From: Paul B. Webster VK2BZC >John Mullan wrote: >> There is actually a device that does this already. They pump a signal >> through the one wire and compare it to what is received at the >> receiving wire. ... > Some twenty or more years ago, I think the magazine was Popular >Electronics, featured a "people sensor" which was simply an oscillator >connected to a single antenna sitting in the middle of the room. > Nothing surprising really - just the simple UHF version of the SHF >(doppler radar) sensors used to open doors and such. Detects variations >in the tuning of the oscillator as people move about in the near field >in the room. I don't see these two effects as being the same. The VHF unit is sensing a capacitance change caused by the person present. If the person is stationary the effect is still present. The (usually) UHF/SHF doppler radar unit radiates an RF signal which is reflected by the moving person. The movement causes a change in wavelength so that when the signal is mixed with the original signal a difference frequency is detectable. A very good (I'm told) moving people detector is achieveable by filtering the difference frequency to detect the "joint articulation frequency" present when people's bone joints rub on each other as joints move. This distinguishes people from pets and falling boxes, flapping curtains etc. I've never tried it myself. I've still got 2 radar units - one of these days ... :-) MANY years ago Wireless World mag did a doppler radar unit using AFAIR a push pull VHF (NOT UHF) transmitter using 2 transistors on PCB and their own aerial - a lot cheaper than UHF purpose built units. UHF has the advantage of having a wavelength rtather less than a person-size but VHF will do. Russell McMahon _____________________________ >From other worlds - www.easttimor.com www.sudan.com What can one man* do? Help the hungry at no cost to yourself! at http://www.thehungersite.com/ (* - or woman, child or internet enabled intelligent entity :-))