On Tue, 2 Dec 1997 10:25:07 +1300, Andrew Mayo wrote: >It is a resonant circuit, which is triggered by a transmitter in the >doorway. By passing a strong magnetic field over the device, it can be >deactivated - I am not quite sure how this works but I think what >happens is that part of the strip is ferromagnetic. If it is magnetised, >it affects the resonant circuit, either by changing the frequency or >introducing harmonic distortion. > I have dissected some of these and found a small printed metallic coil, which I presume is part of that resonant circuit. > >Some variant devices use (I think) a small low-signal diode which is >destroyed by an electromagnetic pulse - until this is done passing the >circuit through the door will generate a strong second harmonic signal >which will trigger the alarm. > Gee Whiz - does this mean I could defeat these things by carrying a small EMF pulse generator, such as an electric butane BBQ starter wand? ;-) CIAO - Martin. Martin R. Green mrgreen@NOSPAMbigfoot.com To reply, remove the NOSPAM from the return address. Stamp out SPAM everywhere!!!