Loop Alarm

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

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.

U3:C and U3:D form an S-R (Set-Reset) Latch. This is cleared by the Exit Delay timer when the unit is switched on. If the output of U2:B should go high, even momentarily, the latch will change state, triggering the alarm.

The Entry/Exit input is shown in figure 2A. In the normal state, pin 1 of U2:A is held high by the N/C (Normally Closed) sensors, and pin 2 is held high by R12. If a sensor on the N/C circuit should operate, pin 1 will be taken low by R13, while of a N/O (Normally Open) sensor were to operate it would pull pin 2 low. Note that both N/O and N/C sensors may be used simultaneously (providing they are connected correctly), and either will trigger the alarm. C6 and C7 remove any noise which may be picked up along these connections. U3:A and U3:B form a latch which operates in the same manner as that used for the Loop input.

The active-low Loop Only signal on pin 12 of U2:D comes from the key-switch. When the line is low it prevents the signal from the latch from reaching the Entry Timer.

U5:A is the Entry Timer, which is triggered by the negative going output of U2:D. RV1 sets the entry delay period from 5 to 30 seconds. The output on pin 5 of U5:A is high during the entry delay period. U7:B gates the signals from U2:D and U5:A to give an output that is high when the Entry Latch has operated and the Entry Timer has timed out. R20 and C12 provide a very short delay to compensate for the propagation delay of U5:A.

The Alarm Timer and output are shown in figure 1B. When either the Entry Alarm or Loop Alarm line goes high, the output of U7:A goes low, triggering the Alarm Timer (U4). The values of R7 and C3 give a calculated time period of 16 minutes, although in practice this may be a minute or two longer due to the minute leakage current in C3. Even in this case the alarm period will be less than 20 minutes, which is now a legal requirement.

During the alarm period, Q1 is turned on, driving the alarm sounder connected to terminals 1 and 2 of SK2 and the optional relay (RL1:A). D1 protects the transistor from the back-EMF produced by the relay when it is switched off. The relay contacts are bought out to terminals on SK2, together with the circuits 0V rail, for connection to a remote sounder or alarm system if required. If this option is not used, RL1 and R21 should be omitted to conserve battery life.

Gate U7:D is unused, so both inputs are connected to the 9V rail and the output is left unconnected.

The battery (9V PP9 type) connects to SK3 and the key-switch to SK4. When the switch is set so that terminals 1 and 3 of SK4 are linked, power is connected to the circuit via D2 and the active low Loop Only line is taken high via D3. When the switch links terminals 2 and 3, power is connected directly to the circuit and the active low Loop Only line is low.

The Power-On Reset circuit and Exit Timer are shown in figure 2B. When the unit is switched on C15 will be discharged, holding the inputs of U6:A high. C15 will charge via R19 within half a second. The output of U6:A will therefore be low for a brief period when the unit is switched on, giving a suitable signal to trigger the Exit Timer (U5:B), and reset the entry and alarm timers. The device used for U6 has Schmitt trigger inputs to give reliable operation with an input signal that does not conform to digital levels.

U6:D inverts the signal from the Loop input, so that the Exit Timer (U5:B) is held reset and therefore disabled, if the loop is not connected. RV2 sets the Exit period from 5 to 30 seconds (approx).

The Warning Sounder is shown in figure 2C. During the Exit and Entry periods the output of U7:C will be low. This is inverted by U6:C, giving a high level to U6:B. This enables the oscillator formed by U6:B, R18 and C13, which operates at a frequency of about 2Hz. This drives a self-contained audible sounder (AWD1) via Q3, so that the sounder beeps twice every second. For this type of oscillator to operate an IC with a Schmitt trigger input is specified.

The Status LED is shown in figure 1B. The output of U2:C goes high in sympathy with the warning sounder, and also when the alarm has operated. Thus the LED flashes during the Exit and Entry periods, and lights steadily when alarm is (or has been) triggered. In the normal operating state it remains off to conserve battery life.

D8 (4V7 zener) in conjunction with the forward voltage drop of D4 (red LED) act as a basic low battery indicator. The LED brightness will reduce as the battery runs down, and it will not illuminate if the battery voltage is below about 6.5V, although the Warning Sounder will continue to operate. The remainder of the circuit will operate with a battery voltage as low as 6.0V, but prompt battery replacement is recommended when the LED fails to light or becomes very dim.

