Lie detector. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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True or False, or What is a Lie Detector?. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electronics in medicine

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"Lie detectors" have become quite real assistants in the field of inquiry. According to their creators, they should reveal whether the suspected subject is telling the truth or not. This is very important when conducting interrogations during the investigation.

Investigative practice shows that lies and denial of guilt by criminals during interrogation are associated with various physiological "phenomena. First of all, this is an increase in blood pressure, a change in skin moisture, as well as uncontrolled swallowing of saliva, a feeling of dry mouth, deep sighs. These phenomena were used by physiologists and psychologists to develop a polygraph ... or “lie detector.” The first experiments in this direction were started in the 90s of the last century, modern electronic devices allow you to correctly evaluate answers in 85 or more cases out of a hundred.

"Lie detector" consists of a set of sensitive measuring devices connected to the following sensors: an elastic bracelet, fixed above the elbow, which gives a record of changes in pulse and pressure on a movable tape, a system placed at chest level for transmitting and recording the rhythm of breathing and an electronic device for measurement and recording of changes in skin resistance, the electrodes of which, in the form of bracelets, are fixed on the hands of the person being examined. The speed of the paper tape during recording is 160 mm per minute.

The study is based on a series of questions that must be answered in the form of "yes" - "no" (denoted on the chart by entries + or -). First, questions are asked on extraneous topics in order to determine the normal reaction of the "innocent" - then questions related to the course of the investigation, while observing the deviation of the shape of the recorded curve from the norm. This is shown in Fig.1.

Polygraph
Fig.1. Recording of "lie detector" signals:
a - the result of the interrogation of an innocent person suspected of stealing money;
b - a record of the offender's reactions (questions 1, 2, 5 are not related to the investigation,
3 - does he know who took the money?", 4 - "For whom did he steal the money"? (reaction to questions 3 and 4 is visible); x - blood pressure;
y - skin resistance; g - breathing.

An additional test of the nature of the reactions of a given person is an experiment with playing cards. The subject is shown 10 different cards and offered to memorize one of them, and then, shuffling them, lay them out one after another. In this case, the interrogated person must answer “no” each time, that is, once he is lying. This will be recorded on the record and serve to correctly decipher further records. This is how the work of a lie detector basically looks like, which in many cases allows you to identify the main culprit, accomplices and those who knew about the crime. However, only about days the results of decoding the polygraph records cannot serve as evidence of the guilt of the accused before the court. Nevertheless, in the United States lie detectors are used by 19 large government agencies (including the army, police and the FBI), which back in 1964. had 512 similar devices. During the year, up to 20 people are examined with the help of lie detectors. Many such devices are also used in private firms, such as banks. The honesty of bank personnel is regularly checked with the help of lie detectors. The company that manufactures lie detectors organizes a six-month training course for personnel serving the detectors. It includes psychology, law and other subjects.

A simplified lie detector (Fig. 2) has such a high sensitivity that even with an occasional weak excitement of the subject or in the case of a deliberate lie, the arrow of the device deviates. Here are the approximate readings of the device (in microamperes) when reacting to expressions that excite emotions and are indifferent: kisses - 73, love - 59, marriage - 58; carrot - 18, flower - 16, pencil - 15. Designations: A - to the electrodes, B - to the recording device, etc.

Polygraph
Fig.2 A simplified version of the "lie detector". Transistors - KT3102 and KT3107 (corresponding conductivity)

Two electrodes (Fig. 3), which look like closed loops of wire without insulation, are fixed on the hand or on the wrist. In the first case, greater sensitivity is provided, but interference caused by hand movements may occur. If this is the case, then the electrodes should be attached to the wrist. If the sensitivity is insufficient, connect two or three loops together in parallel. Loops are attached to the skin with a rubber or rubber band or just a bandage. The electrodes can also be attached to the foot.

Polygraph
Fig.3. conventional electrodes.

Skin resistance, which under normal conditions can be 3 ... 100 kOhm, under the influence of excitation changes by 5%. The device provides information about this in 1,7...2,4 s after exposure to a light, sound or pain signal.

Polygraph
Fig.4. Universal electrode. (1- may be paper from the packaging of photographic film, medicines, etc.); microporous rubber should be moistened with an aqueous solution of common salt.

The sensitivity of the device (Fig. 2) is regulated by a variable resistor R1. To the output terminals, in addition to a galvanometer, you can connect a recorder. You can also add an op amp with a relay or actuator, or connect the entire device to the input of a radiotelemetry system. Resistors R serve only to protect transistors from overload and in case of a short circuit at the system input or at the time of power-up.

The device can also find other applications, for example, in engineering in the study of processes with an ohmic sensor in the frequency range from thousandths of a hertz to tens of kilohertz.

At leisure, with the device, you can take funny "measurements of temperament". On the fingers of the partner and the partner, spring metal clips are strengthened. At the moment of the kiss, the arrow of the device deviates, touching another subject will give a different result.

It is interesting to try to use this device as a lie detector. To do this, metal clips are fixed on the partner’s index fingers and, asking him questions, follow the readings of the pointer device. The range of questions can be expanded, for example: "What is your name?" "Where do you live?", "Do you drink wine"?, "Do you smoke?", "What did you do on Saturday night?". Zeroing should be done before measurements. Both clamps must already be secured. It is desirable to have a device with a two-sided scale.

Literature

J. Voitsekhovsky. Electronic toys

Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Electronics in medicine.

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