How to Make a Lie Detector Machine

Written by stanley goff
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How to Make a Lie Detector Machine
Homemade lie detector (bad guys lie image by robert mobley from

Polygraph machines, also called lie detectors, measure certain physiological responses that are created by various applications of stress. A professional apparatus measure heart rate, blood pressure, and galvanic skin response. Most homemade devices measure skin response only. As long as the machine's limitations are understood--it can't actually detect lies, just variations in physiological responses--they can be an interesting, even entertaining, pastime.

Skill level:

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Things you need

  • Electrical box enclosure
  • 4.7K Resistor
  • 82K Resistor
  • 0.01uF Capacitor
  • 2N3904 Transistor
  • 2N3906 Transistor
  • 2-AA Battery holder
  • Small circuit board
  • Spare PC speaker
  • Velcro
  • Aluminium foil
  • Wire
  • 2 x AA batteries
  • Soldering iron
  • Solder
  • Screwdriver
  • Hot glue
  • Drill
  • Small drill bit
  • Wire strippers

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  1. 1

    Build the ciurcuit board. The wiring schematic for building the circuit board is available at This is the step where the most patience is required. Make clean, solid solder joints.

  2. 2

    Place the circuit board into the box enclosure. Mark and drill holes for the wire to the finger pads. Drill a few small holes near the speaker.

  3. 3

    Cut two strips of Velcro long enough to reach around a large adult finger. Attach two corresponding strips of aluminium foil onto the sticky “soft” side of the Velcro strips. Glue the finger pad leads that extend from the box to the aluminium foil strips on the finger pads. Trim half an inch from the hook-side of the Velcro strips so the finger pads will overlap and attach.

    Start asking questions. If the circuit board is wired correctly, the speaker will make a noise if it detects a spike in galvanic reaction, or how electricity flows across the skin.

Tips and warnings

  • Power tools and soldering equipment are required. Take appropriate precautions.

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