Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!ames!oliveb!apple!xanadu!michael From: michael@xanadu.COM (Michael McClary) Newsgroups: sci.electronics Subject: Re: How can I fool the motion sensors connected to our lights? Message-ID: Date: 5 Aug 89 15:21:45 GMT References: <358@gorn.santa-cruz.ca.us> Reply-To: michael@xanadu.UUCP (Michael McClary) Organization: Xanadu Operating Company, Palo Alto, CA Lines: 34 In article <358@gorn.santa-cruz.ca.us> logo@gorn.santa-cruz.ca.us (David Kiviat) writes: > >I have just started work somewhere where the lights are controlled by a system >of motion sensors-if nothing moves in a pre-determined amount of time >the lights go off. I find this system most annoying as I am often >sitting at my desk working when the lights decide to go out and I >have to wave my arms to get them to go back on again. What I would >like to do is to build a device with a timer where when the timer was >wound some sort of signal would be broadcast that would fool the >motion sensors into thinking something was moving. Motion detectors are x-band doppler radar. I don't recall the exact wavelength, but it's well under an inch. So a corner reflector a couple inches on a side will do a dandy job of spoofing it. Imagine a hollow metal cube, and you cut off one corner, so you have three triangular metal plates at mutual right angles - a triangular pyramid. Microwaves going in through the open base of the pyramid have one component of their direction vector reversed by the bounce off each surface, so they emerge going right back at the source. Just like roadside reflectors do to light. At any significant distance from the transmitter, a small corner reflector returns as much signal as a very large object, because it reduces the signal loss from inverse fourth power of distance to about inverse square. (It would be exactly inverse square if the reflector were perfect and diffraction were neglected.) But this is a MOTION detector. You must move the corner reflector around a little bit to doppler shift the radar signal. So build one out of heavy foil (like TV dinner trays) and mount it on a moving thing of some sort. (Nitinol "space wings" are ideal.) Motion toward/away from the detector is what counts, and it doesn't take much. Much easier to build than an active microwave gadget.