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In these days of stolen birds and equipment, I have come across a very cheap method of' making a burglar alarm with untold uses.

Cost? Under £10 fitted.
All you need is: one smoke alarm detector, one 12 volt car accessory switch, one or more car interior door light switches and a length of bell Wire.
Beneath the test button on the smoke alarm is a switch, usually a tab of metal that when pushed makes contact with another and the alarm will sound. Having found these, connect the bell wire to each side of these contacts so that when you touch the other two ends of wire together, they make the alarm sound. Now you

will need to fit the on/off switch in one of the strands of  bell wire,, allowing enough for it to be mounted in a discreet position, then on to the door switch which you can get from Halfords, etc., or most scrap cars will provide some.
You will have to mount these, in a very similar position as, In a car door, close to a hinge. The, wiring should be one to the end connection, the other to the fixing screw on the earth connection. You can add further door switches by connecting to points A and B, this may be all you need to detect intruders. 1 hope you never get to need it, but by then it may be too late.
The detector also still works as a smoke alarm with the test button fully operational. ……...end