Build this DIY intruder Alarm for your home - small business premises - shed - garage - lock-up - project etc. It has provision for normally-closed and normally-open input devices such as - Magnetic-Reed Contacts, Foil Tape, Movement Detectors, PIRs, pressure mats etc.

COMMENTS SUGGESTIONS	Multi-Zone Transistor Alarm - Support Material	MORE ALARM CIRCUITS

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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For A
Multi-Zone Burglar Alarm

The Exit/Entry zone trigger consists of R3, C3, D2 and the normally-closed loop.

While the loop remains closed - the current flowing through R3 is diverted to ground.

But - when the loop is opened - the R3 current flows into the base of Q3 instead.

This switches Q3 on - connects the negative lead of the Buzzer to ground - and the Buzzer sounds. 

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During the Exit Delay - the job of diverting R3 current to ground - is taken over by Q1.

When you switch the alarm on - C1 begins to charge through R2 - and the voltage at the junction of C1 & R2 starts to fall.

However, until C1 charges - this voltage will continue to be high enough to supply base-current to Q1 - through R1.

This switches Q1 on - and it connects R3 to ground.

While the transistor remains switched on - you can open the loop and leave the building without activating the alarm.

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The purpose of C2 is to postpone the onset of the Exit delay for a fraction of a second.

Without C2 - the Exit delay would start the moment the alarm is switched on.

If the Exit/Entry loop were faulty - you wouldn't know.

You could switch on the alarm - leave the building and drive away - before the Siren started to sound.

Q1 cannot disable the trigger switches until C2 charges.

It only takes a fraction of a second to charge - but the delay is long enough to allow an open loop to trigger the zone.

This causes the Buzzer to sound - so you know straight away that you have a problem.

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About 30-seconds after you switch the alarm on - when C1 has fully charged - Q1 switches off - and the normally-closed loop becomes active.

When you return and open the door - the trigger circuit is broken - and R3 current switches Q3 on.

It's important that Q3 should remain switched on - even after the loop has been restored.

Otherwise you could defeat the alarm by simply closing the door behind you when you enter.

Because the R3 current is once again diverted to ground - we need to supply Q3 with an alternative source of base-current.

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Q2 and Q3 are connected to form a ''Complementary Latch"

When Q3 switches on - it connects the base of Q2 to ground - through R6.

This switches Q2 on - and current through R4 and Q2 flows into the base of Q3.

In other words - Q3 uses Q2 to latch itself on.

It doesn't matter now if you re-close the trigger circuit. 

Q3 no longer needs the R3 current in order to remain switched on - because it has provided itself with an alternative supply.

When Q3 switches on and the Buzzer sounds - the Entry delay begins.

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Note that Q3 connects one side of R7 to ground - through D5. In so doing - it starts the Entry delay.

The length of the delay depends on the values of C5 and R7.

When Q3 switches on - C5 starts to charge through R7 - and the voltage at the base of Q5 starts to fall. 

As the voltage on the base of Q5 falls - it causes the voltage on the base of Q6 to fall. 

Consequently - the voltage at the Q6 emitter falls - and the voltage across the relay coil rises.

It takes about 30 seconds for the voltage across the coil to reach a high enough level to energize the relay. 

The exact time depends on the characteristics of the actual components you've used.

You can make your own adjustments by changing the value of R7

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When the relay energizes and the normally-open contacts close - three things happen.

Firstly, the siren sounds.

Secondly, for a brief moment - while C6 charges through R10 - Q4 switches on.

It connects C5 to ground through R8 - and so completes the charging of the capacitor.

Thirdly - D3 switches Q2 off by connecting its base to the positive line. This unlatches the complementary latch.

D3 allows the relay to switch Q2 off - but it prevents leakage through the siren from turning the transistor on.

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If the Exit/Entry loop is still open - Q3 will remain switched on - because R3 is supplying it with base current.

So the Siren will continue to sound - and the alarm will not reset.

While Q3 remains switched on - R6 prevents Q3 from shorting D3 to ground. 

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If the Exit/Entry loop has been restored - Q3 will switch off.

This cuts off the C5 charge path through R7 & D5.

The diode only allows current to pass through R7 - in one direction.  

D5 prevents C5 from discharging quickly through the Buzzer and R7.

Instead, the capacitor begins to discharge slowly - through R9.

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As C5 discharges - the voltage at the base of Q5 rises.

This causes the voltage at the base of Q6 to rise - and its emitter voltage follows suit.

As a result - the voltage across the relay coil falls.

After about 10 minutes - when it has fallen to a sufficiently low level - the relay will drop out.

The siren will switch off and - the inhibit placed on the complementary latch by D3 - will be removed.

In other words - the alarm will reset and return to standby mode.

