Radio Shack sold a Lie Detector Kit that used dry electrodes. The great thing about the instruction manual for this product is that it included 18 Physiological and Psychological Experiments and are repeated elsewhere on this blog. All of these experiments could be carried out using the Smartphone, LEGO, or GSR2 versions. I think this same basic product was also sold for Biofeedback (right) and you can still get the instructions for that product from the Radio Shack support page.
The schematic for the kit version is typical for this kind of “resistance to frequency” converter. Two transistors (Q2 and Q3) are used has a high gain non-inverting amplifier which oscillates due to a capacitor C1 connected for positive feedback. You can find schematics for simple oscillators like this all over the place. A third transistor Q1 is used to alter the frequency of the oscillator by messing with the bias of Q2. Q2 was originally a germanium, but I’ve found that a silicon PNP works about as well.
Other versions of the Radio Shack product: