All the information you need to build a practical and fully featured burglar alarm system.

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Circuit Description

Click Here For A Photograph Of The Prototype

The trigger circuit is coloured blue in the diagram. 

R1 supplies base-current to Q1. This keeps the transistor switched on.

You can think of Q1 as an extra normally-closed switch - connected in series with the normally-closed loop

In standby mode - the current flowing through R2 is diverted to ground through Q1 and the normally-closed switches.

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If one of the switches in the normally-closed loop is opened - the R2 current can no longer flow to ground through Q1.

Instead, it flows into the base of Q4 - through D2 & R8.

The same thing will happen if a normally-open switch is closed.

If the base of Q1 is connected to ground - the transistor will switch off.

And the R2 current will flow into the base of Q4 - through D2 & R8.

The current flowing through D2 & R8 - switches Q4 on.

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Q4 does three different jobs - the first of which is to latch itself on.

Q3 & Q4 are connected to form a modified version of a Complementary Latch.

When Q4 switches on - it connects the base of Q3 to ground - through R11.

This switches Q3 on - and current through R7 & Q3 flows into the base of Q4.

It doesn't matter now if the trigger circuit is restored - and the R2 current is re-diverted to ground.

Q4 no longer needs the R2 current to remain switched on.

It has provided itself with an alternative and constant source of base current - from R7 - through Q3.

In other words - Q4 has used Q3 to latch itself on.

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Q4's second job is to sound the Buzzer.

It does this by connecting the negative lead of the Buzzer to ground.

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The third and final job is to start the Entry Delay.

When Q4 switches on - it connects D5 to ground.

And C7 starts to charge through R12 & D5.

As C7 charges - the voltage on the base of Q7 falls.

As the voltage on the base of Q7 falls - the voltage on the base of Q8 falls.

This causes the voltage on the emitter of Q8 to fall also.

As the voltage on the emitter of Q8 falls - the voltage drop across the relay coil rises. 

After about 30-seconds - the voltage drop across the relay coil is sufficiently high to cause the relay to energize. 

When the relay energizes - the relay contacts supply power to the Siren.

And the Siren sounds.

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As well as sounding the Siren - the relay contacts also connect the positive line to the base of Q3 - through D3.

This switches Q3 off - so the R7 current can no longer flow into the base of Q4.

Without the R7 current - Q4 is no longer latched on.

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If R2 is still supplying current through D2 & R8 - nothing will happen. 

Q4 will remain switched on - and the Buzzer and Siren will continue to sound. 

But, if the trigger circuit has been restored - and the R2 current is once again diverted to ground - Q4 will switch off. 

The Buzzer will stop sounding.

And the C7 charge path to ground - through R12, D5 & Q4 - will be cut off.

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When this happens - C7 begins to discharge slowly through R13.

This is the Bell cut-off timer.

As C7 discharges - the voltage on the base of Q7 starts to rise.

So the voltage on the base of Q8 starts to rise.

And the voltage on the emitter of Q8 will rise also.

As the voltage on the emitter of Q8 rises - the voltage drop across the relay coil falls. 

After about 10 minutes - the voltage drop across the relay coil is too small to keep the relay energized. 

So the relay drops out - power to the Siren is cut - and the Siren is silenced.

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Since the relay is now de-energized - D3 is no longer connecting the positive line to the base of Q3.

So the latch is no longer disabled.

In other words - the alarm has returned to standby mode and is ready to be re-triggered.

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The Exit delay is provided by Q2. 

During the Exit delay - the job of diverting the R2 current to ground is taken over by Q2.

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When you switch the alarm on - it begins by checking that the doors and windows etc are closed.

To do this - it postpones the start of the Exit delay by a second or two.

To begin with - C5 is in a discharged state.

This means that the base of Q2 is at zero volts.

In other words - Q2 is switched off.  

If there's a door or window open - the R2 current will flow through D2 & R8 into the base of Q4.

This will turn the transistor on - and the Buzzer will sound. 

When this happens you should switch off again - and check for the cause of the activation.

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If everything is in order the Buzzer will not sound - and C5 will charge quickly through C4 and R6. 

In less than 2 seconds - current through C4, R6 & R5 - will flow into the base of Q2.

Q2 will switch on - and connect the collector of Q1 to ground.

While the R2 current is diverted to ground by Q2 - you can leave the building without tripping the alarm. 

After about 30 seconds - when C4 charges through R4  -  current is no longer available from C4 & R6. 

So Q2 switches off - the Exit delay ends - and the trigger switches become active.

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Q7 & Q8 are connected together to form a Darlington Pair.

