A Simple Lie Detector
Nigel Thompson, 2002
Introduction
This article describes a simple lie detector based on measuring skin resistance. The original was created as a science fair project.
The machine operates by sending a small current through the fingers of the subject and measuring the voltage dropped across them. As the subject lies, he begins to sweat more and his skin resistance drops. The reduction of the resistance causes the potential across the fingers to decrease and this is what we see on the analog meter used to display the effect.
The prototype
The picture below shows the prototype which was build on a
Global Specialties board. To the right of the board is the analog meter which
displays the results and in front of the board are the finger sensors.
The next picture shows the prototype circuit in more detail.
You can see the LM324 which is the heart of the system and the two potentiometers used to control operation of the machine. The prototype was powered by a +/- 12V power supply.
The next picture shows the circuit diagram.
The Circuit
A small amount of current from the batteries flows through the two finger contacts. The actual amount is governed by the two 100k resistors and the 2M potentiometer. The first op-amp is configured as a high impedance input buffer with a unity gain. It feeds a simple inverting voltage amplifier who's gain is adjustable by the 100k potentiometer. In operation the 2M potentiometer is adjusted to give a center scale reading on the meter with the gain set about half way. When the subject sweats, the current through the finger contacts increases and so does the voltage across the 100k resistor and 2M potentiometer. The voltage amplifier's output goes more negative causing more current to flow through the meter and its limiting resistor.
The meter limiting resistor is calculated to give 50% deflection on the meter at about 9 volts. I used a 200 uA meter so:
R = 9 / 100 x 10 -6 = 90 k ohm\
I picked the next smallest resistor in the E12 series which is 82k.
The Finger Connectors
The following picture shows the detail of the finger connectors
The connectors were made by cutting two small strips of aluminum ducting tape about ½ inch wide by 2 inch long. The tape was wrapped around a Velcro cable tie. The wire was bared about 2 inches and wrapped around the aluminum tape. Another small piece of aluminum tape was used to secure the wire.
The construction is not all that important.
Parts
Many of the small parts such as resistors were already available
at home. Other parts came from OEM Parts in
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Qty. |
Description |
Part # |
Price |
Supplier |
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1 |
200uA analog meter |
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$19.95 |
OEM Parts, |
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1 |
8x6x3 Project enclosure |
270-1809 |
$6.99 |
Radio Shack |
|
1 pkt. of 4 |
1" knobs |
274-416 |
$2.99 |
Radio Shack |
|
1 pkt. of 5 |
9V battery connectors |
270-324 |
$1.99 |
Radio Shack |
|
1 pkt of 2 |
14 pin IC socket |
276-1999 |
$0.99 |
Radio Shack |
|
1 |
PC board |
276-168 |
$2.79 |
Radio Shack |
|
1 pkt. of 2 |
Banana jack post |
274-550 |
$3.99 |
Radio Shack |
|
1 |
DPDT switch |
275-636 |
$3.99 |
Radio Shack |
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The Case
I used a Radio Shack project box for the case. I discarded the plastic top and used the aluminum one. The diagram below shows how the front panel was drilled.
Front Panel
The front panel legend was drawn with Microsoft Visio and printed on ordinary white paper. The paper was glued to the aluminum front panel with spray adhesive and then given a generous coat of acrylic lacquer to seal it.
Here is a picture of the front panel with the paper applied and holes cut.
The next picture shows the back of the panel after the hardware has been assembled.
Note that the circuit board is held in place by the meter connections. The following picture shows the circuit board after the components have been soldered into place.
The following two pictures show the battery holder and the battery holder and batteries in the case.
The next picture shows the instrument in its case.
Subject setup
The following picture shows the finger contacts.
Operating Instructions
1 With the machine turned off, attach the electrodes to the first and second fingers of one of the subject's hands.
2 Set the Sensitivity control to minimum (counter-clockwise) and the Center control to mid way.
3 Turn the machine on and adjust the Center control until the meter reads center scale.
4 Turn up the Sensitivity and re-adjust the Center control until you can see the meter creeping up or down. As the subject relaxes, the meter should move to the left indicating that the skin is drying out. Try to keep the meter about in the center when the subject is relaxed and not answering questions.
5 Do not let the subject see the machine.
6 Ask a series of simple questions to which you and the subject both know the answers. Ask the subject to respond 'Yes' to each one. Watch the meter move to the right a second or two after you know the subject answered incorrectly.
7 After a 'lie' you may need to ask a few questions with truthful answers to get the subject quiet again and possibly re-center the meter.
Note: This is not an exact system of measurement and you may need to experiment a bit with the sensitivity control and how you ask your questions to get good results.