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Touch switch help!

Discussion in 'General Electronics Discussion' started by hcccs, Mar 4, 2013.

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  1. hcccs

    hcccs

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    Mar 4, 2013
    I'm trying to make a simple touch switch work but it doesn't seem to want to. I have taken it from the numerous examples on the internet and stripped it down to the bare bones just to make it work before I move on and add stuff to it. Here it is:

    [​IMG]

    The switch is not a switch but a pair of pads and when I put my finger on it the LED should light up but, of course, it doesn't. Q1 is a Darlington transistor. What is wrong with it?
     
  2. CocaCola

    CocaCola

    3,635
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    Apr 7, 2012
    You can try lowering the value of R1, move the contact closer together or wetting your fingers but this type of switch is really not reliable, it's way too much of a balancing act (dependent upon your fingers resistance) to work reliably...
     
    Last edited: Mar 4, 2013
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Try connecting the two touch pads together with a piece of wire. If the LED still doesn't light, there's something wrong with your circuit. Upload a photo of your construction. We may be able to see what's wrong with it.
     
  4. hcccs

    hcccs

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    Mar 4, 2013
    I forgot to mention that it works if I press the leads on my wetted finger but when the finger gets dry after a few seconds it stops working. The circuit is not working the way I want so I'll try another one. Any comment on this one?

    [​IMG]
     
  5. BobK

    BobK

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    Jan 5, 2010
    Did you try lowering the resistor, to say 1K?

    Bob
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    The circuit with the JFET input looks good. Post the component values though, to be sure.

    If your original circuit wasn't sensitive enough, you could use a Darlington with higher gain, such as the MPSA14. The BD675A is a 4A type. An MPSA14 has at least ten times as much gain, and is still fine for driving an LED.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Google for "touch switch schematic" and look at the images.

    There is one using a Sziklai pair with the load in the collector of the pass transistor that looks pretty simple to build (and which will have additional gain over a Darlington pair, although you could do the same thing in a Darlington arrangement by taking the first transistor's collector back to the supply and adding a base resistor).
     
  8. hcccs

    hcccs

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    Mar 4, 2013
    Components:

    R1 10 Meg 1/4W Resistor
    R2 47K 1/4W Resistor
    R3 120k 1/4W Resistor
    R4 470 Ohm 1/4W Resistor
    C1 15uF Electrolytic Capacitor
    D1 1N4007 Silicon Rectifier Diode
    Q1 2N5458 N Channel Field Effect Transistor
    Q2 2N2222 NPN Transistor (2N3904)
    Q3 2N3906 PNP Transistor
    K1 Relay w/12V Coil, Contacts To Suit Application
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Looks good, hcccs
     
  10. hcccs

    hcccs

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    Mar 4, 2013
    Touch switch again

    I stripped down the circuit for the purpose of testing its function and this is what it looks like:

    [​IMG]

    The PAD1 is in fact a touch pad made from a piece of laminate, 1x1 cm. When I tap it with my finger the LED flickers very faintly and if I put my finger on it firmly nothing happens at all. Some component values are probably wrong but which ones?
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Those component values look OK.
    The circuit relies on pickup of mains hum and other noise. When the pad is not touched, the 10M resistor keeps Q1 biased ON, which keeps Q2 turned OFF. When you touch the pad, Q1 doesn't turn completely OFF; what happens is that Q1 turns OFF briefly each time Q1's gate goes negative because of the noise and mains hum coupled into it.
    So if you remove the R-C circuit, the LED will not turn on cleanly when you touch the pad. If you test using the full design, you should see better performance.
    The negative rail of this circuit should be connected to actual ground. Alternatively, you can increase its sensitivity by connecting its common rail to a safe AC voltage that is returned to ground (e.g. the secondary of a mains transformer).
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You started this thread with a touch switch that works by finger resistance, using two pads. This design works by noise pick-up. There are other methods.

    Have you done a thorough Google search? Try keywords touch switch methods techniques design.

    Your original design using the Darlington transistor should work OK for a simple touch sensor with two pads. You may just need a transistor with higher gain. That's why I suggested the MPSA14. It's rated to carry up to 500 mA, which is plenty of current to drive an LED and/or a relay coil. The one you originally specified is a 4A device; the process used to manufacture these higher-current transistors yields a lower current gain. Higher current gains can be achieved with smaller transistors such as the MPSA14.

    You can also make a Darlington from two discrete transistors. Two 2N3904s or BC547Bs will make a good Darlington.
     
  13. hcccs

    hcccs

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    Mar 4, 2013
    Touch switch update

    The problem with the touch switch is now solved. This is the circuit which I have managed to get working!

    [​IMG]
     
  14. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Put a flyback diode on that relay...
     

    Attached Files:

  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK, good.

    That circuit should have a diode across (in parallel with) the relay coil. A 1N4001 or similar will be fine. Connect the anode to the grounded end, and the cathode to the top end of the coil (= the cathode of D1). This diode is needed to suppress the "back EMF" or "inductive kickback" from the relay coil so it doesn't damage the 555.

    If you find that the circuit isn't sensitive enough, i.e. if it doesn't always detect your finger, you can replace Q1 with a Darlington transistor such as MPSA14 which has a much higher current gain. The sensitivity is also affected by the value of R1; increase the value to increase the sensitivity.
     
  16. davenn

    davenn Moderator

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    I'm suprised you got the relay to operate with a LED in series with it ???

    not really the way to go

    Dave
     
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