A Darlington Pair has a much higher input impedance than a single transistor.

The input impedance of a transistor - can be thought of as resistance the transistor offers to the flow of base current.

If a single transistor were used in the circuit - C7 would tend to discharge through the relay coil and the transistor's emitter-base junction.

This would speed-up the rate of discharge - and shorten the Cut-off time.

It would also effect the length of the Entry delay.

While R12 is trying to charge C7 - the relay coil and the transistor would be trying to discharge it.

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The input impedance of the Darlington Pair is very high.

It's roughly that of the single transistor - squared.

It's so high that for present purposes it may be ignored.

Consequently - the time it takes for C7 to charge and discharge is effectively controlled by the values of R12 and R13 alone.

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The relay will energize when the base of Q7 reaches - say - 4 volts. 

That is - when the voltage across C7 reaches about 8 volts.

At this point - D3 will switch off the Complementary Latch.

So the C7 charge path through R12, D5 & Q4 will close.

And C7 will be unable to complete its charge.

If C7 does not complete its charge - the time it takes to discharge through R13 - will be reduced.

In other words - the Cut-off timer will not last the full 10 minutes.

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The purpose of Q6 is to complete the C7 charge.

When the relay energizes - current through the relay contacts, R17 & C9 flows into the base of Q6.

This switches the transistor on.

The transistor connects the negative plate of C7 to ground - through R14 - and completes the charge.

The transistor only switches on briefly - while C9 charges through R17.

But it's on for long enough to complete the C7 charge - and provide the full 10-minute Cut-off time.

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The relay will de-energize when the base of Q7 reaches - say - 8 volts. 

That is - when the voltage across C7 falls to about 4-volts.

C7 will continue to discharge slowly through R13.

And it will end up completely discharged - eventually.

But - if the alarm is re-activated before C7 is completely discharged - the time it takes to charge through R12 & D5 will be reduced.

In other words - the Entry delay will not last the full 30-seconds.

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The purpose of Q5 is to complete the discharge of C7.

When the relay de-energizes, current through the emitter-base junction of Q5 flows to ground through C8, R17 and the relay contacts.

This switches Q5 on. 

The transistor connects the positive line to the negative plate of C7 - and completes the discharge.

The transistor only switches on briefly - while C8 charges through R17.

But it's on for long enough to complete the C7 discharge - and make the full 30-second Entry delay immediately available.

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Because there is a possibility that - for a short period - both Q5 and Q6 could be switched on together - R14 is included to prevent a short circuit through the two transistors. 

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For the various timers to operate correctly - when you switch the alarm on - the electrolytic capacitors must all be in a discharged state.

If they still hold any charge - left over from the previous occasion - the length of the time intervals would be unreliable.

When you switch the alarm off - most of the capacitors will discharge rapidly through the existing circuitry. 

However - this is not the case with C4. 

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It would take a significant time for C4 to discharge through the existing circuitry - and so reset the Exit delay. 

For this reason - C4 is supplied with its own rapid discharge path - through R3, the negative line and D1. 

Because it passes through ground - the path only exists when the power is switched off.

It rapidly discharges C4 - resets the Exit delay timer - and makes the alarm ready for immediate re-use.

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Relay coils and some sounders can produce high reverse voltage spikes that will destroy sensitive electronic components. 

D4, D6 and D7 are there to short-circuit the spikes before they do any damage. 

Although there is nothing in the alarm circuit itself that's likely to be damaged - I can't tell what other electronic equipment will be connected to the same power supply. 

So I included the three diodes as a precaution.

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The remaining diodes create one-way paths.

When the power is switched off - D1 bypasses R4 - and allows C4 to discharge rapidly through R3.

D2 allows R2 to supply base current to Q4.

But  - when the trigger switches are restored - it prevents Q1 from connecting the base of Q4 to ground - and possibly switching off the latch.

D3 allows the relay contacts to connect the positive line to the base of Q3.

But - while the relay is de-energized - it prevents current through the emitter-base junction of Q3 - from finding a path to ground through the Siren.

D5 allows C7 to charge through R12 and Q4 - but it prevents C7 from discharging rapidly through R9 & R12.

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R9 is connected to the base of Q3 - through R11.

By connecting its base to the positive line - the resistors keep Q3 switched off until required.

It doesn't matter if the Buzzer is not connected or not working. 

R9 ensures that - in standby mode - the base of Q3 will always be connected to the positive rail.

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R7 controls the size of the latching base current supplied to Q4 by Q3. 

R11 allow the circuit is to be unlatched by D3. 

