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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:
Click Here to View
Circuits
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.
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. |