The circuit consumes under 1mA in the armed state. Most of this is due to U1, U4 and U5. The unit will give typically six months continuous operation from a high capacity 9V battery such as a PP9. A large battery is also more able to operate the alarm sounder.

PCB Construction
The circuit is constructed on a single sided PCB which is designed to fit into the guides in the recommended case. PCB assembly should be carried out following the normal guidelines and recommendations.

There are several wire links which should be fitted first. IC sockets should be used for the IC's as they are static sensitive. Do not fit the IC's into the sockets until all other components have been fitted.

SK1 to SK5 are PCB mounting terminals, and should be fitted with the cable entries towards the edge of the PCB. The LED needs to protrude through a hole in the case, so you may prefer to wait until after the case has been drilled so that you can get the leads the right length.

AWD1 must be fitted with the correct polarity. You may prefer to leave this out until after the circuit has been tested as the noise can become rather irritating (particularly for others). If the relay output is not required, omit RLY1 and R21.

When construction is complete the PCB should be cleaned with a suitable solvent to remove the flux. At this stage it is a good idea to check your work, in particular the soldering.

The general layout of the components in the case may be seen from the photographs. Looking from the rear, the alarm sounder is positioned as close as possible to the left to leave sufficient room for the battery to stand beside it.

The sounder is held to the base of the case with two M3 countersunk screws and nuts. Part of one of the PCB guides should be removed so that the sounder sits level. Fitting the sounder will require a pair of long-nosed pliers to hold the nuts - and some patience! A pattern of holes should be drilled in the case in front of the sounder (before the sounder is finally fitted) to let the sound out.

The PCB is fitted in the second set of slots from the top with the components downwards. A 5mm hole is required for the LED. The key-switch may be fitted midway between the PCB and the sounder. On the prototype the phono sockets for the Loop were fitted to the side of the case above the sounder.

Additional holes will be needed for the cables connecting to the remote sensors. The layout of the components within the case is not critical and may be varied to suit individual needs. Check that everything will fit inside as you intend before drilling any holes.

The prototype was attached to the wall by means of two key-hole shaped holes in the rear panel (lid) of the case. The unit can therefore be fitted over two suitable screws once the back is fitted.

The interwiring is shown in figure ?. The leads from the alarm sounder will probably be excessively long and may be shortened to suit. Because of the noise, you may prefer to leave this disconnected until after the rest of the unit has been tested.

The battery is connected using a pair of PP9 type battery connections leads, ensuring correct polarity. All other connections are made with 7/0.2mm hook-up wire as shown.

The connections on the key switch can be confusing. In any position the centre contact is connected to the pin towards the back (flat) edge of the key. On the prototype the first (anti-clockwise) and third positions were Off so the relevant tags were not connected. The second position is "Loop & Entry", so the relevant tag is connected to terminal 1 of SK4. The fourth position (fully clockwise) is "Loop Only", and the relevant tag is connected to terminal 2. The centre tag on the switch connects to terminal 3. The two operating positions on the key-switch MUST be separated by an "Off" position to ensure the unit resets correctly when changing mode.

You will also need to make up a lead for the loop cable. This should be made using cheap single core screened cable, fitted with a phono plug at either end. To prevent an intruder from simply unscrewing the covers of the plugs to link out the cable, secure the covers with a small amount of super-glue once the unit and cable have been tested. If a long length is required, it may be more convenient and flexible to make up two or three shorter leads, and join them with in-line connectors having a phono socket at either end.

The unit does not require any setting up, apart from adjusting the entry and exit delay periods to suit your installation. The testing therefore involves nothing more than checking the various functions of the unit.

Connect a loop cable between the two Loop sockets. Also link terminals 1 and 2 of SK5 (The N/C entry sensor) with a short piece of tinned copper wire. Set RV1 and RV2 fully anti-clockwise.

