Touch Alarm Problems

Discussion in 'The Projects Forum' started by kazz, Jan 10, 2014.

  1. kazz

    Thread Starter New Member

    Dec 27, 2013
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    [​IMG]

    Can anyone suggest or help me how to convert the circuit with one touch plate only, Using the same components? I'm still new to electronics, and i just understand a little bit on this circuits and functions of each components. BTW im using 3-9 V, is it OK ryt?

    Thanks for Suggestion/Help.
     
    Last edited: Jan 10, 2014
  2. Alec_t

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    I don't see any way to use a single touch plate if you can use only those components.
    Any simple circuit using a single touch plate is likely to be triggered by interference.
     
  3. #12

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    Nov 30, 2010
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    I've used a jfet to pick up finger hum. It would suffer from not being universally dependable as a retail product, but it might work for you. Google fet touch switch.
     
  4. kazz

    Thread Starter New Member

    Dec 27, 2013
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    you can add any components as long it have a atleast 3 transistors,....and the easily trigerred, im trying to fix it by changing the values of the resistors near in the plates. Am i doing it right?
     
  5. kazz

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    Dec 27, 2013
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    your circuit was good, thanks for it, i will try this after my project,.... .but my project neede d atleast 3 transistors
     
  6. MrChips

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  7. Alec_t

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    Since this is appears to be a Homework project, we won't give you a complete solution; but consider how you would modify your circuit around T1,T2 to incorporate #12's suggestion of using a jfet (e.g. a 2N3819).
     
  8. kazz

    Thread Starter New Member

    Dec 27, 2013
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    if im going to change T1 and T2 experimentally, it cost lots of expenses which might damage other components...and i cant try this to simulation since it needs to touch to work. What if i change the value of resistors? but i dont know which of the resistors.
     
  9. kazz

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    Dec 27, 2013
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    thanks to this, but im new to electronics,...i still need to study more about on it... u have any simple/basic suggestion?
     
  10. MrChips

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    Yes, the simplest circuit begins with an N-FET or MOSFET as the first stage as #12 suggested.

    Bias the FET correctly and you can pick up 50/60Hz line frequency interference whenever you touch the gate.
     
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  11. kazz

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    Dec 27, 2013
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    i still need to study the J-FET and MOSFET, i still dont understand its analogy
     
  12. MrChips

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    Actually, a FET is easier to understand than a BJT.

    A BJT is a current device. The base-emitter current controls the collector-emitter current.

    A FET is a voltage device. The gate-source voltage controls the conduction channel and hence the resistance of the channel. The gate input impedance is very high and hence is more sensitive for applications such as touch sensors.
     
    Last edited: Jan 11, 2014
  13. Alec_t

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    You can simulate touch, by applying a voltage (50/60Hz hum signal) via a high resistance (representing a finger).
     
  14. kazz

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    Dec 27, 2013
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    what simulation can i try? with a complete components, im just using circuit wizard but lack of components like the transistors,....any good simulators that have all components in the given diagram?
     
    Last edited: Jan 16, 2014
  15. radiohead

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    May 28, 2009
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    Here's a circuit that uses only one touch pad. Maybe you can find a way to incorporate this with your 3 transistors that you must use.
     
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  16. Alec_t

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    I use LTspice; a free download from Linear Technology, with extra libraries of models freely available from the Yahoo LTspice User Group.
     
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  17. kazz

    Thread Starter New Member

    Dec 27, 2013
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    @Alec_t, i have a question, why does the a 0.3W speaker is not producing a sound when i supply a 9volts source,....it just vibrating, but when i supply a 3V source it make sounds,.. i dont see whats the logic of the speaker,....And about the other component i try to simulate it and change it values, it seems nothing change, i cant point out whats the real purpose of each of the resistors and capacitors

    thanks for the reply
     
  18. kazz

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    Dec 27, 2013
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    how can i import a transistor,...2n6107 pnp transistor model is not in the LTSpice.,
     
  19. #12

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    Try a 2N4403 or a 2N3906
     
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  20. Alec_t

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    Whether or not the circuit works seems to depend critically on component values selected, particularly on the impedance and voltage of the battery powering it.
     
    Last edited: Feb 3, 2014
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