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How does this circuit work?

Discussion in 'General Electronics Discussion' started by Asdf, Dec 20, 2009.

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  1. Asdf

    Asdf

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    Dec 20, 2009
    Hi, can someone help me understand how this circuit works, I know what transistor, collectors, and resistors are, but what are their function in this circuit, and how does it work part by part? And how accurate is it? I am doing project on this, to find out if skin perspiration changes when one lies. I also don`t understand what the difference between an electrode and a regular wire is, please help.

    http://www.aaroncake.net/circuits/lie.asp

    Thank You!
     
  2. kyjosh

    kyjosh

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    Dec 15, 2009
    Well, when it talks about electrodes, it basically just means probes. So a regular bare wire stuck to the skin with tape would work.

    I'm really not sure how that is supposed to work though. Admittedly I'm fairly new to this stuff, and someone like Resqueline would know better; but that schematic just does't look like it would really work.

    What I can see from it, is it is supposed to run a small voltage across the skin. When you lie the skins resistance goes down and causes its curret to go up. The transistor amplifies this small amount of current into the milliamp range that the analog ammeter can read. So when the meter goes up, theoretically the person it lieing. I dunno if that principle is true though.

    I don't know why there are two power supplies, and I guess the capacitor is just for filtering. Really, even at such a low voltage, I'm not sure I'd feel comfortable hooking myself up to that thing!
     
  3. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    I was unable to connect with the site yesterday so I couldn't comment on it then.
    The circuit is very simple yet incorporates most aspects of a professional unit.
    Professional electrodes incorporate silver oxide & conductive gel, but any piece of metal will do for experiments.
    C1 is there to remove noise from the mains and from physical movements.
    R1 limits the current that can pass through you and into the base of Q1.
    Q1 amplifies the skin current to something suitable for M1.
    R2 balances the (center zero) meter and so makes it possible to make the circuit very sensitive.
    R3 limits the maximum current that can go into M1.
    It's a so-called bridge circuit, where the two 4.5V batteries & 1.5k R3 could just as well be replaced with one 9V battery and two 3k resistors.
    It works by the principle (theory) that perspiration increases when a person tells a lie and this increases the conductance of the skin.
    It's only as accurate as the human psyche; it can't be relied upon. The manner in which the questions are asked is important and you have to place control questions to see what the "normal" responses for that person are.
    Also, a person that doesn't give a **** about lying or consequences or doesn't find the situation threatening in any manner will be hard to reveal.
    It completely safe, 9V only tickles the tongue and wouldn't harm the skin even if applied directly.
     
  4. Asdf

    Asdf

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    Dec 20, 2009
    Why is that helpful?
    I am not sure what a 5K Pot is, what is it?
    Why is that helpful also?

    Except for that, I understand everything. And why couldn't I just use an ohm meter directly, wouldn't that work?

    Thank You!!!!
     
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    R1 helps set the point & range of operation of Q1.
    Without it you could end up with a circuit behaving like some fuel instruments; staying on full for 50 miles then quickly dropping to empty and staying there for another 50 miles before the tank is empty for real.

    A pot is short for potentiometer; a variable resistor.

    Moving coil instruments have a limit to the current they can take before burning up. Without R3 you'd have a free path for the current from the battery through M1 and then Q1. Shorting the probes could then damage M1.

    Yes, you can use an Ohm-meter directly instead, provided it has enough resolution (# of digits).
    The circuit described could be called an expanded scale Ohm-meter.
     
  6. Asdf

    Asdf

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    Dec 20, 2009
    I understand the fuel analogy, but I don't understand how it relates to the circuit.

    What exactly is the difference between a potentiometer, and a resistor?

    But couldn't R2 help with that?

    So what would be the advantage of doing a more complicated circuits like this one and others, if you could just do it with an Ohm-meter directly?

    Why should it be an analog meter and not a digital meter?
    How accurate is it at measuring skin resistance?
    And how accurate would an Ohm-meter directly be?


    Thank You!!!!
     
  7. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    I'm sorry but I question myself what planet you're on.

    Please try to read some simple transistor amplifier theory.

    I don't know how to explain the details any closer to you, and I already answered some things you ask again.

    This thing is an old, simple & cheap "invention", go find the price of a 6-digit ohm-meter.
    It would be useful if you actually tried using a 3-1/2 digit multimeter and reported back.
    Accuracy, what's it good for here? The skin is not "calibrated" in any way.
     
  8. Asdf

    Asdf

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    Dec 20, 2009
    I am not very experienced with electronics :-( I need help. I really appreciate your help.

    Sorry, it is not on purpose, I just really have to understand this.

    Why wouldn`t it work?
    I need accuracy for a project. If this is accurate, great! But if it is not, how can I make something that would measure it accurately?

    Thank You!!!!
     
  9. kyjosh

    kyjosh

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    Dec 15, 2009
    A potentiometer is a resistor. It is a variable resistor that allows you to change its resistance value.

    Well, electricity takes the path of least resistance. So if there was no resistor between M1 and the power supply, then R2--assuming it is a higher resistance value than the analog meter--would actually cause all of the current to pass across M1. So R3 is needed to make sure the current that reaches the meter is low enough to prevent damage.

