Build this DIY Intruder Alarm System using cheap off-the-shelf components. It has automatic Exit and Entry delays, a timed Bell Cut-off. Use it in your Home - Business Premises - Garage - Workshop - Shed etc. It has provision for the usual input devices (Magnetic-Reed contacts, Foil Tape, Movement Detectors).

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Garage/Shed Alarm - Test Procedure

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The prototype of this alarm was built using only the Stripboard Layout as a guide. So - if you have faithfully reproduced that layout - you will have a working circuit.

Once you're satisfied that your layout is correct - and you have made a careful and thorough check of the underside of the board - it's time to power-up the circuit and test its operation. This is always an anxious moment. If you construct a lot of circuits - you might consider building the Current Limiting Power Supply - or alternatively - you could add the Simple Current Limiter to your existing PSU. Both will let you set an upper limit on the amount of current supplied to your circuit - and so protect it from any serious damage.


You can simulate the trigger-switches using two short lengths of flexible multi-stranded wire - such as alarm cable. An LED and resistor is all that's needed to demonstrate that the Siren output is working properly. And any small single-pole two-way switch will serve as Sw1.

If You Find a Problem

This circuit uses tried and tested components and techniques. When it's assembled as described - it should work first time. If you've designed your own Stripboard / PCB layout - or you're simply trying the circuit out on a breadboard first - check your work carefully.

If you haven't used the specified transistor - make sure that the one you have used is an NPN. And - check its Pin Configuration. Just because it looks the same as the BC547 - don't assume that its pins are arranged in the same order.

If you haven't used the specified relay - make sure that your relay has the same Pin Configuration. Just because it looks the same - and fits into the layout - don't assume that its connections are arranged in the same pattern.

If you've built your circuit using the specified components - and you've followed the step-by-step construction guide described on the Support Page - then the chances are that any bug will be caused by something minor - a component connected the wrong way round - a missing or unwanted solder bridge - an incomplete cut in the track etc. If an LED is not lighting - check that it's the right way round.

If you've cut the board to size - examine the edges where the copper tracks end. Look for loose particles of metal - left hanging by the saw. These particles can short pairs of adjacent tracks together. Remove them with emery paper - or a small file.

Next - double-check that all of the cuts in the tracks have been made - that they're In The Right Place - and that they sever the tracks completely. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem.

When you're satisfied that the tracks have been severed in all the right places - check that you have made - and correctly placed - the ten solder bridges. These are just small blobs of solder - used to join pairs of tracks together. Mark each bridge with a felt-tip pen - or something similar - so that it can be easily identified later.

Next - carefully examine the full length of each track. Look for unwanted solder bridges. Your felt-tip marks will tell you which ones should be there - and help you to identify any that shouldn't be there. If you backlight the board during the examination - it makes potential problem areas easier to spot.

If all else fails - and you still haven't found the cause of the problem - work your way through the assembly instructions on the Support Page. Check each individual component and link - to make sure that it's present and correctly positioned.

Print out the drawings and mark off the components as you go. Take your time and examine each individual component carefully. If you do it right - you'll only have to do it once. Pay particular attention to the orientation of the diodes - the transistor - and the electrolytic capacitors.

Garage / Shed Alarm - Circuit Simulation
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