I bought them because I wanted a warning light when the battery was getting low – my old ones failed here, so the first I knew was that someone said they visited and I didn’t answer.

They also have great battery life – claimed to be five years – and excellent range – the one in the workshop at the bottom of the garden is reliable.

However, the instructions are cryptic – almost another feature, if I liked puzzles.

So can you work out why the one in the house bongs twice and the one in the workshop once?

Darned if I can.

Actually, can you work out any single thing on the instruction sheet – except the power plug in one, even I understood that.