Be sure standard PIRs terminate against walls within their detection range.

We look after a site that has an older PIR sensor in a warehouse – the sensor has a 40m range, but the warehouse is longer than this – around 70m. The zone works ok most the time, but we’ve found it has a higher tendency to false alarm than the office areas. What could the cause be?

A: When putting in passive infrared sensors without digital signal processing, in particular long-range sensors, make sure they’re targeted to terminate against walls or floors within their specified ranges and don’t just gaze off into deep space.

If large IRE-reflective objects appear outside specified PIR ranges – deeper into the scene – it’s likely they’ll exceed sensor thresholds and generate an alarm. Consider that a standard PIR is designed to detect a person-sized object at say, 10m. But the same sensor may also detect intense IRE from a floor to ceiling window every afternoon at 25m.

It’s worth standing under a PIR location, turning out the lights and flashing a torch around to see if you can pick out reflective surfaces in the target area when faced with a false alarming sensor – pay particular attention to surfaces that might receive direct sunlight through windows for a short time each day.