IC2 acts as a negative voltage generator
IC1/A amplifies the probes signals by 120 times.
C2 memorizes the "cal" level, and IC1/B is a sample&hold buffer
IC1/C compares the two signals from A and B and lights the leds accordingly.
IC1/D is a variable oscillator from the result of IC1/C
IC2/A is a one minute delay to switch-off the system if not in use
ELV short circuits detector KD1