Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 192

Circuit 169: Lie detector with moving-coil instrument
Setup information
Install the two transistors and the three resistors as shown in Fig. 2.169 followed by the seven jumper
wires. Two bare wire ends serve as skin sensor. Check the placement on the board again and only then
connect the battery.
Experiment
Our circuit is a simple DC voltage detector. As long as the two sensors are isolated from each other, the
needle is not moving. If you touch the sensor with your wet (but grease-free) hands, the needle deflection
will be more or less strong. If the anxiety level rises when a person lies intentionally, the sweaty palms of
the hands could be taken as an indication that the person is not telling the truth.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.169.
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Components required:
2 x 2N3904 transistors,
1 x 1 kΩ resistor,
1 x 22 kΩ resistor,
1 x 1 MΩ resistor,
1 x moving-coil instru-
ment, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.169: Lie detector with
moving-coil instrument