The author wanted a very cheap and simple alarm for some of his possessions, such as his electrically assisted bicycle.
This alarm is based on a cheap window alarm, which has a time-switch added to it with a 1-minute time-out. The output pulse of the 555 replaces the reed switch in the window alarm. The 555 is triggered by a sensor mounted near the front wheel, in combination with a magnet that is mounted on the spokes. This sensor and the magnet were taken from a cheap bicycle computer.
The front wheel of the bicycle is kept unlocked so that the reed switch closes momentarily when the wheel turns. This triggers the 555, which in turn activates the window alarm. The circuit around the 555 takes very little current and can be powered by the batteries in the window alarm. There is just enough room left inside the enclosure of the window alarm to mount the time-switch inside it.
The result is a very cheap, compact device, with only a single cable going to the reed switch on the front wheel. And the noise this thing produces is just unbelievable! After about one minute the noise stops and the alarm goes back into standby mode. The bicycle alarm should be mounted in an inconspicuous place, such as underneath the saddle, inside a (large) front light, in the battery compartment, etc.
Hopefully, the alarm scares any potential thief away, or at least it makes other members of the public aware that something isn’t quite right.