensor is wired. Many sensors, especially those that fit into a switch box, connect with just two wires: there is no neutral, so the sensor powers its own electronics with a tiny current that passes through the lamp circuit.

For an incandescent bulb this current is nothing: the cold filament has low resistance, the current passes through and no light is produced. An LED bulb's driver, however, contains a capacitor. The small leakage current slowly charges it, and when the voltage reaches a certain level the driver briefly starts, the bulb flashes for an instant and the capacitor discharges. Then the cycle repeats. The result is either a faint constant glow or a blink every few seconds.

Ways to fix it:

• switch to a three-wire sensor that uses a neutral connection, which is the most reliable option;
• fit the load module (compensator) recommended by the sensor manufacturer in parallel with the bulb;
• if the circuit has several bulbs, keep one of them as an incandescent or halogen lamp for now, as a temporary workaround.

Minimum and maximum load

Look for two figures in the sensor's documentation. The first is the minimum load: older sensors do not work properly below a certain wattage. One or two low-wattage LED bulbs may fall under that threshold, and the sensor behaves erratically.

The second is the maximum load, and here there is a paradox. A sensor may allow a large wattage of incandescent lamps but a much smaller wattage of LED lamps. The reason is inrush current. At switch-on, the capacitor in an LED driver draws a brief but very strong surge. In a relay-based sensor that surge can spark-weld the contacts shut, and then the light never goes off. So check the sensor's separately stated limit for LED loads and stay within it.

Sensors with thyristor or triac electronic switching, on the other hand, can clash with some LED drivers, causing flicker or buzzing. For such sensors the documentation should state LED compatibility explicitly.

The light turns on by itself or goes off too soon

Sometimes the problem is not electrical at all but in the sensor's "eye".

Passive infrared (PIR) sensors react to changes in heat radiation. Typical sources of false triggers:

• warm air from a heater, air conditioner or ventilation outlet;
• sunlight through a window that changes as clouds pass;
• pets;
• curtains, plants or, for outdoor sensors, trees moving in the wind.

Microwave sensors can "see" through thin partitions and glass and react to movement in the next room. With these, the sensitivity needs to be turned down.

Another typical case: the light comes on, goes off after a while and immediately comes back on. If the sensor has a light level (lux) setting and the lamp's own light falls on the sensor's photocell, the sensor decides it is already bright enough and switches off, then switches on again once it is dark. Position the lamp or the sensor so that the light does not shine straight at the sensor, or change the lux setting.

Sensor placement and settings

A PIR sensor detects movement across its field of view far better than movement straight toward it. A sensor at the end of a corridor facing people as they approach notices them late, so the light comes on a few steps too late. Mounting the sensor so that it looks across the path of movement solves this.

If the time delay (how long the light stays on after movement stops) is set too short, someone standing still in a bathroom or storeroom is left in the dark. That is a setting, not a fault.

Troubleshooting order

• test the LED bulb in an ordinary fitting that is not on the sensor; if it behaves normally there, the bulb is fine;
• find out how many wires the sensor uses (with the help of an electrician);
• find the maximum and minimum LED load in the sensor's documentation;
• if the light never switches off, suspect welded relay contacts; the sensor needs replacing;
• for false triggering, check heat sources, sunlight and the direction the sensor faces;
• reset the lux and time delay settings according to the documentation.

Connecting or replacing a sensor means working with mains voltage, so leave it to an electrician.

Zaferoğlu Elektrik does not make sensors; its plant in Ganja produces LED lamps under the Zəfər İşığı brand. If the fix turns out to be a different bulb rather than a different sensor, note the fitting's cap before you shop.