I found an old Polycell door alarm in the rummage drawer and rather than throw it away I decided to make a letterbox alert with it.
When I say “old” Polycell, it is ancient. I bought it at least 15 years ago for my previous property, but never got around to fitting it. I inserted a new 9v battery and it still worked!
The pictures on this page show the result. I opened the unit and drilled out the plastic fixings holding the tiny circuit board in place. Then I unsoldered the buzzer wires using a solder pump. The magnetic ‘keep’ was mounted on the letterbox inner flap (I had to drill two small holes through the flap). The unit itself was attached to the door using sticky pads. These are supposed to hold up to 1kg each, yet are tiny, about 20mm x 10mm. They really ARE sticky!
I then ran bell wire along the small gap between the carpet and the skirting board to the kitchen (where I hang out most in the mornings). I bought a small plastic case from Maplin and fixed this to the wall, again using the sticky pads.
Next morning I ‘armed’ the detector on the door (by switching it on) and forgot all about it. But at around 10:30am I suddenly heard a loud peep-peep from downstairs. The post had arrived! It works!