wireless home security

[billdar]

I'm thinking about making myself a wireless home security system. I was leaning towards a very small transmitter sits idle until a sensor (either laser or magnetic) detects a break. <p>The several transmitters would feed a central reciever that can be armed/disarmed and sound an alarm.<p>I've seen some small transmitters that use two transisters (one to occilate, the other to amp) that send out fm signals. But would the same idea work for a 555 timer feeding a bjt transistor and connecting the collector an antenna? Then the reciever just "listen" for that particular frequency?<p>Is this a workable idea, or am I defining my own physics here?

[bobsRAC]

A 555 timer provides a square wave output. This is unacceptable as a reference oscillator for RF because a square wave contains harmonics on frequencies other than that of interest; Particularly odd order harmonics (i.e. 3*f, 5*f, 7*f, etc.). Another potential problem with the solution is that 555s only operate up to 1Mhz or thereabouts. Radio signals are sent at lower frequencies, but the wavelength is so long that you'll likely end up needing a ferrite core antenna, which is more trouble than you probably want to go through.
The most common bands for hobbyist xceivers are 433MHz, 900MHz, and 2.4GHz. One main advantage of these bands is the simplicity of antenna design. A 1/4 wave whip antenna is 17cm at 433MHz, 8.3cm at 900MHz and 2.7cm at 2.4GHz. You'll notice that most portable phones as well as other consumer devices, use these bands.
If you are interested in investing in a pre-built transceiver, digikey (www.digikey.com) sells 433MHz and 900MHz transmitters, radios, and transceivers for under $50. If that's too much money, companies such as MicroLinear (www.microlinear.com) sell inexpensive transceiver ICs that need only a small amount of support components.