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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 Silicon Chip Binders REAL VALUE AT $12.95 PLUS P&P These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf.  Hold up to 14 issues  80mm internal width  SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A12.95 plus $A5 p&p. Available only in Australia. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 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 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my ❏ Bankcard ❏ Visa   ❏ Mastercard Card No: ________________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ JUNE 1999  85