I've seen a few examples of short circuit detection circuits using BJTs that activate a LED and buzzer but that's not what I want. they also have next to no explanation
https://www.eevblog.com/forum/projects/semi-conductor-safe-continuity-tester/msg1725182/#msg1725182reading from left..
1) power supply (9V battery)
2) power led indicator
3) Vtest emitter follower BJT Q2 to provide DUT voltage testing. adjustable by changing R4, R7 voltage divider. 2V in the picture is set as default, i guess it should be safe for most semiconductor (10Kohm output impedance from R5 hence maximum 200uA shorted/drain current to 0 ohm DUT)
4) this is the heart of the adjustable continuity tester (see below)
5 upper) comparator (based on continuity result at Ip and In) that will turn on Q1 to provide power to buzzer.
5 lower) simple self oscillating circuit to beep the buzzer (i got this from just googling)
6) input protection (to some degree) can be omitted if DUT is passive device such as resistor.
4) explained...
it is some form of bridge or twin voltage dividers if i may say...
we adjust Radj to what range we want to beep. in the schematic its using 200 ohm trimpot so it basically can be adjusted to beep up to 200 ohm continuity. if we put trimpot to center position (100 ohm) anything lower resistance on probe point 1 and 2 will make the device beep. this Radj trimpot with R6 forms voltage divider that will act as voltage reference to comparator negative input In.
we put our probe on the second (twin) divider. if point 1 and 2 (probe) has lower than Radj resistance, comparator will make the circuit beep because voltage at Ip is lower than voltage at In, so comparator 101, it will pull the Q1's base PNP low and turn it on and then power the oscillator circuit hence beeping.
i cant think of any simpler circuit, this should ranked in the first beginner class of this forum. just a comparator, oscillator circuit to beep, and voltage dividers bridge. you can go mcu route but it should ends up pretty much similar (or more) complexity than this, you will need voltage dividers for adjustability, and beep circuit anyway. fwiw...
