Threshold voltmeter


This is not an actual voltmeter, it just “detects” if the input voltage is below or above a certain threshold. The threshold voltage is the breakdown voltage of a Zener diode.

I have seen this in a magazine described as an rudimentary car battery voltage indicator, recently I have used it as a protection circuit, just replaced the red LED with a relay, acting like a switch (closed when the voltage is under the threshold value and open if the voltage is grater).

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