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

COMMENTS SUGGESTIONS	Four-Zone Module	MORE ALARM CIRCUITS

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

Instant Zones - Circuit Description

The basic normally-closed  zone is the simplest to understand.

The trigger consists of Rx - the Green LED and the normally-closed loop.

It 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.


                 ==============

When you switch the alarm on - current flows through Ry. 

Initially, this current is diverted to ground by the normally-closed loop. 

But when the loop is opened - the current will flow into Gate 2 instead.

This will turn Ty2 on - and the Siren will sound immediately. 

                 ==============

After about 10-minutes - the alarm will attempt a reset.

If the normally-closed loop has been restored - the attempt will be successful.

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

                 ==============

The two diodes are there to create one-way paths.

Dx allows a closed loop to connect Ry to ground.

But when the loop is open - it prevents the LED from finding a path to ground - through Gate 2.

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

In other words - the LED could not fulfil its primary function.

                 ==============

Dy isolates the normally-closed loop - from Gate 2.

When the loop is open - Dy allows current through Ry to trigger Ty2.

But it prevents a closed loop from connecting the gate of Ty2 to ground.

This means that any number of extra zones may be connected to Gate 2. 

Any and all the zones can trigger Ty2 - because none of them are holding its gate at ground potential.

                 ==============

The second circuit works in exactly the same way - except that there's a transistor switch in series with the normally-closed loop.

Base current through Rz keeps the transistor switched fully on.

So the current through Ry is diverted to ground - through the transistor and the normally-closed loop.

You can think of the transistor as just another normally-closed switch - in the normally-closed loop.

                 ==============

If a normally-open switch is closed - it will divert the base current to ground - so the transistor will switch off.

The current through Ry can no longer flow to ground through the normally-closed loop - so it flows into Gate 2 instead.

                 ==============

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".

The 100nF capacitors are there to short-circuit high-frequency signals to ground. 

                 ==============

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 9 resistors, the 5 capacitors and the Four 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 Nine Resistors -
The Five Capacitors - And
The Four Wire Links

Add The Nine Resistors -
The Five Capacitors - And
The Four Wire Links

The next stage is to fit the diodes, transistor, and LEDs. Pay particular attention to the orientation of the diodes and LEDs.

You'll probably want to mount the LEDs on the front of your alarm panel. Connect them to the circuit board using pairs of flexible wire - the cores from a piece of alarm cable are ideal.

The wires may be connected to either the front or the rear of the circuit board. Leave them long to start with - and twist them into pairs to keep them tidy. You can cut them to size later - when you come to assemble your alarm panel.

Add The Eight Diodes - The Transistor -
And The Four LEDs

Add The Eight Diodes - The Transistor -
And The Four LEDs

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 Four-Zone Module

Four-Zone Expansion Module - Veroboard Layout

Four-Zone Module - Photograph	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Four-Zone Module

Instant Zones - Circuit Description

Parts List

Construction Guide

External Connections