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Simple Lie Detector

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colin55

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You can get ECG electrodes from a hospital that stick on your skin to create a more-reliable connection. You can then measure the internal resistance of your body.
Most of the resistance of your body is in the small distance between the outside of your skin and the inside of the skin.
If you press harder on an electrode, the resistance decreases enormously.
This effect is much larger than the change in resistance due to sweating.
That's why lie detectors are so inaccurate.
I have a circuit for a "super-sensitive" lie detector for those with dry skin.
A person with dry skin does not activate a simple lie detector AT ALL.
 
Actually lie detectors are inaccurate because a knowledgeable tester can manipulate the subject and a knowledgeable subject can manipulate the test. Last time I checked lie detector tests were inadmissible as evidence in US court because the test is so easily manipulated.
 
Actually lie detectors are inaccurate because a knowledgeable tester can manipulate the subject and a knowledgeable subject can manipulate the test. Last time I checked lie detector tests were inadmissible as evidence in US court because the test is so easily manipulated.
And even at best the interpretation of test results are very subjective.

I recall some decades ago the show 60 mins did a report/expose on lie detectors and the agencies that contract out their services. It was a setup where they pretended to be a small company that was having some office thefts and that while they suspected a specific employee who they did identify to the testers, they had no proof so wished to have all the office employees tested. The results from three different agencies were all the same and that was that the suspected employee was lying and only he was lying. Of course the suspected employee was an actor (as were all the 'employees') and was told not to lie. Funny that results equaled expectations! Of course all three agencies denied subjectivity (or worst, fraud) and said the tests were flawed because all the actors were not under real life stress. Also turns out that the lie detector association is a very profitable organization that 'sells' certifications for their 'member' agencies and help keep out independent and 'non licenced' and 'unqualified' testers.

As practiced, at least in that N.Y. city area, it is or was at least then a scam and just a way for some people to make a rather sleazy living.

Lefty
 
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Hopefully the trend is nearly spent =) I remembered from my 5 in 1 sheet from work.
Employee Polygraph Protection Act
Interesting read. Did you notice all the exemptions from this law, nearly all government and military are not restricted in anyway from using polygraph testing. :rolleyes:

Always strange how a goverment for and by the people can often pass laws that apply to normal people but exempt themselves.

Lefty
 
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And for the person looking for the M1 mA reader to make the schematic in pPsice... Just draw a short circuit and put your current probes on that node and have it output that to a graph. I'd take a digital multi meter and measure your skin, and use that as a load across where the electrodes would go. Then graph that current. Then you could increase and decrease the load resistance a little be and see how this effects the current. i.e. you would be modeling it after how a person's skin resistance could increase or decrease and the effect it would have on the current that you are motoring.
 
Hey guys I have a question was hoping could be answered. I'm an EE undergraduate student so I have been exposed to many type of transistor circuits.

Can you tell me what role the capacitor plays in this circuit? I do not understand why there needs to be a capacitor connected in between the two electrodes.

Thanks
 

Roff

Well-Known Member
Hey guys I have a question was hoping could be answered. I'm an EE undergraduate student so I have been exposed to many type of transistor circuits.

Can you tell me what role the capacitor plays in this circuit? I do not understand why there needs to be a capacitor connected in between the two electrodes.

Thanks
The cap is probably not necessary. Human bodies will generally have a lot of common-mode mains-related voltage (primarily 50 or 60Hz, depending on where you live), but this is relative to earth ground. I don't think there will be enough differential-mode noise to drive the transistor into saturation on the peaks, which would screw up the bridge. The cap is probably cheap insurance though, just in case.:)
 
Any comments if it really works fine? It seams to simple to really work well detecting lies, but it could be possible if the lier is a big one and the person is very nervous.
Kind regards
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Any comments if it really works fine? It seams to simple to really work well detecting lies, but it could be possible if the lier is a big one and the person is very nervous.
Kind regards
Of course it doesn't really work, it's a skin resistance meter, that's all - it will only detect is the person sweats when he lies.
 

audioguru

Well-Known Member
Most Helpful Member
People who lie a lot are calm and do not sweat when telling lies. But maybe they get nervous and sweat when telling the truth.

An expert uses a skin resistance test plus heart rate plus breathing rate plus tremmors in the voice and still are not always correct to determine a lie.
 
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Ultrasonic Vision

Nice Project sir ,If u hav some projects on ultra sonic vision please mail me............





Here's a simple lie detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie.

 

HiTech

Well-Known Member
OMG!!! The last two pages of this thread are simply hillarious to read! I think Audio Guru has posted more replies here than in any other thread! Hey buddy, you're preaching to a choir that apparently has the collective intelligence of a door knob. I also think we have a troll lurking about since many of the replies are 1-2 post wonders.
As for the pleading by some of the boneheads to so desparately want to build this basic circuit, by all means go ahead, have at it. In the end you will have accomplished the task of building a basic electronic circuit. However, it's not up to performing the job or specification of it's mis-represented name. You built a circuit, go have fun with it, but don't base your spouse or girlfriend's alleged infidelity on it.

And to think that some of these nations have peaceful and tactical nuclear capabilites!!!:eek::(
 
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