Bio-feedback devices allow individuals to train on control an aspect of their autonomic physiology. Autonomic, in other words, means automated and controlled by lower brain functions and therefore not under out conscious control. This belief, however, is outdated – it proved to be a fallacy. Training a person to control such physiology as EED (brain waves), EKG (heart rate), blood pressure, and tension level proved to be possible once a method became established of showing a person (feedback) the physiological changes they are trying to control in real time.
It is interesting that the control of these functions cannot be accurately taught verbally. It is the training or the conditioning of the body using a bio-feedback device to feel a particular way or to get a feeling that controls these functions.
The types of bio-feedback devices available are numerous. The type that we will build in this chapter is a galvanic skin resistance bio-feedback device, see Figure 1. Galvanic skin resistance is a good indicator of stress level in the subject. But perhaps its most famous attribute is its use as a lie detector.
Figure 1
Constructing GSR meter for BioFeedback
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.
Electrodes
Silver is considered one the best electrode materials around. We can capitalize on this by using two U.S. dimes for our electrodes. The cable we’ll use is a shielded 2-conductor. The shielding is a copper braided wire that surrounds two insulated wires in the center of the cable. Remove about 2 inches of the outer cable jacket, and separate the shielding from the insulated wires. Strip ½ inch of insulation off the center wires.
Soldering the wires to the dimes is a little tricky if you haven’t done much soldering, see Figure 3. Place the tip of your soldering iron on the coin, and keep it there till the coin becomes hot enough to melt solder on it. This takes about 1-2 minutes of continuous heating. At this point melt a small puddle of solder on the coin, and place the bare end of one of the insulated wires into the puddle. Remove the soldering iron from the coin. Keep the wire in place until the solder solidifies. Repeat the procedure for the other coin, but solder the shielding along with the wire to this coin. This will be the ground electrode
The palm of the hand is very sensitive to galvanic changes, so it is therefore the area of choice. To secure the dime electrodes to the palm of the hand I made a small palm-fitting electrode holder out of ¾ inch pinewood. You only need a couple of square inches practically any piece of scrap can be used, see drawing. After cutting the wood to the proper shape, drill a ¼ inch hole through the center as shown to feed the cable through. Then glue, epoxy or hot glue the electrodes to the wood block as shown. To finish off the hand electrode, attach a rubber band or elastic material to the base so that it covers the electrodes. This material is what will secure the holder to your hand.
GSR BioFeedback Device Construction
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.
Using GSR BioFeedback Device
Bio-Feedback
To use this device in a bio-feedback mode for relaxation and tension reduction, set the balance pot to light the graph in the upper portion. As you relax or reduce tension the body’s resistance increases, which will be seen as a gradual downward sloping of the graph. When you reach the bottom you can re-a just the balance pot to bring it back up and then try to bring it down again.
Lie Detector
To use this device as a lie detector set the graph on the lower portion of the graph. Any stress will cause the graph to rise. There is a delay between question and response of about 1.5 seconds. Remember, this device is for entertainment purposes only. Even full-fledged lie detectors are fallible, and it could be the nature of the question regardless of the answer that would cause a stress reaction.
| Parts List | ||
| Item | Source | |
| IC1 | CMOS OP-Amp | Images Company |
| R1 | 68Kohm | Radio Shack |
| R2 | 100Kohm Potentiometer | Radio Shack |
| R3 | 2.2 Meg | Radio Shack |
| R4, R5 | 2.2Kohm | Radio Shack |
| C1 | .047uf | Radio Shack |
| SW1 | DPDT | Radio Shack |
| Misc. | 1.5V Battery and Holder, 2 Dimes for use as electrodes, Case with PC Board | |