Parts chart for building the DIY home alarm system.
MOTHER EARTH NEWS STAFF
Statistics show that a house protected by an intrusion
alarm is less likely to be burglarized than one that lacks
such a device. And, beyond the personal security that a
burglar alarm can provide for you and your family, many
insurance companies offer reduced rates to homeowners who
install them. In fact, some underwriters even demand that
an alarm system be present before they'll issue a policy.
Recognizing that many of our readers would be interested in
building their own effective, economical security alarm, we
put electronics expert TJ Byers to work designing one.
After several prototypes (the schematics for which would
make your head spin), TJ has come up with an alarm that's
relatively easy to assemble, inexpensive, and does the job.
The DIY Home Alarm System
Just because this is a comparatively simple device to build
doesn't mean that it's a stripped-down, bare-bones alarm
system. MOTHER's Security Alarm has features normally found
only in more expensive security systems — including an
adjustable exit and entrance timer, a built — in loop
test circuit, an optional 12-volt triggered output, and a
set of external relay contacts.
The exit timer allows you to set the alarm and leave
through a protected door without tripping the alarm. The
entrance timer is very similar to the exit timer, except
that it allows you to enter the building through a selected
door without triggering the alarm. Both timers are
individually adjustable from a few seconds to several
minutes — more than enough time for you to get your act
together and make a graceful exit or entrance. If the
system isn't secured within the specified time periods, a
warning bell or siren sounds, thus preventing unwanted
entry by persons not familiar with the disarming procedure.
The timed exit and entrance features save you the hassle of
carrying a special alarm key. This approach is also more
secure, because there's no key to lose or have stolen.
Furthermore, only one portal is monitored by the exit and
entrance timers. Should anyone attempt to enter through any
other protected opening, the alarm would sound immediately.
MOTHER's Security Alarm includes another timer that resets
the system five minutes after it's been triggered. The
system can only be reset, however, if all the protection
devices are intact and working. A faulty device or door
that is left ajar locks out this timer, so the alarm will
continue to sound until the situation is corrected.
How the DIY Home Alarm System Works
MOTHER's Security Alarm is a wired alarm system. Such
systems — as opposed to new wireless systems — such as
ultrasonic detectors — have been around for many years and
have proved themselves very reliable.
Basically, the concept of a wired system is that of a
sensor wire stretched around the perimeter of the area to
be protected. The wire loop is routed so that it bridges
doorways and crosses windows. A small current runs through
the loop; as long as it isn't interrupted, the perimeter is
secure. In this state, the alarm is in the standby, or
armed, mode. Opening a protected door or window, however,
interrupts the flow of current, which activates a relay in
Building the System Board
The heart of the security alarm is a system board that
contains all the necessary electronics. While it is
possible to construct the system board using different
wiring techniques, I recommend that you use a printed
circuit board. Printed circuit boards minimize wiring
errors while providing a standard layout that has been
proved stable and reliable. You can duplicate the circuit
board yourself using the pattern shown in Figure 1 (see MOTHER EARTH NEWS issue
84, page 136 for instructions on making circuit boards), or
purchase one from the supplier mentioned in the parts list.
To make your own, simply locate the parts according to the
layout in the photo, and solder them in place. Pay
particular attention to the polarity of the capacitors,
LEDs, and integrated circuits. There are two ways in which
they can be aligned with the holes in the board, but only
one orientation is correct. Also note that the transformer
is secured on the opposite side of the board from the other
The completed board mounts in a plastic box. The board
rests parallel with the lid of the box, which becomes the
front panel of the alarm. You'll need to drill five holes
in the lid to accommodate the reset switch and the four
indicator lamps. Fastening a terminal strip to the bottom
of the case completes construction.
Installing the DIY Home Alarm System
Installation consists of nothing more than running two
loops of wire-one for the timed entry/exit and the other
for the rest of the openings in your house-to and from the
central controller. The main loop, of course, must include
all parts of the perimeter to be protected.
Before beginning to wire your structure for alarm
protection, take a few minutes to map out the areas to be
protected. This step is particularly important because it
forces you to consider the protected area as a whole. Look
for all possible entrances from the outside — even those that
seem trivial, such as skylights — and place them on
your map. At this time, you should also pick out the door
you'll use for the timed entry, and consider carefully what
the most practical (yet hidden) location for the central
unit might be.
Once you have a plan, you'll need to decide which type of
security device is right for each entrance. Doors, for
example, require a reed switch-magnet combination. The
sensor is a reed switch mounted to the door frame; a magnet
to activate the switch attaches to the door. Only when the
door is securely closed will the magnet engage the switch
and complete the loop.
Windows can be protected using a number of devices,
including reed switch/magnet combinations or microswitches.
Adhesive foil or vibration sensors are often used to detect
a broken window.
