Fun With a Lie Detector!

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Introduction: Fun With a Lie Detector!

About: Tinkerer, hackster and prankster. Hit me up on Facebook: https://www.facebook.com/kipkayvideos/ Thanks for checking out my Instructables!
Is your friend or spouse lying to you? Build this Lie Detector and find out the truth! Great for parties too!

This is the FIRST Lie Detector project on Instructables.

Watch the video and then build it...


Step 1: Parts You Will Need...

I picked up all of the components at Radio Shack except for a few I list at the end.

1. Project Enclosure
2. 4.7K Resistor
3. 82K Resistor
4. 0.01uF Capacitor
5. 2N3904 Transistor
6. 2N3906 Transistor
7. 2-AA Battery holder
8. Small circuit board

Spare PC speaker, velcro, aluminum foil, wire and AA batteries in my extra parts bin.

Step 2: Basic Tools...

You will need a soldering iron/station, solder, screwdriver, hot glue, small drill bit and wire strippers.

Step 3: Build the Circuit...

Following the schematic shown here, build the circuit. Take your time and make good solder joints.

Step 4: Make the Finger Pads...

Install everything in the project enclosure leaving just the Finger Pad leads exposed through drilled holes. I also drilled holes in the box so you can hear the speaker.

I got the finger pad design from this article.

Using a strip of Velcro (soft side), cut a piece of aluminum foil the size of the veclro. Make sure the velcro is long enough to go around a big, fat finger. Some liars are chubby. Remove the sticky side of the velcro and attach the aluminum foil with one of the Finger Pad leads under the aluminum. Then use a smaller piece of the other part of the velcro (the 'hook' side) and attach it to the end so the whole pad can be wrapped and secured.

Step 5: Test It Out...

This works on the galvanic skin response. When you lie, your skin becomes moist. While this does work, it's not as accurate as a true Polygraph machine. Hope you enjoyed this Instructable and for another really good DIY Lie Detector, check out Jason Bradbury's!

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    132 Discussions

    great tutorial. used a breadboard instead of a circuit board, worked fine. the computer speaker had too much resistance, though, so i just bought a tiny eight ohm one from radioshack.

    1 reply

    Im making this for a school science project and I could really use some insight on which parts work and which dont

    what type of transistors do we have to use: npn or pnp ?

    Does any one have a picture of everything soldered together so I could get a better Idea of the connections?

    i have been having headache on downloading these pdf for these lie detectector cause its my project ,, somebody ,, how can i downlaod these lie detector pdf

    I did it and the sound wasn't clear and as i see there are many people who have the same problem. Anyone can help us ?

    hello agian how are you sir i have built the portable lie detector when i drop water on it it changes tone do you think i can try 100 k ohm poterntiometer in the place of the 82 k resistor.

    How do you tell if someone is lying to you? Does the speaker change tone or something like that?

    Pienso, que el final de R2 es tu dedo, cuando aterrizas con R1.

    It goes to a finger pad. The line sticking out in between C1 and R1 goes to the other finger pad

    An 82k resistor? Is that what that is? Because it doesn't at all look like it? PLEASE HELP!

    I tested this with a breadboard, and when I put on the finger pads the speaker makes noise, but the tone slowly goes up and up and it doesn't stop for, like, 30 seconds. I tried lying with it but the speaker didn't change tone :/

    Is there a switch? Or does it turn on when you put on the finger pads?

    2N3906 are NPN transistors, and no no other transistors can replace it in this particular circuit