I'm just back to breadboarding the LM324 continuity mode addition to a benchtop DMM. I have lots of cheap ebay ones I might aswell use. I set it up with split rails, and set it to trigger on 2 ohms resistance. Which gives an output of +1V, then a difference amplifier compares that to an adjustable reference so I can change it if I want, with a poteniometer poking out of the case. Seems to work good.
I can see the output of the inverting amplifier go from 920mV to 840mV when the diff.amp goes from high to low. In fact just connecting/disconnecting a 1x probe to the diffamp output, drops the inverting OA output from 860mV to 800mV. I'm only using 2of4 of the LM324, the other 2 have NonInv pins on GND and INV tied to their outputs.
I have plenty of fine details to learn about op-amp's, but I don't know if that's typical, it sounds huge, but my gain is ~1000x, so IDK. Some of it might be the breadboard and wires back to the PSU, the whole thing with a PWR LED and continuity LED uses 35mA for now. IDK what the buzzer circuit will use, but I hope it won't be too much for the little transformer in the DMM.
The only path from the outside world to my added circuit is via a 100k resistor, to the op-amp. I'll use the DMM on 2Kohm mode, and the DMM has a buffer as part of the resistance mode, that senses the DUT from across 100kohms also, with no added safety protections, so I'll just have to be as careful when using it, no different than before tho.
On the breadboard, there's about 920mV at the output of the 1st OA from the 2 ohms, and then about that from the divided down 5V1 zener.