There's a broad selection of sensors and protection devices
available, and each is specifically designed to fill a
certain security need. Just make sure that the devices you
use are of the normally closed type. That is, they form a
complete circuit in the protected state and go open when
After you have a map showing your security points, start
from the central controller and draw a continuous line that
connects all the points (except the timed door) and returns
to the controller. Make the line as short as possible; it
will actually become your loop wire, and planning will save
you both effort and expense.
Wherever possible, hide the loop wires. The less obvious
your installation is, the harder it will be for a would-be
thief to defeat the system. Besides, there's no need to
deface the decor of your building simply because you're
installing a burglar alarm.
Finally, return the line to the controller. This is your
security loop, with all points within the loop wired in
series starting from the controller and returning to it.
All devices within this loop are wired to the MAIN LOOP
The timed loop is wired in a similar fashion, but goes from
the TIMED LOOP connectors out to your timed entry and back
Using the Alarm
The alarm has been simplified to the point where only a
single control switch is needed to test and arm the system.
This switch is located to the left of the indicator lamps.
In the OFF position, the alarm is completely disabled. The
center position of the control switch is used to TEST the
security loops. A semiconductor indicator called an LED
(Light Emitting Diode) is wired into each loop. When all
security devices and the timed entry are in the "safe"
state, both LEDs glow.
If there's a break in the system, though, one or both lamps
will fail to light, and it will be impossible to arm the
alarm until the break is closed.
The ARM position of the control switch actually turns the
system on. Until this point, no amount of tugging on the
sensors can make the system sound. Moving the switch from
the TEST position to the ARM position starts the exit
timer, which delays the actual arming of the system by 60
seconds. If all is secure after the 60-second time-out
period, the alarm arms itself and begins the job of
monitoring the perimeter.
A break in the main loop automatically trips the alarm. If
this breach is secured, the alarm resets itself within five
minutes, and the vigil begins anew. Entry through the timed
door, however, initiates another sequence of events. A
break in this loop starts an entrance timer. This feature
gives you an opportunity to unlock the door, put down your
packages, and still have time to reach the disarm switch
before the alarm sounds. Failure to reset the alarm before
the entry time expires triggers the alarm relay.
The relay contacts are wired so that there are several
options to signal when the perimeter circuit has been
broken. First, MOTHER's alarm contains a pair of
uncommitted relay contacts. These contacts can be used to
switch external devices on or off. Among the potential
applications is the use of these contacts to turn on indoor
room lights or outdoor floodlights. For the more
sophisticated user, the relay contacts can be used to dial
a telephone or activate other security devices.
In addition to the relay, the security system provides a
triggered 12-volt output Vol tage. [EDITOR's NOTE: A
separate 12-volt transformer and rectifier are required for
this option.] The voltage is available only after the alarm
is tripped and may be used to sound bells, sirens, or
whistles. Anything that can be powered by 12 volts may be
operated by this output.
MOTHER's Security Alarm is as electronically sophisticated
as systems costing many hundreds of dollars more. By
building it yourself, you can enjoy the personal
satisfaction and the peace of mind of knowing your home is
protected-and all for less than the price of an evening
EDITOR'S NOTE: We'll freely admit it, there's just no way
we can fully detail a project of this magnitude in a few
magazine pages. If you've dabbled in electronics before, we
think you'll be able to build MOTHER's Security Alarm from
the information in this article. (You experts can also
obtain a schematic for free by sending your request and a
self-addressed stamped envelope to TMEN Reader's Service,
Hendersonville, NC) If you're new to
fiddling with circuits, however, a little more assistance
might well be appreciated. Consequently, we're offering a
set of plans for MOTHER's Security Alarm that provides
much more detail on points that were, because of space
limitations, condensed in the article.
The plans are in an 8 inch by 10 inch pamphlet format and include
instructions for adjusting the exit and entry timers,
troubleshooting the system, and wiring optional
configurations for the alarm output. They also provide a
comprehensive list of sensor devices normally used with
closed-loop alarm systems, along with a list of retail
suppliers, many of whom are willing to lend assistance
should you run into problems. The plans will be available
June 1, 1986, and you can get a set by sending $10 to
MOTHER's Security Alarm Plans, Hendersonville,
If you'd rather not be bothered with soldering irons,
circuit boards, and the associated electronic hoopla and
legerdemain, there's yet another option available. At the
bottom of the parts list, you'll find information on
ordering an assembled circuit board at a very reasonable
An etched and drilled printed circuit board is available
from Danocinths, Inc.,Westland, MI, for $17 postpaid. Request part number RW-132. A kit of
parts (RW-132K) costs $44, and an assembled board (RW-132A)
is $55. Please add $5 for shipping to the price of the kit
or the assembled board. Also note that neither the kit nor
the assembled board includes the case; installation in the
case and wiring to sensors is required. Sensors are not