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I would like to generate computer controlled touches on a capacitive touch screen (e.g. iphone) using the following setup: I have a conductive patch (size of finger) that I place on the touchscreen which is connected to a wire. When I touch the other end of the wire, it generates a touch at the location of the capacitive patch. However, also when I do make contact with the wire, it also generates a touch. I think this is because the wire (25cm) acts as an antenna and the capacitive signal is influenced by noise. I do not encounter this problem when the wire is shorter (e.g. 5cm).

My idea now is to place an electronic component close to the patch, which I can control digitally to connect/disconnect the wire (switch). So when the "switch" is open the wire is very short and will not generate a touch. When the "switch" is closed a touch will be generated because it is connected to the longer wire which generates a touch because of noise in the environment or/and the user is touching the end of the long wire. From my basic knowledge of electronics and transistors, I though that a transistor might be suitable as this can be as a switch that is controlled digitally. However, I did not come that far as I encountered the following problem using a PNP BC327:

I directly soldered the collector to the conductive patch that makes contact with the screen. When touching the screen with this setup (leaving the base and emitter disconnected) does not generate any touches. However, when connecting a loose wire to the base or emitter, a touch is generated without even controlling the transistor. I do not measure any conductivity between the Collector, Base, or Emitter using a multi-meter so I am very surprised that connecting these wires causes a change in capacitance on the collector.

Any help to beter understand this problem would be useful as well as input on other techniques on how to make the desired system/setup.

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    \$\begingroup\$ So far, nobody whom I am aware of has come up with a reliable method of digitally controlling a capacitive touch array (like your iPhone screen) without using a mechanical means, or tapping directly into the circuitry of the array. However, if you think you're up to the task; there would be a "less mechanical" possibility if you digitally controlled the magnetic field applied to a capsule of ferrofluid near the array. In theory, if you use a non-polar suspension, then relaxing the field & allowing the suspended metal particles nearer the array... \$\endgroup\$ – Robherc KV5ROB Apr 19 '17 at 22:05
  • \$\begingroup\$ should effectively simulate "a touch", and strengthening the field to pull the suspended metal away from the screen should stop the sensing. ... Aside from that, I don't know that anyone has come up with a plausible switching mechanism for what you're wanting to accomplish. \$\endgroup\$ – Robherc KV5ROB Apr 19 '17 at 22:06
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You wouldn't measure a current going through the glass in your capacitive touch screen, either – but it would still flow!

Measuring the capacity at points on the screen usually works by applying a high-frequency signal to that point and seeing how much energy gets absorbed/stored in a (hypothetical) object at the other side of the screen. Basically, think of your finger detuning an LC-oscillator when touching the capacitor.

Now, the PN-junction in your bipolar transistor actually might be a good isolator for DC, but that doesn't say much about the behaviour at higher frequencies. Essentially, you'll have to assume that though the charges are separated in the "non-conducting" case, they're still pretty close – and thus form a capacitor!

You might counteract that with a larger transistor, but I doubt that would work out great.

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