Originally published by Paul Stenning in Electronics and Beyond (The Maplin Magazine), January 1997

• Diagrams and Schematics

Introduction
This alarm system was originally designed to protect the contents of a garden shed, although it would also be ideal for other outside buildings. In addition it would offer a good basic security system for small residential areas, such as flats and bedsits. It may be installed without drilling or attaching to the building, which could be an advantage in rented accommodation.

The unit features two separate protection arrangements. The entry/exit is protected using standard normally-open and/or normally-closed sensors such as magnetic switches and pressure mats. Entry and exit delays are independently adjustable, from 5 to 30 seconds, during which time a warning sounder operates and a status LED flashes.

Valuable items are protected by a wire loop system similar to that used in shops etc. This uses a two core cable which is threaded through the items to be protected, and the alarm will be triggered immediately if either core becomes open-circuit or if the two cores are shorted together. Suggestions are given later for making the system even more tamper-resistant.

The loop system can be used independently from the entry/exit system. This would be useful if a shed or garage needs to be left open when working in the garden, while still providing protection for valuable bicycles and power tools.

The unit is battery powered, and will give at least six months continuous operation from a PP9 battery. The alarm sounding period is limited to about sixteen minutes, to conserve battery life and reduce annoyance, but the status LED remains on after this period to indicate that the alarm has been triggered. An optional relay output can be connected to an extra sounder or household alarm system.

Block Diagram
Referring to the Block Diagram, the Loop Input and Entry/Exit Input are both followed by Latches to hold the alarm state even if the input reverts to normal. The Entry/Exit Latch can be disabled by setting the key-switch to the Loop Only position.

The Loop Latch operates the Alarm Timer and Driver immediately. The Entry/Exit Latch triggers the Entry Timer which drives the Warning Sounder. If the unit is not switched off by the end of the Entry Timer period, the Alarm Timer and Driver are operated.

When the unit is switched on, the Power-On Reset circuit triggers the Exit Timer. During this period the two Latches are cleared and the Warning Sounder operates. If the Loop is not connected correctly the Exit Timer will not operate.

The Status LED flashes while the Warning Sounder is operating, and is lit steadily when the Alarm is or has been sounding.

Circuit Operation
The circuit diagram (figures 1 and 2) contains a number of inter-connected sections. To simplify the diagram the various sections are shown separately, with interconnections indicated by labeled arrows.

The loop input and latch are shown in figure 1A. The far end of the loop is terminated by a 100K resistor (R5) which, when the loop is connected, forms a potential divider with R4 to give a voltage equal to half the power supply voltage on pins 2 and 6 of U1 (4.5V with a 9V battery). C1 removes any noise that may be picked up along the loop wire.

U1:A and U1:B (LPC662 dual op-amp) form a window comparator. R1, R2 and R3 set the top and bottom ends of the acceptance range. With a 9V battery the voltage on pin 3 of U1:A is about 5V and that on pin 6 of U1:B is about 4V. If the voltage from the loop potential divider should become outside this range, the appropriate output of U1 will go low, giving a high level on pin 4 of U2:B. The LPC662 op-amp has the lowest current consumption in the range of devices offered by Maplin that will operate down to 6V.