5 Zone Alarm System
Description
This is a complete alarm system with 5 independent
zones suitable for a small office or home environment. It uses just 3 CMOS IC's
and features a timed entry / exit zone, 4
immediate zones and a panic button. There are indicators for each zone a
"system armed" indicator. The schematic is as follows:
Circuit
Notes:
Each zone uses a normally
closed contact. These can be micro switches or standard alarm contacts (usually
reed switches). Suitable switches can be bought from alarm shops and concealed
in door frames, or window ledges.
Zone 1 is a timed zone which must be used as the entry and exit point of the
building. Zones 2 - 5 are immediate zones, which will trigger the alarm with no
delay. Some RF immunity is provided for long wiring runs by the input
capacitors, C1-C5. C7 and R14 also form a transient suppresser. The key switch
acts as the Set/Unset and Reset switch. For good security this should be the
metal type with a key.
Operation:
At switch on, C6 will charge
via R11, this acts as the exit delay and is set to
around 30 seconds. This can be altered by varying either C6 or R11. Once the
timing period has elapsed, LED6 will light, meaning the system is armed. LED6
may be mounted externally (at the bell box for example) and provides visual
indication that the system has set. Once set any contact that opens will
trigger the alarm, including Zone 1. To prevent triggering the alarm on entry
to the building, the concealed re-entry switch must be operated. This will
discharge C6 and start the entry timer. The re-entry switch could be a
concealed reed switch, located anywhere in a door frame, but invisible to the
eye. The panic switch, when pressed, will trigger the alarm when set. Relay
contacts RLA1 provide the latch, RLA2 operate the siren or buzzer.
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