|Circuit and board designed by Phil Townshend 1994
The circuit is based upon a 555 IC, wired in a monostable configuration. The circuit is standard with the addition of an RC network, (R4 and C3) to keep the monostable reset for approximately 10 seconds after the power is switched on. The output at pin 3 connects to a transistor driver which provides the current to drive the siren.
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 transistor 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.
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Example of PCB layout