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How to activate capacitive touchscreens without bio-electricity?

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peter:
Hey does anybody have suggestions on activating capacitive touchscreens without bio-electricity? i would like to have a 'touch node' sitting on the touchscreen and only activate the screen when a circuit is completed. Any ideas what the circuit/material would look like?

Peter

amspire:
I don't think touch screens are anything to do with bio-electricity. I don't think those mobile phones can read our minds yet. :)

The capacitive screens just work on capacitance, and for that, you basically need a body with a significant self capacitance which is dependant on the size of the conductive body - like a human body, a car body, or a wire to ground (mains earth).

I would imagine that a lightweight electrode on the screen connected to a lead to ground via a switch or relay near the electrode would probably work. Give the electrode a diameter of, say, 5 -10mm.

If you have access to the touch screen circuitry, use a switched wire from the 0 volts.

If it is something completely encased - like a mobile phone or tablet, you can use a flat conductive sheet in contact with the back of the unit as the ground.

Worth a try.

Richard

peter:
Thanks for the quick response Richard! I think most capacitive touchscreen styluses transfer the capacitance of the body-- what I meant by bio-electricity (maybe it's bio-capacitance?).

Is an electrode just a metal surface with current flowing through it? like this: https://skitch.com/shacklebolt/f6ek3/circuit

If I have a 5V battery, would I need a resistor?

amspire:
I had just added to my previous reply before I saw your response.

You don't need any battery.

Just try different size electrodes until you find the size that works the best. The closer the relay (if that is what you want to use)  is to the electrode, the better it will probably work.  It is probably possible to use electronic switches, but a small relay is probably the easiest thing to use.

Richard

amspire:
Just to explain a bit more, the capacitive touch screens have a grid with AC voltages applied under the top plastic layer.

When you touch the screen, you are conducting away a tiny current at the touch point. The screen is detecting that. It is not detecting any voltages coming from you.

So all you have to do is to provide an alternate way for the AC volts to flow from the screen.

Aluminium Foil would be excellent for making electrodes for initial experiments. You just have to make sure that when you are doing tests, you are not close to the screen or foil. Your fingers have to be a few centimeters away at least.

Richard.

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