Detecting Motion and Body Heat With Motion Detectors

A motion detector attached to a garage.
A motion detector used to control an outdoor, automatic light. By CHG - Own work, Public Domain, Link

Motion detector lights have long been used as a first step to stopping intruders from approaching a home or business. Conventional security lights with motion detection sense objects and people crossing the path of the motion detector's sensor, triggering the lights to go on. But now there are more sophisticated sensors that actually detect body heat. Here's a look at the benefits of them and how they work.

Problems with Motion Detection

First, let's consider regular motion sensors. Yes, they detect motion and turn on lights. And you can adjust the sensitivity so little things don't set them off. The problem I see with them is that things like tree limbs, tall grasses, heavy snow or rain, and perhaps most commonly, cats and raccoons, keep setting them off in the middle of the night. Wouldn't it be great if the lights could distinguish between those kinds of "disturbances" and humans approaching? 

How Body Heat Sensors Work

These newer style sensors actually check for body heat using infrared rays. When someone approaches, the sensors turn on the lights. So what about things like cats, raccoons, and other small animals that also have body heat. Well, they could set off the sensor if the sensitivity is set high enough. And who knows, maybe you want to know if a stray dog enters your yard... But for people, the sensitivity can be set to detect only larger bodies, thus ignoring small creatures of the night.

Setup Tips for Body Heat Sensors

Keep the following considerations in mind when choosing and setting up a body heat security light: 

  • Check the wattage limit on the light fixture to make sure the combined wattage of the light bulbs doesn't exceed the limit. This should be no problem with energy-efficient LED bulbs, and LEDs aren't affected by low temperatures the way compact fluorescent bulbs are.
  • Adjust the sensor for a range of 30-60 feet to start. This gives you plenty of heads-up for approaching people while not being too broad-reaching. Experiment with different sensor settings to find what works best for your situation.
  • Mount the fixture between 6 and 12 feet from the ground. Anything higher and the ground coverage is diminished. The ideal height is around 10 feet because it's high enough so that the lights can't easily be tampered with. 
  • Remember that this is a heat source detector, so keep it away from heat-producing objects like swimming pools, windows, reflective items, dryer vents, heater exhaust, etc.

Aiming the Sensor

You might think that facing the sensor straight toward the intended target, maybe along an entrance sidewalk, would be the logical direction to face the unit. However, there is a dead zone in the area coming straight towards the sensor, and someone could possibility walk right up to the house undetected. Instead, mount the sensor to the right or left of the area being monitored to establish a crossing area of coverage that someone would have to walk through, say, at a 10-degree angle. That way, people will walk through the lobes of detection, not between them. 

Depending on how your lighting scheme is wired, you may light only the front door area, or there may be a combination of lights that are triggered when someone approaches. The idea is to give you and anyone approaching a warning. For you, it tells you someone is approaching your home's door. For the visitor, it sends a message that they have already been seen and that you are turning on the light to see who it is. If it is a burglar, you may just deter him or her from proceeding!