How To Defeat a PIR Motion Detector Jackson Research

How To Defeat a PIR Motion Detector

There are dozens of ways to defeat PIR motion sensors, many of which are quite trivial. Actually, PIR motion sensors are useless at ambient temperatures above 95°F. The easiest way to defeat them is to either turn up the heat in the facility or turn off the air conditioning or both. In many parts of the world, just opening the windows and allowing enough time for thermal equilibrium to be achieved with the environment is sufficient. This is simple to explain and trivial to demonstrate.

The graph shows the blackbody radiation curve of the human body at 98.6°F. The peak emission wavelength is about 9.4 µm. Approximately 52% of the power lies in the 5- to 14- µm wavelength band, which corresponds to the wavelength range of the detector sensitivity.

A typical motion detector arrangement uses a combination slotted screen and Fresnel lens to project the incoming thermal radiation onto a pyro sensor specifically designed for this purpose. The pyro sensors can be single, dual or quad devices all with a typical output signal as shown.

This is a typical signal conditioning circuit schematic commonly found on the Internet. There are many other examples. Some of them use embedded processors or MCUs. The presumption is that the ambient temperature is much lower than the human body it is intended to detect. If the ambient temperature is near to or above the human body temperature, the device is totally blind. Optically speaking, one would say that the field of view is all the same color. Motion detectors are a commodity and easy to find. An electronic store near you probably has them. Test it for yourself.

Obviously, if the ambient temperature cannot be adequately elevated in anticipation of the intrusion, a specialized infrared radiation source of the necessary spectrum is required to blind the sensor. Inquire...