Without R11 - there would be a short circuit to ground through D3 & Q4.

R10 keeps Q4 switched off in standby mode. 

It also reduces the sensitivity of the latching circuit - by keeping C6 discharged.

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There is nothing special about the transistors. 

So long as the pnp/npn requirement is satisfied - any small transistors with a gain (hfe) greater than 100 and an Ic(max) of at least 100mA should do.

But remember that the pin configuration of your transistors may be different from that of the BC547 and BC557.  
 
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The wires going to the switches on the doors, windows, etc. can act like a radio antenna.

They pick up stray signals that can cause 'false alarms'.

C1 and C2 are there to de-tune the wires - and short-circuit any high frequency signals to ground. 

C3 does a similar job for the supply.

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Learn More About The Complementary Latch

Power Supply

The circuit itself should not use more than a few milliamps. It is only when the relay is energised and the siren is sounding that any real current is required. The actual current consumption depends on the sounder you use. A conventional bell uses up to about 400ma. An electronic siren generally uses less. In any case, a 1-amp power supply should be sufficient. The Alarm Power Supply is ideal.

Inertia Sensors

Inertia Sensors are devices that react to vibrations. They do not react to constant low-level vibrations of the sort you would get from loud music or heavy road traffic. Instead they have a certain amount of inertia that must be overcome before they react. In other words they need a sharp blow such as would be required to break a pane of glass.

Essentially - they are small normally-closed switches - the contacts of which are joined by a specially weighted bar. It's the weight of the bar that provides the inertia. When a blow is struck - the bar bounces - and the circuit opens briefly. The length of time the switch is open depends on the strength of the blow - i.e. how high the bar bounces.

How Inertia Sensors Work

The alarm circuit sensitivity is adjusted by altering the speed at which it reacts:- i.e. the speed at which C6 charges through R8. At its most sensitive - the value of R8 is zero and C6 will charge very quickly. The alarm will react to a very short opening of the circuit. So a light tap on the sensor will do. With R8 set to maximum value - a heavy blow is required.

The Control Switch

Any 1-amp SPST switch may be used to control the alarm. However - you may also use either the 4-Digit Keypad or the 5-Digit Keypad. The guide to the Alarm's External Connections includes details of how to attach the keypads.

Parts List

Suppliers

Worldwide RS Components

UK & Ireland Maplin

Construction

Click here if you're new to constructing stripboard projects.

The terminals are a good set of reference points. To fit them - you may need to enlarge the holes slightly. Then turn the board over and use a felt-tip pen to mark the 21 places where the tracks are to be cut. Before you cut the tracks, use the "actual size" drawing to Check That The Pattern is Correctly Marked .

When you're satisfied that the pattern is right - cut the tracks. Make sure that the copper is cut all the way through. Sometimes a small strand of copper remains at the side of the cut and this will cause malfunction. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem. If you don't have the proper track-cutting tool - then a 6 to 8mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine.

Pattern for Cutting
The Tracks on the
Underside of the Board

Pattern for Cutting
The Tracks on the
Underside of the Board

Actual Size Of Pattern

Pattern for Cutting
The Tracks on the
Underside of the Board
ACTUAL SIZE

Next make and fit the Six Wire Links. I used bare copper wire on the component side of the board. Telephone cable is suitable - the single stranded variety used indoors to wire telephone sockets. Stretching the core slightly will straighten it - and also allow the insulation to slip off.

Then fit the 17 resistors. They're all shown lying flat on the board. But those connected between close or adjacent tracks are actually mounted standing upright. See the Photograph Of The Prototype

Single Zone Alarm
Construction Details

Now fit the transistors and diodes. The BC557 (pnp) transistors are those with the emitter arrow coloured red. Pay particular attention to the orientation of the diodes. Note that D1 and D2 face in a different direction from the rest - and that D2 is mounted standing upright.

Single Zone Alarm
Construction Details

Finally - fit the nine capacitors and the relay. Pay particular attention to the orientation of the electrolytic capacitors. They generally have a light coloured stripe down the side next to the negative terminal. Note that all the electrolytic capacitors have their negative terminals facing the bottom of the board. See The Photo Of The Prototype

Single Zone Alarm
Construction Details

Examine the underside of the board carefully - to make sure that there are no unwanted solder bridges or other connections between the tracks. If you backlight the board during the examination - it makes potential problem areas easier to spot. When you're satisfied that everything is in order - add the 5 solder bridges.

You Are Now Ready To Test Your Finished Circuit Board

Connections

Connecting External Devices
To The Single Zone Alarm

Modification

General Information