Home > GCSE Projects > Bag Alarm
Modified: 17:58, 26 October 2013
Circuit and board designed by Phil Townshend 1994

This circuit is based upon a standard 555 monostable circuit. The trigger for the monostable is a vibration switch rather than a tilt switch. The output can be a siren or audible warning device such as a siren or loud buzzer.

The choice of transistor TR1 depends on the type of siren you are using. Find out the current drawn by the siren you want to use. If it is less than 300mA then use a 2N3704 or similar. Otherwise it is better to use a high power type Darlington such as TIP122. You can select your own NPN transistor by looking at the transistors specifications and checking Ic (collector current) against the siren current and noting the pin connections.

Set the duration of the alarm by adjusting the preset PR1. You can alter the timing more by selecting different values for C1. The values shown will produce an alarm duration of between 2 seconds and 4 minutes.

A standard 555 timer IC can be used but for longer battery life, the CMOS version is a better option.

Download PCB and circuit details

Example of PCB layout

Circuit Diagram - pretty standard stuff
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