Bio-electronic interface for biofeedback devices

A bio-electronic interface that can be used in tests ECG, EMG and EEG can be performed using electronic diagram below.
All this refers to the measurement and display of electrical potential produced by the heart (ECG), muscle (EMG) and brain (EEG).
Three copper plates used as electrodes are connected through shielded cables to the amplifier differential input circuit forming. Circuit made with A1 ... A3 can be described as an instrumental amplifier: Differential amplifier with operational amplifiers and two high impedance inputs. The output signal of this entry floor is filtered low-pass active filter, A4, before being sent to the optocoupler emitting diode.
This electronic project require to be powered by a two 4.5 volts batteries.
Receiver of optocoupler transistor transfer signals to IC2, which are transformed using a pulse modulated signal in time.
Filling factor of the output signal is set at 50% with the potentiometer P2. Output signal frequency can be chosen with the potentiometer P3. Amplification factor of the input signal can be adjusted with the potentiometer P1.