Circuit Description
The circuit (Figure 2) is broken down into two main parts. The front end consists of the op-amp and resistance bridge. This is the actual bio-feedback circuit. The back end of the circuit, the 3914 IC and ten LED’s, make up the display section.
Bio-Feedback Section
The advantages this galvanic resistance device has over previous designs is that the op-amp we are using requires a single ended +9V power supply, in contrast to the standard 741 op-amp that requires a bi-polar 9V power supply, This really simplifies out circuits power requirements. We do require one 1.5V AA battery top supply power to the resistance bridge.
Circuit operation is straightforward. Looking at the schematic, examine the resistance bridge that consists of the electrodes, 1.5V battery, 68 Kohm fixed resistor and 100 Kohm potentiometer. When the electrodes are attached to the subject, the subject’s resistance becomes part of the bridge. The bridges can be balanced using the 100 Kohm potentiometer. The output from the bridge is fed to the input of the op-amp. The op-amp is set up as a difference amplifier. Once balanced, this amplifier will amplify any minor change in the subject’s resistance.
The output of the op-amp is directed to the input of the 3914 chip. The 3914 chip reads the voltage from the op-amp and converts it into a digital display using the ten LEDs. Two types of display are available from the 3914; bar and dot.