Passive IR Mechanics: How to Make a Motion Detector

Here’s what I’ve found out about passive IR motion detectors…

First, they do not emit infrared light. In fact, the only light they emit is from that little red LED that indicates it’s detected motion.

That which has been referred to as a ‘sonic’ detector a few times here does not use sound waves at all – they use microwave signals generated by a Gunn diode in a tuned cavity resonator with a horn antenna. The signal is transmitted into the target area, then reflected back, and combined by a mixer with the TX frequency. Motion is indicated when a beat frequency caused by the combination of the TX freq and the RX frequency with Doppler shift is returned.

You can detect the signal from these motion sensors with a radar detector, making them easy to avoid. A radar detector in your car may trigger in the parking lot of a store with motorized doors triggered by microwave sensors! (Do they make these with the ability to run off internal batteries?)

I don’t know offhand of any jamming methods… other than to place a similar sensor nearby. The signal from the second, if it’s a few hertz off from the first, will cause it to trigger continuously. Eventually it’ll wind up being ignored or replaced. Most common applications for these are as door openers, and they can be recognized by lack of any translucent front panel. Usually there is only a black plastic dome, or a flat black plastic plate. I’ll try to find documentation as to what frequencies they use later.

Second, the way they detect motion is pretty nifty. Here’s what I’ve determined about them from a bit of reverse engineering…

The front of the sensor, a whitish plastic sheet, has a neat multi-zone Fresnel lens on it.

Inside the detector’s housing is an assembly containing two infared-sensitive photo diodes or photo transistors (I’m not sure offhand). These are bonded to a thick piece of metal to ensure that their temperature and sensitivity are about the same.

The Fresnel lens on the front of the detector focuses incoming infared light from several different zones in the detection area onto the sensor assembly. From there, an amount of current is passed through either side of the sensor proportional to how much infrared is landing on that section.

Motion detection is a function of finding the difference between the signal on both sides of the sensor. If the overall level changes, that is just a temperature change in the detection area, and is discarded. A person walking through the area will cause an increase in signal on one side of the detector or the other, but not both simultaneously.

I believe the best strategies for disabling one of these sensors would be either the freezing method mentioned earlier in this thread (turn a can of ‘canned air’ upside down and fire on the sensor), or adding something to the sensor to block out incoming IR. If there’s a detector right near an entrance, where you can easily get in, mess with it for a moment, and get out, do that. Obtain some of the aluminum foil duct tape (the variety that has the removable backing over the adhesive) and cut a few strips to about the size of the detector’s window, plus a bit on either side. Slit the backing so you can remove only the sides leaving the center intact. Then, fold back those ends so you have something like this:

Remove the backing on the two ends, and stick it over the detector with the silver side towards the lens. What you should wind up with is the white release paper surface out, which looks a bit less obvious than the silver tape surface. Don’t stick it to the lens… this may be impossible to remove after some time, and we don’t want to damage the detector! Hopefully, all the security person who comes out to investigate will do is just look around for anyone in the area, then wander off. If they do look at the detector, all they will see is the white strip, which should look fairly normal unless they are expecting to see the red LED shining from behind the window (depends on how the thing’s designed).

If you’re feeling really daring, know you have a little more time, and the motion sensor’s front can be removed without tools, pop it off and tape right over the round sensor with a black window inside, then reinstall the cover. It’s very doubtful that a security guard will open the detector and look inside, and will simply report that the detector malfunctioned then died.