I noticed that my father-in-law struggles to hear the feeble high pitched squeak from DMM continuity testers. It’s not just years and years of working with noisy machinery sans ear-defenders — in a noisy environment, they really are difficult to hear!
This device uses a PICAXE microcontroller because they’re cheap (Arduino doesn’t like me), easy to programme, and I have oodles of ’em. The system runs from 9V, so there’s a small 5V regulator to run the chip. The speaker however gets the full 9V via a power transistor.
I selected ‘concert A’ as the tone at 440 Hz, added some start-up tones (old skool) and there’s reminder tone after 5 minutes of not being used.
The programming socket is accessible via a pop-off cover, for upgrades or fixing errors!