A Transistor Based – Single Zone Alarm

This circuit features automatic Exit and Entry delays – timed bell cut-off – and system reset. It has provision for normally-open and normally-closed switches – and will suit all of the usual input devices (Pressure Mats, Magnetic Reed contacts, Foil Tape, PIRs and Inertia Sensors).

If the building is not secure – when you switch the alarm on – the buzzer will sound. You should turn off the alarm – and check for an open window or door etc. If everything is secure – when you switch the alarm on – the buzzer will NOT sound – and the Exit delay will begin.

You have about 30 seconds to leave the building and close the door behind you. When you return and open the door – the buzzer will sound – and you’ll have about 30 seconds to switch the alarm off. If you don’t switch it off in time – the siren will sound. It will go on sounding indefinitely. However, if the building is re-secured – then the siren will stop after about 10 minutes – and the alarm will reset.

The Exit delay, Entry delay and Bell Cut-off times can be changed by altering the values of R4, R12 & R13 respectively. Q5 and Q6 ensure that the Entry delay and Bell cut-off timers always start with C7 either fully charged or fully discharged as required. If you can live with slightly less precise time intervals then leave out Q5, Q6, R14, R15, R16, R17, C8 & C9. If you don’t want a Bell Cut-off at all – then leave out D3 as well.

The sensitivity of the Inertia Sensors is adjusted by R8. Set to minimum value, a light tap will activate the alarm. Set to maximum value, a heavy blow is required. If you are not using Inertia Sensors then replace R8 with a 27k fixed resistor. If you are not using normally open switches then leave out R1, C1 & Q1 and fit a link between R2 and C2.

The [url=http://uk.geocities.com/ronj_1217/sza.html]Support Material[/url] for this alarm includes a step-by-step guide to the construction of the circuit-board, a parts list, a detailed circuit description and more.

For the layout of the circuit see: sza2l.png

[url=http://uk.geocities.com/ronj_1217/sza.html]Detailed description and expansion modules[/url]
[url=http://uk.geocities.com/ronj_1217/circ.html]Ron J’s Circuit Page[/url] – updated regularly.

Copyright [url=http://uk.geocities.com/ronj_1217/mail.html]Ron J[/url]Parts:1x Entry buzzer
1x Relay
1x Siren
Misc: diodes, resistors, capacitors, transistors, switches, etc.

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