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A single zone alarm circuit with entry and exit delay
and other facilities.
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Circuits
The 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 the usual input devices (Pressure
Mats, Magnetic Reed contacts, Foil Tape, PIRs and
Inertia Sensors). When the power is applied, if there's
a fault the buzzer will sound and you should switch off
again and check for open doors, windows, etc. If
everything is in order the buzzer will NOT sound and the
exit delay will begin. You have about 30 seconds to
leave the building. When you return the buzzer will
sound. You then have about 30 seconds to switch off;
otherwise the siren will sound. It will go on sounding
indefinitely. However, if the building is re-secured the
siren will switch off 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.
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Circuits |
