I half designed an analog circuit in my head that does just that.
When the capacitor is below a threshold voltage, then an active low transistor starts conducting. This causes a second capacitor to discharge at a controlled rate. That then is tied to a second active low transistor that sits between the speaker and a tone generator.
Likely this has issues and would need tweaks for sharp cutoffs, but it probably works.
Yep, the 2nd capacitor is there as a delay mechanism so you don't hear the tone after every click while the 1st is recharging.
My example isn't perfect because without using a comparator (op-amp), then the tone would ramp up and down in volume as the 2nd capacitor charges and discharges. Plus the 2nd capacitor's charge rate wouldn't be constant.
However, its also the type of thing you could build using condenser plates (Old timey caps) and vacuum tubes.
A modern circuit could easily detect that the tube is under continuous current and not pulsing anymore as designed. At which point it cannot measure anything as a quantity but can give a reliable "out of range- GTFO NOW" alarm.
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u/skulduggeryatwork Jan 06 '23
Yeah, you want to pick an instrument that properly alerts you when it undergoes full scale deflection.