Miniature Loop Alarm

By Tomaz Lazar – Ljubljana, Slovenia

A few months ago, I decided to build a compact, yet effective alarm. My demands were:- simple construction, reliable operation, very small power consumption, and, most of all, small size. I started with CMOS logic gates, but was soon forced to abandon the concept after a few unsuccessful (and far too complicated) attempts. Then I suddenly realized that a simple transistor switch might do the job and I was right.

As you can clearly see from the schematics, the circuit is utterly primitive and consists of two identical transistor switches. Each has its own alarm LED and they’re coupled to a neat 82dB buzzer. The two 1N4148 diodes are used to prevent a signal from one sensor from triggering both LEDs. The sensors used are either wire loops or normally closed reed switches or even a combination of both. You could, for example, tie a wire loop to your suitcase and place a reed switch to the door of your hotel room.

Since this little alarm is intended to be kept in arms reach at all times, there aren’t any provisions for automatic shutdown after a certain period of time. The buzzer will sound until you turn the whole circuit off or connect the wire loop back to the jumpers. The same goes for the two LEDs, each indicating its own zone.

Construction is not critical and there aren’t any traps for the novice. The two 100n capacitors aren’t really necessary, I just included them to make sure that there is no noise interference coming from the long wire loops. For transistors, you can use any NPN general-purpose audio amplifiers/switches (BC 107/108/109, BC 237/238, 2N2222, 2N3904…). Assemble the circuit on perf board. Together with the buzzer and a 9V battery, it should easily fit in a pocket-sized plastic box smaller than a pack of cigarettes. A fresh battery should suffice for weeks of continuous operation.

Source :: http://www.zen22142.zen.co.uk/Circuits/Alarm/alarm.html

Auto Burglar Alarm by IC 555
This alarm circuit is based on two 555 timers. The alarm will sound your car horn if anyone opens the car door while the circuit is armed. The timers will allow you to leave the car without soundi ...
Easy kick out steal
Although the prevent steal or Circuit this be model to be easy the system is simple. but I thinks it may help can protect your assets. The principle is simple be the system Close Wire Lo ...
Single Zone Digital CMOS Alarm
Circuit :Ron J Email Ron Description: This circuit features automatic Exit/Entry delays, timed Bell Cut-off and System Reset. It has provision for normally open and normally closed switches a ...
Enhanced 4 Digit Alarm Keypad by And Gate
Circuit : Ron J Email Ron Description: This is an enhanced 5 digit keypad which may be used with the Modular Alarm System. 4 digit keypad Notes: The Keypad must be the kind with a co ...
Gate Alarm by IC 4011
Figure 1 represents a cheap and simple Gate Alarm, that is intended to run off a small universal AC-DC power supply. 4011 quad nand gate IC1a is a fast oscillator, and IC1b a slow oscillator, w ...