This is a circuit
for a simplified meter - a no bells and whistles version for home construction
and use in basic auditing or just to experiment with. I've cut the circuit
back to basics using a standard wheatstone bridge and instrumentation amp
circuit. The arrangement with diodes D1 and D2 is just to stop the needle
moving too far when the sensitivity is adjusted with the needle at 1/3rd
of a dial. C1 and C2 cut out radio signals picked in the leads and wiring.
The LED is there as just a primitive voltage stabilizer. The bridge circuit
can be optimized for dual cans, solo or fingertip electrodes by varying
Here's the layout diagram
for 0.1" veroboard. I'd suggest leaving space for mounting and maybe 8
rows at the top if you're thinking of adding better voltage stabilization
with another chip later.
R1 22k for 2 can use, 43k
for solo, 62k for fingertip electrodes.
Led small red
Meter movement 100ľA
Op amp LM324, MC33174. Most
quad op amps should work ok.
D1,2 small signal diodes,
VR1,2 50k linear.