Description:
A single
zone
alarm
circuit
with
entry
and exit
delay
and
other
facilities.
Notes:
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.
