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This circuit features automatic Exit and
Entry delays and a timed Bell Cut-off. It has provision
for both normally-closed and normally-open contacts, and
a 24-hour Personal Attack/Tamper zone. It is connected
permanently to the 12-volt supply and its operation is
"enabled" by opening SW1. By using the expansion
modules, you can add as many zones as you require; some
or all of which may be the inertia (shock) sensor type.
All the green LEDs should be lighting before you open
SW1. You then have up to about a minute to leave the
building. As you do so, the Buzzer will sound. It should
stop sounding when you shut the door behind you. This
indicates that the Exit/Entry loop has been successfully
restored within the time allowed. When you re-enter the
building you have up to about a minute to move SW1 to
the off position. If SW1 is not switched off in time,
the relay will energise and sound the main bell. It will
ring for up to about 40 minutes. But it can be turned
off at any time by SW1. The "Instant" zone has no Entry
Delay. If you don't want to use N/O switches, leave out
R8, C8 and Q2; and fit a link between Led 3 and C7. The
24 Hour PA/Tamper protection is provided by the SCR/Thyristor.
If any of the switches in the N/C loop is opened, R11
will trigger the SCR and the bell will ring. In this
case the bell has no time limit. Once the loop is closed
again, the SCR may be reset by pressing SW2 and
temporarily interrupting the current flow. The basic
circuit will be satisfactory in many situations.
However, it's much easier to find a fault when the alarm
is divided into zones and the control panel can remember
which zone has caused the activation. The expansion
modules are designed to do this. Although they will work
with the existing instant zone, they are intended to
replace it. When a zone is activated, its red LED will
light and remain lit until the reset button is pressed.
All the modules can share a single reset button.
The Modular Burglar Alarm Support Material includes the
stripboard layout of the prototype, a parts list, a
complete circuit description and more.
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Circuits |