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The Instant Zone is "always" connected to the 12-volt supply - even when the alarm is switched off.

If the LED is not lighting - you know that there's a door or window open - somewhere in the zone.

If the LED is lighting - you know that the zone is secure - and it's safe to switch the alarm on.

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While the loop remains closed - current through R12 is diverted to ground - through D9. 

But when the loop is opened - the current will flow into the base of Q4 instead - through D8 and D7.

This will turn Q4 on - C5 will charge rapidly through R8 - the relay will energize - and the Siren will sound. 

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D9 allows a closed loop to connect R12 to ground.

But when the loop is open - it prevents the LED from finding a path to ground - through D8, D7 and the base-emitter junction of Q4.

Without D9 - the LED would light all the time - even when the loop is open.

So you wouldn't know whether the zone was secure or not.

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D7 and D8 do two jobs.

The presence of D9 means that - with the loop closed - the voltage at the junction of R12 and D9 will be about 0v7.

This is high enough to turn Q4 on.

D7 and D8 increase to about 2-volts - the threshold voltage required to switch the transistor on. 

D7 and D8 also isolate the normally-closed loop - from the base of Q4.

When the loop is open - the diodes allow current through R12 to turn Q4 on.

But they prevent a closed loop from clamping the voltage on the base of Q4 at about 0v7.

This means that any number of extra zones may be used to switch Q4 on - and so trigger the alarm. 

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After about 10-minutes - if the normally-closed loop has been restored - the alarm will reset itself in the usual way.

However - while the loop is open - Q4 will remain switched on - C5 will remain charged - the Siren will continue to sound and the alarm will not reset.

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Relay coils and some Sounders produce large reverse voltage spikes that will destroy sensitive electronic components. 

D4, D6 & D10 are included as a precaution. They short circuit the spikes at source - before they can do any damage.

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I specified a relay with a minimum coil resistance of 270 ohms. Use a higher value if you wish.

With a 12-volt supply - the maximum current through Q6 will be about 45mA.

This is well within the limits of a BC557 - which has an IC(max) of 100mA.

C7 makes the voltage at which the relay energizes a bit more predictable - and consequently the length of the entry delay is more reliable.

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There is nothing special about the transistors.

Provided the NPN/PNP 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 & BC557.

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D1 bypasses R2 to provide a rapid discharge path for C1 - through R3 - the normally-closed loop - and/or R5.

Thus, the moment the alarm is switched off - the exit-delay timer is reset - and ready for immediate re-use.

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D2 allows R3 to supply current to the base of Q3 - but prevents the normally-closed loop from shorting the base of Q3 to ground.

Without D2 - you could defeat the alarm by simply closing the door when you enter.

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The wires going to the doors and windows act like radio antennas.

They pick up interference that could cause false alarms.

C3 and C8 short circuit the high frequency signals to ground.

R5 and R11 keep their respective transistors switched firmly off until required.

C4 further de-sensitises the trigger circuit by slowing down the speed at which the complementary latch operates.

R8 and R10 limit the peak current reaching Q4 through C5 and C6.

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Q5 & Q6 are connected to form a "Darlington-Pair". 

The input impedance of a single transistor is relatively low.

Current through the relay coil and emitter-base junction would have a significant effect on the rate at which C5 charges and discharges.

The input impedance of a Darlington-Pair is very high indeed. Roughly that of the single transistor - squared.

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

Consequently - the rate at which C5 charges and discharges is controlled by the value of R7 and R9 alone.

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It's only when 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.

Parts List

Suppliers

Worldwide RS Components

UK & Ireland Maplin

Construction Guide

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 17 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 fit the 13 resistors and the Ten Wire Links. For the 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.

The resistors are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright.

Add The Thirteen Resistors -
And The Ten Wire Links

Add The Thirteen Resistors -
And The Ten Wire Links

The next stage is to fit the diodes, transistors and LED. Note that the PNP transistors (BC557) are those with the emitter coloured red. Pay particular attention to the orientation of the diodes.

Add The Ten Diodes -
The Six Transistors -
And The LED

Add The Ten Diodes -
The Six Transistors -
And The LED

Fit the remaining Components - the eight capacitors and the relay. Note that C2 has its positive terminal pointing downwards.

Add The Remaining Components
And The Four Solder Bridges

Add The Remaining Components
And The Four Solder Bridges

Turn the board over and examine the underside 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 4 solder bridges.

You're Now Ready To Test Your Circuit

External Connections

Connecting External Devices
To The Transistor Alarm

Four-Zone Expansion Module - Veroboard Layout

Transistor Alarm Veroboard Layout	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Multi-Zone Transistor Alarm - Support Material

Construction Guide

Actual Size Of Pattern

External Connections