Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 191

Circuit 168: FET lie detector with LED indicator
Setup information
Install the two transistors, the diode and the three resistors as shown in Fig. 2.168 followed by the eight
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 LED
remains dark. If you touch the sensor with your wet (but grease-free) hands, the LED will shine brightly. 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. 3.168.
Components required:
2 x N3904 transistors,
1 x red LED, 1 x 1 kΩ
resistor, 1 x 22 kΩ resis-
tor, 1 x 1 MΩ resistor,
1 x breadboard, 1 x 9V
battery with clip con-
nector
Fig. 2.168: FET lie detector
with LED indicator
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