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Biofeedback monitor - PC
This circuit uses an LM555 to form a biofeedback monitor that connects to a personal computer. Probe A and B are simply two foil strips glued to the cover of the case the parts are mounted in. This circuit measures the galvanic skin response (GSR) which indicates tension. As you relax, your skin resistance increases. The LM555 is configured as a pulse generator. The width of the pulse proportional to the product of C1, R1, and the skin resistance between probes A and B. PL1 is the printer port of a PC. Pin 16 , the INIT line, connects to the trigger input of the LM555. Sending a short pulse INIT pulse causes the output (pin 3) to go high, Pin 3 goes low at the end of the pulse. This time, from high to low, is an indicator of the GSR. Pin 20 is ground. Software must be written (or see Radio-Electronics Oct, 1988 page95) to operate the unit. To run the monitor, place your index finger on one probe and your second finger on the second probe. Do not move your fingers or change their pressure. The unit may also operate as a rudimentary lie-detector.