A Home Security Alarm

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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.

by TJ Byers

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.