Bicycle Alarm Schematics

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.

Bicycle Alarm Schematics Diagram:

 
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.

Caution: The installation and use of this circuit may be subject to legal restrictions in your country, state or area.