I wrote before about squeezing the last electron out of batteries with voltage too low to power some equipment...
Squeezing the last electron from AAs
Sadly, my supply of geriatric, not dead but tottering, batteries is not yet exhausted. While looking for other electronic items, I stumbled across a miniature 3 volt buzzer. It gave me an idea to build an audible continuity tester. Now I've built such beasts before...
Audible resistance checker
but such sophistication is not always required or desirable. I'd hate to accidentally poke the that one into a powered circuit and blow it.
How about a dead simple battery and a buzzer ? Touch the leads to something conductive and it makes a noise; you don't have to be watching it to see if you have a circuit, you hear it*. Get it across a powered connection and you blow maybe $1 worth of parts. Well not really a whole buck. Every part in this device was found lying around the house in various boxes of parts and wires - even the empty pill container that houses it. Total time to construct was 27 minutes (yes, I timed it), and that involved soldering and using the hot air gun to shrink some shrink tube.
Nitpickers may notice I'm using 4 AA batteries, ie 6 nominal volts, to power a 3 volt buzzer. Remember, all these batteries are sub-power, the open-circuit voltage of the four pack is about 4.7 volts. Beyond that, the buzzers are very tolerant of input power and, if it's a bit loud, I'm an old man with poor hearing and tinnitus. :-)
* If you're checking continuity inside a live box, this property could well save your life. DAMHIKT.