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Author Topic: Continuity via capacitive coupling.  (Read 4662 times)

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alb1985

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Continuity via capacitive coupling.
« on: January 21, 2013, 20:19:18 PM »
I thought your article "the Capacitive touch switch" was excellent.
I am doing a similiar Circuit design for work (PCB).
My project is basically to enable something whenever a blade touches a conductor via capacitive coupling.  Does anyone know some kind of frequency comparator circuit that I can experiment with other than a microcontroller?
Is there a way that I can do this circuit so that the duty cycle changes when there is capacitance coupling.

Can anyone help?

kam

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Re: Continuity via capacitive coupling.
« Reply #1 on: January 21, 2013, 20:25:34 PM »
The simplest way i managed to make this without a microcontroller was with the 555. It was not very reliable for small surfaces, but if you talk about metallic surfaces then it may work for you. The idea is to have a frequency of around 100K or more, using a small capacitor for the 555 timer, a few pF, not more than 50 pF. At the output of the 555 you wanna add a low pass filter. Select the resistor and the capacitor so that when the frequency changes (from a touch) the voltage will change as well. You may also wanna use a frequency to voltage chip along with the 555 and make it more reliable.
The touch sensor will be connected across the small capacitor.

alb1985

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Re: Continuity via capacitive coupling.
« Reply #2 on: January 21, 2013, 20:28:41 PM »
I am using a 74HC14 configured as a Schmitt Trigger Oscillator would that be ok?

alb1985

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Re: Continuity via capacitive coupling.
« Reply #3 on: January 21, 2013, 20:30:59 PM »
"Select the resistor and the capacitor so that when the frequency changes (from a touch) the voltage will change as well."

That is what I have been having issues with.  How do I do that?

kam

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Re: Continuity via capacitive coupling.
« Reply #4 on: January 21, 2013, 20:32:16 PM »
sure. do you have an oscilloscope? Wire the chip to oscillate at a high frequency, but make sure that the RC network does not add more than 50pF capacitance. Make sure also that you isolate the touch pad with plastic tape (this will ensure reliable operation). Check how much the frequency changes when you touch the pad. Make sure that the pad has a grounding layer to perform a capacitor! And keep in mind that larger touch pads require smaller capacitors. A very large touchpad require no capacitor at all.

kam

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Re: Continuity via capacitive coupling.
« Reply #5 on: January 21, 2013, 20:33:35 PM »
"Select the resistor and the capacitor so that when the frequency changes (from a touch) the voltage will change as well."

That is what I have been having issues with.  How do I do that?

Trial and error. You are prototyping something original - There is no rule of the thumb in capacitance sensors, since each one is unique. You need a batch of resistors and capacitors. I can only tell you that you probably won't need a capacitor larger than 50pF...

alb1985

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Re: Continuity via capacitive coupling.
« Reply #6 on: January 21, 2013, 20:44:54 PM »
This is a broad Circuit That I had designed.  But where you see the dottet LPF there is not voltage change.  The square wave is just more dc (analog).

kam

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Re: Continuity via capacitive coupling.
« Reply #7 on: January 21, 2013, 20:48:55 PM »
between your oscillator and the LPF you should add a buffer, a transistor or fet or op-amp... something to decouple the oscillator from the lpf.

alb1985

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Re: Continuity via capacitive coupling.
« Reply #8 on: January 21, 2013, 20:50:49 PM »
I will just use the next gate of the 74hc14.

kam

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Re: Continuity via capacitive coupling.
« Reply #9 on: January 21, 2013, 20:56:43 PM »
let us know the results  ???

alb1985

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Re: Continuity via capacitive coupling.
« Reply #10 on: January 21, 2013, 21:02:41 PM »
I am using 150Khz operating frequency.  I think that should be OK.

I will let you know the results.
thanks

alb1985

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Re: Continuity via capacitive coupling.
« Reply #11 on: January 21, 2013, 21:57:08 PM »
That extra gate that I used as buffer made a difference.
I had to put a diode in parallel with resistor why is that?

kam

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Re: Continuity via capacitive coupling.
« Reply #12 on: January 21, 2013, 22:02:16 PM »
which resistor and how is the diode facing? and how did you decided to place this diode  ::)

alb1985

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Re: Continuity via capacitive coupling.
« Reply #13 on: January 21, 2013, 22:05:35 PM »
The LPF resistor.  the Cathode is facing the Oscillator. 

alb1985

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Re: Continuity via capacitive coupling.
« Reply #14 on: January 21, 2013, 22:08:35 PM »
There is also something weird going on with the LM358 comparator.  I am using TL431 as the reference of 2.50 v connected to V- of the comparator.  when I create cpacitance to change frequency the voltage increases at the v+ past this 5 volts but my led is not coming on.  I wonder if I have to buffer the low pass filter before V+ input.