Questions on a circuit with CD4013

Discussion in 'General Electronics Chat' started by hazim, May 8, 2010.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    13
    Hello,

    This is a "touch switch" circuit which I have some questions about..
    First, which resistor(s) and capacitor(s) are controlling the frequency at the output of the first flip-flop? R1 & C2?:confused:

    I tried this circuit and it's working well, the second transistor connects just when I touch the "touch button". I'm trying to make some changes in it in order to make it switches on when I touch the first time, and switches off when I touch it a second time. It's easy to do that using a third D flip-flop, but this requires using two ICs. I'm thinking about a way to do it by editing the circuit around the second part of the IC/the second flip-flop if possible.

    [​IMG]

    the truth table:
    [​IMG]
     
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  2. Wendy

    Moderator

    Mar 24, 2008
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    I've seen simpler designs, That one is over complicated by quite a bit. Should be one FET and one flip flip (and a driver after).
     
  3. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    I want the contact to be one a single wire/plate, not two as many designs with FETs are ( they depend on electrical conduction through the finger).
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I didn't say it was two. One FET can make a switch with one plate. The principle is used for doorknob alarms and much more. Can you say body capacitance and AC hum?

    The design you have is way overcomplicated.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    Bill is absolutely right. If you have ever broke your mom's brand new touch dimming lamp, you would know that. ;)

    Forrest M Mimms has a 555 touch circuit on Page 12 of Volume 1 of The Engineer's mini-Notebook. Ive built it. works. Pin 2 is the single pin switch. You can run a lead from pin 2 to a metal plate or chassis (or lamp body) and get a pulse per touch.

    A 555, 1 resistor, and 2 caps. done.
     
  6. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    I think you are talking about this circuit:
    [​IMG]

    I've built it... It works fine, but sometimes it doesn't..I think the body resistance affects here, when I'm barefoot or not maybe...

    I built it first, but I don't want a pulsed output, but I want the output as I said above: touching the first time switches on, and touching the second time switches off. This can be done using a D type flip-flop... We have talked about that in another thread few days ago and found out that it can be done simply using CD4013.

    Regards
    Hazim
     
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    That circuit works by body capacitance changing the time delay to the D pin of the second 4013 section, relative to the clock transition that latches the data level.

    As it's repeating the set or reset operation on every clock pulse (at around 20Hz), you can not make it in to a toggle action without an extra IC.

    If you are trying to keep the size down, you could replace the oscillator - any type that gives a similar logic level pulse should work. That would leave the other half of the 4013 for the toggle action.

    That circuit looks fine in principal to me - I don't like the type that rely on hum pickup, you cannot guarantee them to work under all conditions.
    You can guarantee a capacitance change.


    Oh yeah, the question.. It looks like R1 & C2 control the delay between pulses, with R2 & C2 setting the actual pulse duration.
     
    hazim likes this.
  8. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
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    Thanks rjenkins, actually I was thinking the same in all what you said, and I agree 100% with you. I can see that capacitance type touch switch is better than resistance/dc based ones. Also I've thought about using a 555 timer as an oscillator and use the CD4013... Thank you again.
     
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