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Add-on for a commercial alarm gives up to
32 sensors, each individually monitored . . .
Make Your Alarm
System More Versatile!
Do you have an alarm system fitted? Smart move! However, while
many commercial burglar alarms have the advantage of good
features and presentation for a reasonable price, they often don’t
have nearly enough inputs to cater for larger houses. Here’s how you
can extend a burglar alarm without spending a heap of dollars.
By MAX MAUGHAN
A few years ago I had a new two-stosound for 30 seconds before the ex- which comes with an approved aurey house built. During construction,
ternal siren, so my neighbours didn’t to-dialer. This alarm may now be difthe wiring for all doors and windows
hear accidental alarms.
ficult to obtain but the decoder should
was installed so that they could be
(5). I wanted to be able to arrive and be adaptable to the vast majority of
monitored with reed switches and leave by car, via one of two remote
alarms on the market.
PIR detectors.
roller doors without tripping the PIR
The decoder interface is external to
There are 32 devices to
the alarm box. The circuitmonitor: 22 windows, 6 doors
ry uses high speed CMOS
and four PIR sensors. Every
(74HC series chips) to ensure
switch has its own pair of
low power operation and
wires back to a central point.
this is run from the auxiliary
power output (12VDC) of the
Wiring 32 switches into
alarm.
four serial circuits to match
the four available sectors was
Each of the four input
not a practical option.
sectors of the alarm “sees”
an end-of-line resistor and
There were six problems
a pair of relay contacts. All
to solve and the following
input leads to the 4-sector
explains what these problems
alarm are only 50cm long
were and how they were
which makes them effectivesolved.
ly noise-free. Noise induced
(1). I wanted to use an unon long wiring lines is a
modified commercial alarm
Not exactly the alarm used for the decoder interface,
major cause of false alarms.
system but its battery backup
but similar: this is the new model Altronics S-5920
Fig.1 shows the interface
would also be required to
5-sector alarm. We believe that most commercial alarm
circuit. There are 32 inputs,
power any add-on circuitry.
systems will work satisfactorily with this project.
provided by IC1, IC2, IC3,
(2). A LED indicator panel
IC4 and IC7. Each input is
was required to show which
sensor in the garage.
pulled low by a reed switch (in fact, to
doors and windows were not shut
6. An exterior indicator was rebefore setting the alarm.
quired to show when the alarm was
(3). A delayed entry door was reFig.1 (right): this add-on circuit for a
set.
quired so that the alarm did not need
commercial 4-sector alarm provides
These requirements were met by for up to 32 points to be monitored
to be disarmed and rearmed after
using a decoder interface with an
every entry.
with reed switches, PIR sensors or
(4). I wanted an indoor siren to Altronics S-5490 four sector alarm other devices.
82 Silicon Chip
JUNE 1999 83
λ
0V) so in the armed state only minimal
noise can be induced on the lines.
When the reed switch opens the
associated input is pulled high by a
10kΩ resistor and this signal feeds
through to IC8, a 74HC540 inverting
octal Tri-state buffer, to drive one of
the four sector relays. These relays
control the 4-sector alarm referred
to above.
Eight-wire Category 5 UTP twisted-pair data cable has been run for
the inputs, with one pair of wires for
each separate switch; one cable serves
four separate switches.
Switch monitoring
To monitor every reed switch and
sensor, there is a display panel next
to the alarm keypad. The display has
the house plan on it and LEDs are positioned where every switch is placed.
The total of 32 LEDs is driven by
eight 74HC540 buffers. These can be
arranged to turn the LEDs on or off,
to represent the relevant circuit being
open or closed, depending on the
setting of four jumpers.
With the LED panel circuit powered, you can see at a glance which
windows and doors are not shut before
setting the alarm. This can also be
useful on rainy days to check which
windows are open or shut.
84 Silicon Chip
λ
Of the four zones available, zones
1 and 2 are downstairs and zones 3
and 4 upstairs. Zone 4 is only used
when the house is vacant. At night
when the family retires, three of the
four zones are armed.
With members of the family coming
and going at all hours via the entry/
exit door, it is a nuisance having to set
and reset the alarm repeatedly.
This problem is overcome by the entry/exit delay circuit involving IC5a,
IC5b, IC9 & IC10. This circuit allows
the door to be left open for about 25
seconds before the alarm is triggered.
IC10, the 555 timer, provides the
25-second delay. Its pin 4 is held low
by the reed switch when the door is
closed.
When the reed switch opens, it
allows pin 4 to go high and pin 1 of
IC5a goes low to trigger the monostable IC9. In turn, this delivers a trigger
pulse to IC10. Its pin 3 goes high for
25 seconds and then low, to provide
the entry or exit delay.
Indoor siren
The circuit of Fig.2 provides an
indoor siren facility. This prevents
the external alarm from sounding if
the house occupants inadvertently
trigger the alarm by walking into one
of the armed zones.
Fig.2: below is the add-on circuit which
adds the facility of an indoor siren to
a 4-sector alarm. At left is the circuit
for monitoring the roller-door courtesy
lights.
Instead, the internal siren sounds
for about 25 seconds to allow the
alarm to be disarmed. Depending on
how the jumper at the output of IC4
is set, the circuit can be bypassed so
that both inside and outside sirens go
off together.
The circuit of Fig.2 works in a similar way to the entry/exit delay circuit
described above. The alarm output
of the 4-sector alarm is connected
via a reed relay monitored by IC1a,
connected as an inverter.
When the reed relay closes, IC1a &
IC1b trigger the 74HC123 monostable
and this triggers the 555 for 25 seconds
to drive IC1c and transistor Q1 which
then drives relay RLY1. Its normally
closed contacts open the circuit to
the external siren for the 25-second
period.
The circuit is reset when the alarm
is disarmed.
Garage entry/exit
The garage has two roller doors,
each monitored with a reed switch
and there is a PIR sensor to monitor
the whole area. Pins 4, 10 and 12 of
IC3 are connected to the roller door
switches and PIR sensor.
Each roller door has a UHF remote
control and the reed switches and PIR
sensor must be disabled each time the
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Fig.3: this alarm panel shows the plan of the 2-storey house. There are LEDs
for every door or window being monitored.
remote controls are operated. This is
done by monitoring the two lights
fitted to each door that come on and
stay on for a couple of minutes when
the door is opened or closed with the
remote control.
These lights are detected with a
simple circuit containing two LDRs,
a resistor and a transistor, which pulls
the input pins 5, 9 and 13 of IC3 low.
While these are low, the output of
IC3 is disabled so the alarm cannot
be tripped while the roller door lights
are on.
The LDRs must be shielded from
external light so that daylight does
not defeat the system.
When leaving the home by car, the
procedure is to set the alarm, open
the respective roller door with the key
remote control, drive out and close
the roller door by the remote control.
Then when the roller door lights
go out, the garage is protected by the
alarm. Manually opening the doors, if
you get past the PIR sensor, will trip
the alarm.
To provide an outside indicator, a
LED and 2.2kΩ series resistor were
connected to the programmed output
of the 4-sector alarm.
This was programmed to come on
when the alarm was set. Thus family
members can see whether or not the
alarm is set when they are leaving or
arriving home.
SC
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