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Author Topic: Increasing sensitivity of a simple resistive touch sensor  (Read 2117 times)
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Maryland, USA
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Hello All

I am experimenting with a simple resistive touch sensor, made up by two thumbs tacks. One is connected to +5V and the other to a digital pin with a resistor in parallel to ground as a pull down. I activate the sensor by shortening the two tacks with my finger.
I used a sketch for a Normally OFF momentary push switch.
It took me a while to figure out that it was not working because I was using the recommend (for a switch) 10K pull down resistor (and the fact I am so proud that I figured it out all by myself gives a new meaning to the word "newbie").
I checked the resistance across the shortened tacks (my finger pad resistance) and got about 1.4M.
I replaced the pull down resistor with 2.2M but it did not work too well.
So I replaced with a 4.7M and now it works.
Problem is that you have to cover the two tacks completely in order for it to work. It is not enough to touch them both (with the same finger).
In other words, it is not sensitive enough.
I tried to increase the resistance of the pull down resistor to 6.9M with two resistors in series (2.2 + 4.7) but it does not seem to change much.

Short of licking my fingers or wearing thimbles (one is gross, the other impractical) is there a simple way to increase sensitivity?

Thanks


« Last Edit: November 13, 2012, 07:22:31 am by Thot » Logged

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Brunsbüttel, SH, F.Rep.GERM
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Hi!

Maybe a darlington config?
http://en.wikipedia.org/wiki/Darlington_transistor

can u give us a drawing of ur circuit?

Have u seen this?
http://www.arduino.cc/playground/Code/CapacitiveSensor

Bye
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-Arne

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Thanks for your reply.

I have attached a sketch (in the original meaning) of what I am doing.

I was going to experiment with the capacitive sensor, but my understanding is that it is too sensitive sometimes activating with proximity instead of touch. But I have not tried that yet, so there may be a way.



* Thumb Tacks.png (94.61 KB, 908x977 - viewed 32 times.)
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Brunsbüttel, SH, F.Rep.GERM
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ok
with a transistor (e. g. a npn bjt like 2N4401 (IIRC)) u should get better results...
in darlington configuration it should b even better...
http://www.pcbheaven.com/circuitpages/Resistance_Touch_Button/

i use a self made touch key keyboard when lying in bed and it works quite reliable...
just when i spill juice on it, it behaves funny...
i covered the electrodes with duct tape...
and i use one capacitance multiplier (or what i believe it is) for each key:
http://en.wikipedia.org/wiki/Capacitance_multiplier
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-Arne

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Quote
with a transistor (e. g. a npn bjt like 2N4401 (IIRC)) u should get better results...
I would tend to disagree, a transistor is a lower impedance and is a current device and so will be harder to drive than a FTE. I have used a CMOS logic gate for this sort of thing, like a 4001.

It is working through injection of a signal through your body, so one way to increase the sensitivity more is to have an oscillator on a wire somewhere close so your body can pick that up and your touch transfer it to the input.
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