I protoboarded differential amplifier with opamp MCP6041 (I really love this opamp for low-power applications), with gain 100, resistors R1 = 82Ohm, R2 = 2Ohm, R3 = 2Ohm (see my previous project log) and put ordinary DMM on its output.

When input of amplifier was shorted, I got expected reading a few milivolts - input offset voltage multiplied by circuit gain - with open output I got full output voltage, slightly under 4V supply. So far so good.

Though the input has 4-wire connection to eliminate resistance of connactors and probes, I tried to make it simpler and used only two-wire probe, connected by 4-wire connection - eliminatiing the connector resistance, not probes.

I also prepared simple probes with copper ends.

Though not optimal setup, it works surprisingly good. When sliding the probes through copper track of PCB, the output voltage changed nicely with length and width of copper track - while DMM itself, switched to resistance measurement, showed just some random value around 1Ohm on the same tacks.

So far so good. I'm going to attach trinket pro and buzzer.