Use this DIY Alarm in your home - small business premises - shed - garage - lock-up 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.

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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For
An SCR Burglar Alarm

This circuit uses two SCRs - or "Silicon Controlled Rectifiers". They are also called "Thyristors".

I used TIC106Ds because they are widely available.

But - provided the current, voltage and power ratings are adequate - practically any type of SCR will work fine. 

Of course - the pin configuration may be different from that of the TIC106D.

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You can think of an SCR as a simple on/off switch. 

It turns on when a small current flows into the "gate" terminal. 

Typically - the size of  the current required is about 0.2mA.

Once the SCR has switched on - it no longer needs the gate current.

It will stay on all by itself.

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Consider the alarm's trigger circuit. It consists of R3 and the normally-closed loop.

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

So it cannot flow into the "gate" of Ty1

However,  if the loop is opened even for an instant - current will flow into the gate - and switch the SCR on.

Ty1 connects the negative side 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 the alarm is switched 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 a door or window were open - 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 Ty1.

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

You can turn off the alarm and go check the doors and windows.

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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 flows into the gate of Ty1.

It doesn't matter now if you re-close the trigger circuit. The SCR has switched on - and it will stay switched on.

At this point - the Buzzer sounds - and the Entry delay begins.

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Note that Ty1 connects one side of R7 to ground. In so doing - it starts the Entry delay.

The length of the delay depends on the values of C4, R7 and Z1.

Q2 is normally switched off. It's off because its base is connected to the positive line by R6.

When Ty1 connects R7 to ground - C4 starts to charge through R7.

This causes the voltage at the junction of C4 and R7 to fall.

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To begin with - this fall in voltage has no effect on the base of Q2 - because the transistor is isolated from the junction of C4 & R7 - by Z1.

Z1 is a 5v1 zener diode. Its presence means that the junction of C4 & R7 will have to fall below about 6-volts - before Z1 will conduct and switch Q2 on.

Without it - the junction of C4 & R7 would only have to fall to about 11v3 to switch Q2 on. This would happen very quickly.

In other words - without the 5v1 threshold provided by Z1 - a 30-second Entry delay would require C4 & R7 to have much larger values.

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After about 30-seconds - when the voltage at the junction of C4 & R7 falls to about 6-volts - base-current flows through the emitter-base junction of Q2, Z1, R7 & Ty1. 

This switches the transistor on - and current flows through R8 & Q2 - into the gate of Ty2.

Ty2 switches on - connects the negative side of the Siren to ground - and the Siren sounds.

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Once an SCR is switched on - it will stay switched on.

If we want a timed Siren cut-off and a system Reset - we have to turn the SCRs off.

There are a number of ways to do this - but the most convenient and reliable way is to interrupt the current briefly.

Once the current flowing through an SCR is interrupted - the SCR will revert to its off state.

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You'll see from the diagram that the power to the Siren, Buzzer etc - flows through the normally-closed relay contacts.

When Ty2 connects the Siren to ground - it also connects R11 to ground. 

At this point - C6 starts to charge through R11 - and the voltage at the base of Q3 starts to fall. 

For a reset time of about ten minutes - R11 needs to be one meg.

The current flowing through R11 is very small. If we are going to use it to energize the relay - the transistor switch will have to have a very high gain.

Q3 & Q4 are connected to form a "Darlington-Pair". In this configuration - the gain of Q4 is multiplied by the gain of Q3.

Say each transistor has a gain of 100 - the "Darlington-Pair" will have a gain of 100 x 100 = 10 000 

As the voltage on the base of Q3 falls - it causes the voltage on the base of Q4 to fall. 

Consequently - the voltage at the Q4 emitter falls - and the voltage across the relay coil rises.

It takes about 10 minutes for the voltage across the coil to reach a high enough level to energize the relay. 

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When the relay energizes - two things happen.

Firstly - the normally-closed contacts open. This cuts power to the alarm and resets the SCRs.

Secondly - the normally-open contacts close. This connects R12 across C6 and discharges the capacitor.

As a result - the voltage at the base of Q3 rises - and the transistors switch off. 

So the relay de-energizes and restores power to the alarm.

In other words - 10 minutes after the Siren sounds - the relay energizes and cuts the power to the SCRs.

It will remain energized for about 5 seconds - the time it takes for C6 to discharge through  R12.

Then it will de-energize - and restore power to the circuit.

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If the normally-closed trigger circuit has been restored - the alarm will reset.

But - if the loop is still open - the alarm will re-activate.

The circuit will attempt to reset itself every 10 minutes - until the loop is restored - or the alarm is switched off.

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

D2, D4 & D6 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 Q4 will be about 45mA.

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

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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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SCRs trigger very easily. All you need is about 1-volt at the gate - and a current of 0.2mA. 

In fact - simply touching the gate pin is sometimes enough to switch them on.

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'.

C3 and C5 slow down the speed at which the gates will operate - so they won't be triggered by electrical interference.

R4 and R9 further de-sensitise the gates.

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R5 & R10 each do two jobs. 

To remain switched on - SCRs must have a certain minimum current flowing through them. 

Some Buzzers require a very small current. And some sounders - such as Bells - operate by interrupting their own current flow. This could cause the SCRs to switch off.

The current through R5 & R10 guarantees that - once the SCRs have switched on - they will stay on.

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After the circuit performs a Reset - when the SCRs have switched off - R5 & R10 do a second job.

They complete the discharge of C4 & C6 through D3 and D5. The diodes bypass R7 & R11 - so the discharge is rapid.

With the capacitors discharged - the delays are reset - and the system is ready for immediate re-use.

D1 does the similar job of bypassing R2. It provides a rapid discharge path for C1 - through R3, the normally-closed loop and/or R4. This resets the exit delay.

In other words - the instant the alarm is switched off - the Exit, Entry and Bell cut-off delays are reset. 

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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.

If you intend to draw a heavier current from the Buzzer or Siren terminals - the SCR in question will need to be bolted to a metal heatsink - and the relay contacts may need upgrading.

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 10 places where the tracks are to be cut. Before you cut the tracks - double check the position of each cut.

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

Next fit the 12 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 and diodes are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright. See The Photograph Of The Prototype.

Add The Twelve Resistors -
The Ten Wire Links

Add The Twelve Resistors -
The Ten Wire Links

The next stage is to fit the diodes, transistors, zener and SCRs. Note that the PNP transistors (BC557) are those with the emitter coloured red. Note also that the bottom diode faces downwards.

Add The Six Diodes - The Four Transistors -
The Zener - And The Two Thyristors

Add The Six Diodes - The Four Transistors -
The Zener - And The Two Thyristors

Fit the remaining Components - the six capacitors and the relay. Note that two of the capacitors have their positive terminals pointing downwards.

Add The Remaining Components
And The Three Solder Bridges

Add The Remaining Components
And The Three 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 3 solder bridges.

You're Now Ready To Test Your Circuit

External Connections

Connecting External Devices
To The SCR Alarm

Four-Zone Expansion Module - Veroboard Layout

SCR Alarm Photograph	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

SCR Based Burglar Alarm - Support Material

Construction Guide

External Connections