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Newbe Help with this schematic

Discussion in 'General Electronics Discussion' started by stspringer, May 14, 2019.

  1. stspringer

    stspringer

    89
    5
    May 10, 2019
    Hello All,
    Newbie here.
    I am not clear on this schematic, on how the touch metal, hooks up. Is it just 1 wire going from the 100k resistor to the touch metal? Please advise.

    Thank You
    stspringer
     

    Attached Files:

    Last edited: May 14, 2019
  2. Bluejets

    Bluejets

    3,432
    697
    Oct 5, 2014
    That's it.:)
     
  3. stspringer

    stspringer

    89
    5
    May 10, 2019
    Thanks. I was confused because most touch switches show 2 wires. Why is this one different and why a 100k resistor isn't that pretty high in ohms?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,074
    1,824
    Nov 17, 2011
    Your body works as an antenna. The tiny signal is fed to the base of the transistor Q1. As there are 3 amplifying stages, a very small base current sufices.
    The 100 kΩ on the other hand protects the transistor from high currents in case of e.g. an ESD discharge.
     
  5. stspringer

    stspringer

    89
    5
    May 10, 2019

    Thank you, very nice
     
  6. stspringer

    stspringer

    89
    5
    May 10, 2019

    I just tried it with metal and my finger and it doesn't work. It only works if I jump the gate and the collector of Q1
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,074
    1,824
    Nov 17, 2011
    The "gate " is called base in a bipolar transistor and yes, of course it works then because the base gets a positive current from R1 and R2.

    Did you try barefooted? I'm not joking.Standing on well isolated soles may reduce the amount of noise available below the required minimum.

    A more sophisticated single contact sensor is shown e.g. here.
    A more reliable method would use a sensor with 2 plates which have to be touched (bridges) by a finger. You can use the circuit you found and add a second plate which is connected to +9V. Or use this even more simple circuit.
     
    Bluejets likes this.
  8. Minder

    Minder

    2,807
    583
    Apr 24, 2015
    Would Mosfets work better? They operate on almost zero current and relay on voltage to switch.
    The 2n7000 is cheap.
    Maybe clamp the gate with a zener.
    M.
     
  9. stspringer

    stspringer

    89
    5
    May 10, 2019

    Thanks I will check out your sites. Thank you
     
  10. stspringer

    stspringer

    89
    5
    May 10, 2019

    Hello,
    I am a newbie don't forget :) you are speaking greek to me.
     
    Bluejets likes this.
  11. Bluejets

    Bluejets

    3,432
    697
    Oct 5, 2014
    Nah...He's from Canada so more likely "French". o_O

    You might want to take a peek at Talking electronics web page.
    It has many such circuits and a host of other projects good for beginners as well as others.
    Books, circuits, explanations...all good.

    For your present project scroll down the left side to "touch switch"

    Edit:- just take note that they show some circuits which do not work and the reasoning behind why.

    Cheers Jorgo

    http://www.talkingelectronics.com/te_interactive_index.html
     
    Last edited: May 15, 2019
    Cannonball and davenn like this.
  12. Cannonball

    Cannonball

    170
    47
    May 6, 2017
    This is a poorly designed circuit. It could be made to work but by the time it takes to redesign it. It wouldn't even look the same and not a circuit for a newbie.http://www.talkingelectronics.com/te_interactive_index.html would be the place to find a circuit that would work.

    Don't be discouraged.
     
  13. stspringer

    stspringer

    89
    5
    May 10, 2019
    Thanks to all
     
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