Standard light switches aren’t exactly hard to use. Most of us flip them on without a thought. But what if you walk into the laundry room with an armload of clothes and can’t even see the switch, let alone get a hand free to turn it on? Or when you’re carrying a heavy box or bags of groceries from the garage and can’t reach the light switch to turn it off? You have to make a second trip, or the light just gets left on. So right there are just two very good reasons to make the switch—to motion detection.
There are two basic types of motion sensor switches: active sensor and passive sensor. Active sensors often referred to as radar-based, send out sound waves into the room and wait for the signal to return. (Some garage door openers do the same thing.) If someone enters the room or moves inside the room, the speed of the returning sound waves changes, triggering the switch.
Passive sensors also called passive infrared sensors (PIR) or pyroelectric detectors, detect body heat from humans and animals. The sensor uses a photodetector, which converts light in the wavelengths into electrical current that triggers an alarm in the minicomputer housed in the detector, activating the switch. To prevent nuisance switching, the computer ignores slow changes in room temperature due to sunlight.
Motion sensor switches can automatically turn the lights on or off, or both. Occupancy switches turn the lights on when you enter the room; when you leave, the switch waits a preset time before turning the lights off automatically. Vacancy switches must be turned on manually but turn the lights off automatically when you leave the room. Both types leave the lights on if they detect motion in the room, so if you’re perfectly still, they might turn the lights off.
If you have kids who tend to leave their bedroom lights on despite your loving reminders, vacancy-type motion sensor switches can solve the problem. You can also get motion sensor switches with a built-in dimmer, allowing you to set the desired light level manually. This is a helpful feature for bathrooms and bedrooms, where you might not want full brightness at night or in the morning or when others are sleeping.
Motion sensor switches are designed to replace any standard single-pole wall switch. Specific wiring configurations vary by product. Some switches include a neutral wire connected to power the LED, while others do not (standard switches typically do not connect to the neutral circuit wire). Be sure to check the wiring schematic that comes with your specific model of the switch to ensure you wire it correctly.
In a typical installation, the motion detector switch has three wire leads. The black lead connects to the incoming “hot” wire that supplies the power to the switch. The blue lead connects to the outgoing circuit wire that is connected to the light; this is called the switch leg. The green lead is the ground and connects to the circuit’s ground system.
Source: The Spruce