Introduction: Fun With a Lie Detector!

Picture of Fun With a Lie Detector!
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...

Picture of 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...

Picture of 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...

Picture of 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...

Picture of 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!


rhapsodium (author)2012-02-21

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.

SanderleeA (author)rhapsodium2016-12-01

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

raza896 (author)2011-05-23

Will it help to catch the 9/11 attackers??????????

ShreyV1 (author)2015-10-07

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

SacredDemon (author)2015-06-18

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

shelore (author)2015-06-08

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

souheib.sfaxi (author)2015-02-13

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 ?

michael.gibbs.1656 (author)2014-10-28

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.

ETfonehome (author)2012-12-05

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

punom (author)2010-09-28

where the R2 resistor ends?

tceferino (author)punom2012-09-28

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

dog digger (author)punom2011-01-15

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

Kzummo (author)2012-08-06

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

MrMistermister (author)2012-06-13

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 :/

MrMistermister (author)2012-06-06

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

gmarian (author)2011-09-14

What can substitute 2N3906 PNP transistor?

toast5678 (author)gmarian2012-04-03

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

lovechickens (author)2011-11-13

hey could i use a breadboard instead of a circuit board to build this?

karatekid99 (author)lovechickens2012-02-19


robofish723 (author)2011-08-24

Same here, there isn't even a peep coming from the speaker, anyone haveany help?

mshahzeb (author)2011-08-02

i have same problem no sound is coming from speaker.

acaz93 (author)2007-12-02

Hey , Kip .. Remember The Comment That Says It needs a tuning pot It's actually simple , just put (instead of 82k resistor ) a 50K Potentiometer and a 33k Resistor In series , And That's It , VOILA tuning ready!

instructa-fan (author)acaz932007-12-07

can you get the pentiometer thingy at radio shack as well as the 33K resistor? its kind of convenient I read now cuz thats the one thing i don't have yet! lol!

acaz93 (author)instructa-fan2007-12-08

Yes You can , but it is not vey cheap :S ,
Here are the links by the way 50k Pot And 33k Resistor

acaz93 (author)acaz932007-12-08

resistor's color code is Orange-Orange-Orange-Gold

Kaiven (author)acaz932008-02-08

you know about electronics? i know how to make a tazer from a camera, and found it only gvies 35v. I saw at radio shack a huge capacitor that was 500v. could i replace the 35v with the 500v capacitor? would i need multiple camera boards to sharge it? thanks

XOIIO (author)Kaiven2011-01-29

It actually isn't technically a tazer, it is far from it, and the capacitors in cameras are usually 200-450 volts.

instructa-fan (author)Kaiven2008-02-14

Actually, they put out 350 Volts, not 35. Check the current on the 500V cap and make sure you dont kill anyone. the little camera circuit should charge it just fine, just make sure you have your polarity right. It might take a little bit longr to charge but hey, if it keeps em' twitcing!

Kaiven (author)instructa-fan2008-02-16

ahh... okay do i check the current?

instructa-fan (author)Kaiven2008-02-16

I think amperage is the same as current but im sure some other guy reading this is gonna tell us otherwise, but check that with a volt meter, it might be under Ohm's as well, thats current im sure... and besides amps can be lethal too. Let me guess your age... 9-11?

acaz93 (author)instructa-fan2008-02-16

my age ? i'm 15
Current is checked using an Ammeter (or use the Amp function in your multimeter)
just remember more than 900 mA = Semi-lethal shock

instructa-fan (author)acaz932008-02-17

You're still a smidge off on your digits, I'm pretty sure its just 90ma for death...

bye the way, i was actually talking to cowscankill on the whole age thing.

Kaiven (author)instructa-fan2008-02-18

haha! im twelve, though ussually i think im thirteen. i don't have any a-meters of any kind. maybe i could shock metal and see which leaves a bidder mark?

Stuffses (author)Kaiven2010-05-20

 woo! teens are smart too! im 13 and im buildin a combat robot with a coilgun.

Kaiven (author)Stuffses2010-05-21

 I'm actually 15 now lol. But I do love weapon design...

xproplayer (author)Kaiven2009-02-21

that would be really primitive most all multi meters have amp reading

acaz93 (author)instructa-fan2008-02-17

ok , i understood

evanwehrer (author)instructa-fan2008-07-21

The FCC ham test thing says 100 mA

acaz93 (author)instructa-fan2008-02-17

I Think you're Right , Actually Less Than 7 mA Can severely injure somebody , Awww , Science ...

Stuffses (author)acaz932010-05-20

 i thot it was more than 10mA

acaz93 (author)Kaiven2008-02-08

Well , Just A little , i'm actually a student It's kind of hard , for example , your camera board gives 35 v , it actually shocks you beacuse of amperage not voltage , a simple solution if you have a AC source , you can google "voltage Doubler " Or even better Voltage "tripler " I know how to assemble a Shock Box (giggles lolz) if i just can only find the schematic , gimme a couple of days , I hope this small information helped you

Kaiven (author)acaz932008-02-09

hmm...okay thanks, but you say the camera board charges up to 35v? does that mean it wouldn't charge the 500v capacitor?

xproplayer (author)Kaiven2009-04-25

and yes it would charge the cap but only you would get about 300v not 500v

xproplayer (author)Kaiven2009-02-21

ok on a disposable camera board when you press the charge button it takes a 1.5v batter and runs it through a small style transformer and the output which goes to the capacitor which is about 300v it varies from transformer to transformer and will not kill you but will hurt alot

acaz93 (author)Kaiven2008-02-09

Maybe , The capacitor is just charged half , i actually don't know that much electronics , If its AC the capacitor Is being loaded with about 60 - 75 volts ,maybe...

technodude92 (author)acaz932008-03-24

the 500 volt rating on the capacitor just means its rated for 500 volts. in other words don't put more than 500v into a 500v cap. you can charge it to whatever voltage you want just not more than 500 or you risk explosion. Oh and caps don't store AC, only DC. you can't check the current coming out of a cap only the voltage. but if you have a cap with high capacitance then you run the risk of hurting someone. this is where it gets fuzzy... quote from wikipedia for Farad: "a farad is the amount of capacitance that requires one second for a one ampere flow of charge to change the voltage by one volt" i.e. a 1 farad capacitor charged to 1 volt will put out 1 amp for 1 second now. a farad is a VERY large unit. most caps are measured in pico- or micro- farads. so a 1uf capacitor(1microfarad) charged to 1 volt will put out 1 milliamp for 1 second(i think). this side of electronics makes my brain numb. and trying yo decipher this at 4 in the morning doesn't help either.

knight2067 (author)technodude922008-08-22

A 1uF capacitor will give 1uA (microamp) at 1V for 1 second... I think.

technodude92 (author)knight20672008-08-31

you're correct, I now realize that i said a MICROfarad cap puts out a MILLIamp at 1 volt for 1 second, when it should have been 1 microfarad = 1 microamp at 1 volt for 1 second.

About This Instructable




Bio: Tinkerer, hackster and prankster. Hit me up on Facebook: Thanks for checking out my Instructables!
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