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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SCR Based Burglar Alarm - Support Material

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SCR Alarm
Front Page
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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For
An SCR Burglar Alarm



Parts List

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



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


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


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


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
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