With the key-switch set to one of the Off positions, connect a PP9 battery to the battery connector leads. Set the key-switch to the "Loop and Entry" position. The LED (D4) should flash and the warning sounder (AWD1) should beep in time with it, for about five seconds (this is the Exit period). After this time the sounder should be quiet and the LED should be off. The unit is now armed, and waiting for someone to break in!

Momentarily link terminals 3 and 4 of SK5 with a piece of wire. The LED and warning sounder should operate as before, for five seconds (the Entry period). After this time the main alarm sounder should operate, the relay (RL1) should pull in and the LED should remain on (not flashing). Set the key-switch to off.

Try this again, but this time trigger the alarm by disconnecting the link between terminals 1 and 2 of SK5. The unit should operate as before.

Set the key-switch back to the "Loop and Entry" position, and wait until the end of the Exit period. Now unplug one end of the loop cable. The alarm sounder and relay should operate immediately (no entry delay).

Switch off, and then back to "Loop and Entry" without reconnecting the loop cable. This time the Exit delay will not operate, and the LED should remain on (not flashing) to indicate that something is amiss.

Switch off, reconnect the loop cable, and switch back to "Loop and Entry". After the Exit period, trigger the alarm by short circuiting the inner and screen of the loop cable at one end (bridge the terminals of the socket inside the case with a screwdriver). Again the alarm sounder and relay should operate immediately.

Switch off, then switch to "Loop Only". After the exit period, link terminals 3 and 4 of SK5. This should have no effect. Now unplug the loop cable, which should trigger the alarm as previously.

If you wish you can leave the alarm sounding, and check the timing of the alarm timer - which should be between 15 and 20 minutes. At the end of this time the sounder should silence and the relay should release, but the LED should remain lit to tell the user that the alarm has been set off.

Since every installation is different I can only give some general comments about installation. The main unit should be wall mounted if possible. Alternatively it may be free standing in a steady position. You may wish to hide it so that it is not immediately visible but can still be accessed quickly for disarming.

Cables need to be run from the main unit to the entry/exit sensors being used. In many cases one or two normally closed magnetic reed switch sensors mounted on the entry doors and possibly windows would be adequate. Normally closed sensors must be connected in series, so that if any operate the circuit becomes open. Connect to terminals 1 and 2 of SK5.

If it is not possible or desirable to screw sensors to the doors, pressure mats may be used underneath carpets or rugs. These generally have normally open contacts, and must therefore be connected in parallel to terminals 3 and 4 of SK5.

The connections should ideally be made with 4 core alarm cable, which will fit tidily into the sensors. Alternatively you could use any convenient thin two core cable. Hide the cable out of sight where possible.

The loop cable should be threaded through or attached to the items to be protected. For example, with bicycles thread the cable through both wheels and the frame.

Some items, such as televisions and hi-fi equipment have no obvious gaps to thread the cable through, and will therefore call for some ingenuity. If you are sure you know what you are doing, and the item is out of guarantee, you may be able to remove the back or cover and thread a piece of cable through a couple of the ventilation slots before fitting plugs to the ends. Alternatively you could fix a section of the cable to the item with a suitable adhesive such as Araldite.

If you don't want to mark the item, the best option might be to securely tie the loop cable to the units mains cable. A few cable ties can be useful here. Another possibility is a P-clip under a suitable screw on the unit.

External Alarm or Sounder
The relay contacts may be connected to an additional sounder or a separate household alarm system if required. The additional sounder may be a similar type to that used in the unit, mounted in an outdoor enclosure and powered by it's own battery.

The unit may be connected to any convenient zone on an alarm panel, depending on the type of protection required. The Panic button input would be suitable if you wish the alarm to operate whether or not the main alarm is set. If you have a monitored alarm system you may need to contact the monitoring company before connecting this unit to the system.

Greater Tamper Resistance
No alarm system is 100% secure and completely resistant to any attempt to defeat it, and this unit is certainly no exception. However it should be adequate in most cases.

The following suggestions are offered to more experienced constructors who may wish to customize their system and installation to suit their individual circumstances.

If the unit is connected to an external alarm system, as described earlier, this will continue to sound if an intruder attempts to smash this unit once it has triggered.

