PIR Motion Sensors: What they are & where to put them

Passive Infrared (PIR) sensors have become a key component of smart alarm systems. Here we run through what they are and where they should be placed to maximise room coverage.

boundary pir sensor

What are PIRs?

Passive Infrared sensors work by detecting body heat, or infrared energy.

The sensor views the room as a grid. If it senses a block of temperature moving across the grid, it will interpret this as an intruder.

One of the most famous examples of infrared heat detection was in the movie The Predator. Arnold Schwarzenegger covers himself in mud to conceal his body heat. It makes for a great movie scene, but it wouldn’t work in real life. The mud would quickly be warmed by Dutch’s body heat and he’d be spotted!

How is a sensor pet-friendly?

Some sensors can disregard pets of a certain weight – any mass of infrared movement below a size will be ignored. In Boundary’s case, any pet weighing less than 35kg won’t trigger the alarm.

How do false alarms occur?

False alarms can be caused by pets moving, drafts of heat, shifts in sunlight, insects crawling inside and low batteries. The best way to avoid any of these is to buy a premium alarm and get it professionally installed.

Check whether the alarm conforms to a Grade. To be a Graded alarm, the PIR sensor will be built with certain technical specs. For example, a Boundary PIR sensors has a sealed lens, which stops insects getting into the plastic casing. You’d think this would be relatively rare, but according to Which? bugs account for around 30% of false alarms in the UK. The alarm must also be professionally installed to be graded.

Infrared imaging

Where to place your sensor

  • Corners

The best place to fit a PIR is in the corner of a room. This ensures maximum coverage of the space as the device maps out its grid.

  • Choke-points

Choke-points are areas that an intruder would have to walk through. These could be entry doors or hallways.

  • Upstairs

It’s easy to fall into the trap of keeping your security airtight downstairs whilst neglecting the upper floors. Intruders may enter through an upstairs window and so having PIRs installed on your second floor will protect against all eventualities. They also don’t intrude on your privacy, so you can install them in bedrooms.

  • Visible from windows

Another important thing to consider when choosing a PIR location is whether they are visible from windows. Detecting an intruder is important but deterring them in the first place is better.

Where not to place your sensor

  • Above a heat source

As PIRs work by detecting body heat, a rise in temperature caused by radiators or boilers can confuse the system and affect accuracy.

  • The conservatory

When the sun shines the raised temperature in a conservatory will have a similar effect to a warm radiator, possibly triggering the sensor unnecessarily. Instead, place your PIR at the choke-point between the conservatory and its adjoining room.

  • The bathroom

Over time, the steam from showers and baths could reduce your sensor’s sensitivity. PIRs should be installed where they will stay dry and won’t be subject to rapid changes in temperature.

Motion sensors are the preserve of motion pictures no more. You’re probably already used to them opening automatic doors at the supermarket or even turning the tap on in a high-end restaurant bathroom. When used correctly, home security can also be enhanced by this technology.