My son (and I) designed a novel bike alarm system for his DT project last year....
Consisted of a little box that fit under the saddle, easily modified to include a loop through the back wheel.
This had some clever electronics in it, including a motion sensor.
When armed, and then disturbed or contact lost, it would transmit an RF signal to a second box attached to your belt which would then beep at you. If the disturbance stopped e.g. the bike had simply been bumped and was not in the process of being handled, the thing would reset, otherwise, a loud alarm would sound from the unit under the seat and a continuous tone sounded from your belt unit.
it worked quite well, too! RF range about 20-150 metres depending on obstructions.
To be commercial, though, it would need a proper re-design, probably with a PIC logic chip to do the clever bits instead of the analogue circuitry we slung together.....
The whole thing cost us about £50 in parts...