The main concern will probably be attempts to defeat the unit to prevent it being triggered. Normally this would involve trying to bypass the loop, close to the alarm unit, so that it may be disconnected. The system operates by sensing the resistance of the terminating resistance via the loop. This resistance need not be a single component at the far end, it could be made up of a number of resistors along the loop.

If R5 is reduced to 33K, and two additional 33K resistors are added inside connection plugs along the length of the loop, any attempt to bypass the loop at the alarm end would trigger the unit. I would suggest that one of the additional resistors is in the core connection and the other is in the screen. The only drawback of this is that you have to remember to use all the pieces of loop cable, but this could be ensured by using a different type of connector for each joint. You could use any number of resistors, providing the total resistance is between 90K and 110K.

The other likely tampering method would be to disassemble the box and disconnect the battery. This could be sensed by either a tilt switch or micro-switch suitably positioned within the box. The micro-switch would be held operated by the lid, such that it is released when the lid is removed. Fix it inside the case with glue, as the screw-heads on the outside of the box would be a give-away! In either case the connections that are closed when everything is OK are connected inseries with the Loop connections inside the box.


Zip Files

Number of zones 2 (Entry/Exit and Loop Cable)
Maximum Loop Length Unlimited
Exit Time 5 to 30 seconds (set by internal preset)
Entry Time 5 to 30 seconds (set by internal preset)
Alarm Time 16 minutes (typical)
Entry/Exit Warning Pulsed Buzzer and Flashing Status LED
Alarm Sounder Piezo Siren (110dB @ 1 metre)
Supply Voltage 9V nominal (PP9 Battery)
Low Battery Indicator Status LED extinguished below 6.5V
Supply Current (Armed) 1mA (typical)
Battery Life 6 months (typical)
Remote Output Optional change-over relay
PCB Size 172 x 75mm
Overall Size 177 x 120 x 83mm
Weight 500g

Parts List

Resistors (All 1% 0.6W Metal Film)
R1,3-5,12,13,15,16,18 100K 9
R2,10,17 22K 3
R6,14,20 10K 3
R7 3M9 1
R8 4K7 1
R9 220K 1
R11 330R 1
R19 1M0 1
RV1,2 470K Horiz Preset 2
C1,2,4,6-9,14 10nF Ceramic Plate 8
C3 220uF 16V Radial 1
C5 10uF 63V Radial 1
C10,C11 47uF 16V Radial 2
C12 100p Ceramic Plate 1
C13 4u7 63V Radial 1
C15 100n Disc Ceramic 1
U1 LPC662IN Dual Op-Amp 1
U2,6 4093 Quad NAND Gate 2
U3,7 4001 Quad NOR Gate 2
U4 ICM7555IPA Timer 1
U5 ICM7556IPD Dual Timer 1
Q1 ZTX651 Transistor 1
Q2 BC548 Transistor 1
Q3 BC558 Transistor 1
D1,2 1N4001 Rectifier 2
D3,5-7 1N4148 Diode 4
D4 5mm Red LED 1
D8 4V7 500mW Zener 1
AWD1 PCB Buzzer 1
SK1,5 4 Way 5mm PCB Term 2
SK2 6 Way 5mm PCB Term 1
SK3 2 Way 5mm PCB Term 1
SK4 3 Way 5mm PCB Term 1
  Case MB7 1
  Battery PP9 1
  PP9 Clips 1
  key-switch 4 way 1
  Micro Piezo Siren 1
  Chassis Phono Socket 2
  Single Screened Cable As Req'd
  Tinned Copper Wire As Req'd
  Hook-Up Wire 16/0.2mm As Req'd
  M3 x 10mm Csk Screw 2
  M3 Nut 2
  PCB (9603) 1
Optional Items
R21 68R Resistor 1
RL1 1A MicroMin Relay 5V 1
  In-Line Phono Connector As Req'd
  Phono Plug Black As Req'd
  Reed Switch Surface 1
  Pressure Mat 1
  Alarm Cable 4 Core As Req'd

This project, including all text, images and diagrams, is copyright 1991 - 2003 Paul Stenning. No part of this article may be reproduced in any form without prior written permission from Paul Stenning and WallyWare, inc. All details are believed to be accurate, but no liability can be accepted for any errors.

TAGS: Components
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.