Circuit Construction
The circuit is fairly simple if you look at the photograph of the circuit board (Figure 4) you’ll see I used ribbon cable to connect the LEDs to the 3914. Ribbon cable isn’t necessary but it helps keep the LEDs in proper order. The bar/dot display mode switch simply connects pin 9 of the 3914 to the +V, or lets it float.
Circuit Operation
Attach the electrodes to your subject’s hand by placing the wood electrode holder in the palm and the rubber band around the hand, see Figure 5. Turn on the circuit, then adjust the balance pot so that the LED graph is lit approximately midway. You will notice that when adjusting the balance pot the LEDs jump very quickly when you reach the balance area. A soft touch is required in rotating the pot around this area.
If your subject is a little nervous you may have to adjust the balance a few times as they relax. When everything is stable, have the subject quickly inhale and exhale deeply. This should cause a rise in the LED graph that gradually returns to the previous level. If you get this result the circuit is operating properly and you’re ready to go. If you get the opposite reaction i.e., the LED graph dips, the battery in the bridge section is reversed.
In order to test the circuit you must have some resistance connected across the electrodes or the LEDs will never light.