A dark front yard, secluded backyard or dark garage is an open invitation to unwanted visitors. Home security doesn't start with the door lock, but with the light. The sudden, blinding flash of a headlight is the number one deterrent for any would-be burglar, while also helping you park or find your keys with ease.
The technology that "drives" these projectors is called PIR (Passive Infrared). However, misplacing and adjusting them often results in them turning on every time the wind blows or a stray cat walks by, annoying you and your neighbors.
As engineers, let's see how the "eye" of these devices works and how to set them up for maximum reliability.
The biggest mistake is to think that the detector "sees" the movement like our eyes. In fact, the PIR sensor is "blind" to light. It only "sees" heat.
Detects infrared radiation emitted by warm bodies (people, animals, car engines). When a warm body passes in front of the sensor (which "reads" the colder ambient temperature), the sudden temperature change activates the switch and the light turns on.
80% of safety light problems are due to incorrect installation.
PIR sensors are much more sensitive when a body crosses their field of view (from left to right or vice versa). They are less sensitive if you walk in a straight line directly on them. Therefore, do not put the searchlight in the straight line of the corridor, but on the side wall, so that the attacker "cuts" its beam.
They must be placed at a height of 2.5 to 3 meters from the ground. If you put it too low, someone can easily break it or cover it, and it will constantly be lit by small animals. If you put it too high (eg on the 2nd floor), the sensor loses its sensitivity and may not "catch" movement on the ground.
The projector should point slightly downwards (about 30-45 degrees). If he is looking completely straight ahead, he will blind passing drivers and "catch" movements from the opposite road.
On the bottom of the sensor (the little plastic dome), you'll usually find 3 small rotary switches. Getting them right makes all the difference:
It has a sign with "Sun" and one with "Moon". This sets how dark it must be outside for the projector to turn on. If you leave it on "Sun", the headlight will turn on whenever you pass, even at 12:00 noon (wasting power). The Right Setting: Turn it to the "Moon". Test it at dusk, turning it slowly until it lights up, so you can be sure it only works when you really need light.
Adjusts how far (or how much heat volume) the sensor will react to. If you have it at max (+), it can be lit by a cat or the hot hood of a car passing 10 meters away. The Right Setup: Start in the middle. If you see stray lights, lower the sensitivity (-). If you have to get to 1 meter for it to "see" you, increase it.
Determines how long the spotlight stays on after motion stops. * The Right Setting: For safety purposes and to get to the door, 1 to 3 minutes is the ideal time. Anything above that will just annoy your neighbors if accidentally turned on in the middle of the night.
Since, as we said, the PIR reads temperatures, there are three things you need to keep out of its sight:
An LED floodlight with a motion sensor (typically 20W to 50W for home use) costs a few euros, but offers enormous peace of mind. Choose strictly IP65 waterproofing, screw it up in a strategic spot so it "cuts" the access path, and spend 5 minutes one afternoon to "play" with the LUX and SENS settings.
Next Step: The traffic light is great for safety. But what about the architectural lighting of the garden and facade, which we want to stay on all night to highlight our house? No need to flip switches! Continue to our guide: Photocells (Day/Night Switches): How to make garden lights turn on by themselves at sunset.
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