Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!bbn!mit-eddie!bloom-beacon!hermes!jpexg
From: jpexg@hermes.ai.mit.edu (John Purbrick)
Newsgroups: sci.electronics
Subject: Radio link from intrusion alarm? Via FM?
Keywords: Power, legality
Message-ID: <3259@hermes.ai.mit.edu>
Date: 2 Jan 89 17:43:54 GMT
Organization: MIT AI Lab, Cambridge, MA
Lines: 16


I'm thinking about an intrusion alarm for automobiles which would send a radio
signal if a door were opened. Easily-available transmitters are very
low power--like Radio Shack's 50MHz walkie-talkies give "up to" 1/4 mile 
range, where "up to" can mean anything at all. CB radio would do it (I'd like 
a range of 5 miles) but existing traffic is heavy and unpredictable. At least
the system's bandwidth would be minuscule--one bit every 5 minutes would be 
OK--say a coded digital signal modulated onto an audio-range tone, or use
frequency-shift techniques. The problem is the radio carrier. Would it be 
possible (purely as a design exercise, since this would be blatantly illegal)
to use a broadcast-band AM or FM signal? Then a cheap and highly portable 
receiver could be used, say a credit-card sized receiver with some additional
decoding circuitry.

Radio Shack and Heathkit both have "wireless mikes" which broadcast to an FM
radio, but I assume that their range is almost zero. Could they be souped-up?