    Well, this circuit should theoretically be more sensitive, and thus more accurate than just using an Ohm meter on the skin. Specifically, the transistor Q1, increses the current that reaches the meter; so it should make any changes more obvious.
     
  10. Asdf

    Asdf

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    Dec 20, 2009
    Oooh

    And you might have answered this, but I am not sure if I understand, Why is R2 there?

    I understand all of the rest.

    Thank you!!!!
     
  11. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Thanks for chiming in kyjosh, sometimes it helps to put it in other words.

    I don't mean to be hard on you Asdf but you must try to pay attention, read & understand what's written, & also try to search the net for some basics.

    Digital meters have much better resolution than ordinary linear meters, but they're not very easy to observe a 1 or 2 second deviation with. An expanded scale linear meter can easily compete with the resolution of the best digital meters and keep the easy deflection reading too.

    R2 is a button used to center the needle of M1, like the balance control on a stereo.. Its function can't be interchanged with R3 (ref. my earlier statement about it).

    I still don't understand your need for accuracy. What kind of accuracy? What are you trying to do & achieve?
    A digital ohm-meter is accurate, this circuit is not - but still much better suited to show the physical reactions one is after. If you want to do something other than a lie-detector project then you better tell.
     
  12. kyjosh

    kyjosh

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    Dec 15, 2009
    R2 is so that you can adjust the resistance to control the starting current across the meter. You want the current to start out at zero when the person isn't answering any questions.
     
  13. Asdf

    Asdf

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    Dec 20, 2009
    Ooooh, now I understand.

    I am a doing project with the purpose of figuring out if skin perspiration changes when one lies, so I need to be very accurate, I have to measure (exactly) how much the perspiration changes, is this circuit not accurate? What can I do/change to make it accurate?

    P.S. What I didn't write about, it means that I understood.

    Thank You!!!!
     
  14. Asdf

    Asdf

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    Dec 20, 2009
    Also, I just realized, that I don't understand how R1 is helpful to the circuit, it is the last thing that I don't understand, but except for that, I understand everything in this circuit, and their function.

    Thank You!!!!
     
  15. kyjosh

    kyjosh

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    Dec 15, 2009
    Well, again Resqueline is one who would know for certain, but I believe it is supposed to limit the current and voltage that travels across the skin.

    As for your project... well... I don't really think this is going to tell you how much the person perspires. Its functioning will only tell you if their perspiration increases when they lie.
     
  16. Asdf

    Asdf

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    Dec 20, 2009
    If that is what it helps with in this circuit, then I understand, which makes me understand all of the parts in the circuit and why they are there, but let's see what Resqueline says about it.

    This will be good enough, but it has to tell even if the perspiration increases just a little bit. Does it? I think that the transistor helps with that.

    These questions might be stupid, but please bear with me:
    Why 4 volts batteries?
    What does 0-1 mA in the Analog Meter mean?
    Why a mA-Meter, and not an ohm-meter?

    Thank You!!!!
     
  17. Asdf

    Asdf

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    Dec 20, 2009
    Also, what grade level do you think that this would be in canada? And also, what about PN-Junctins, and semicunductors' stuff, what grade level do you think that would be in Canada also?

    Thank You!!!!
     
  18. kyjosh

    kyjosh

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    Dec 15, 2009
    Well I don't know if it actually works, you'd have to build it and test it to see. But theoretically yes it does tell you if perspiration increases because more sweat makes the skin more conductive, so you get more amps across the meter.

    The batteries are 4.5V because most batteries are rated at 1.5V, so three of them add up to 4.5V; and two of the 4.5V sources added together equal 9V- which is a fairly common voltage to work with.

    I assume the author of that page/circuit means that meter reads through a range of 0-1 milliamp.

    You use an Ammeter because the transistor can be used to increase the amperage to a level the meter can read. The actual current that will travel across the skin will be very, very small. You wouldn't use an Ohm meter because you would have no way to increase the resistance up to a level that you could easily read. If the skin's resistance changed by .0001 milliohm then the meter probably wouldn't even move.
     
  19. kyjosh

    kyjosh

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    Dec 15, 2009
    By grade level do you mean K-12, or college level? I live in Kentucky, but I assume K-12 requirements are about the same between the US and Canada; and I know for engineering related degrees the requirements are basically identical in college.

    I personally never covered anything like this in high school. We covered basic electrical theory in science class in either grade 10 or 11- but we never went into components or how they work. There might have been elective classes over it, but I took all of my high school electives in business and computer technology... lot of good that is doing me now though. lol

    In college you cover this kind of stuff in engineering or engineering technology/electronics classes (and maybe a few physics courses). I don't guess it really matters what year you are in college.

    At my college (and I believe at most other schools) you specifically cover electrical theory (resistance, voltage, and current and how they interact) in a class called Circuits 101/102. Semiconductors and integrated chips in Digital 101/102; and transistors in Devices 101/102.

    I know that isn't exactly what you asked, but I hope it helps.
     
  20. Asdf

    Asdf

    58
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    Dec 20, 2009
    But why 9 volts in this case?

    Oh, sorry about this one, it was kind of common sence.

    Ooooh, I understand.


    Yes, that was very helpful!

    Thank You!!!!
     
    Last edited: Dec 23, 2009
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