Class: BE209
Group: R8
Members: CHIANG, CHING-YU (DORA); Hernandez, Liani; JONAS, RACHEL D; RAMPAT, NITIN; SCHRIVER, MAUREEN P

Date: December, 2002

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INTRODUCTION:

In criminal investigations, polygraphs have a reputation for being an unreliable source of proving guilt or innocence.  In many states, a polygraph exam is not admissible as evidence in a court of law, and polygraph evidence is widely called into question.   The polygraph exams are designed to monitor physiological signals of the suspect and detect physical arousal, since it is believed that a change in arousal during questioning is indicative of deceit.  The motivation of our experiment is to examine these conceptions about polygraphs by building our own polygraph, administering exams on a number of subjects and interpreting the polygraph data to determine the guilt or innocence of our subjects.  Our objective in this experiment is to build a functional polygraph from a simple bridge circuit useful for detecting physiological changes in a subject when that subject falsely responds to a question.
In polygraph exams, the four basic physiological signals that are monitored for changes are brain waves, galvanic skin response, pulse and respiratory rate.  The physiological responses that we will be testing in our experiment will be respiration rate and breath temperature.  Respiration rate is a common response that is monitored during polygraph testing.1  In addition to monitoring respiration rate, we will also look at the effects of breath temperature in evaluating deceit.  The thermistor apparatus that we will be using is a device that has a resistance proportional to temperature and will provide data output for both of these responses.  Using the Steinhart-Hart equation, we will be able to determine the relationship between the temperature of a subject’s breath and the voltage in the circuit.  The circuit is comprised of a simple bridge using two 22KOhm resistors, a potentiometer and the thermistor.  The bridge is used to reduce the amount of noise in the circuit to a negligible amount.