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I saw this video on Vimeo http://vimeo.com/23507315 and am amazed. Does anyone know how this works? Dies it send a pulse or switch ground? What is the wheel he is turning? What is that big gray thing at the top of the screen? Why dies he seem to have to hold the wire at the bottom of the screen?

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Its actually a rather simple circuit, there are 4 things going on at once.

There is a pc speaker (silver blob) connected to an arduino out pin and ground, probably driven PWM from the arduino

There is a potentiometer, connected across the 5v and Ground, with the center tap going to an analog in pin used as a simple voltage divider, he is probably using the value of the analog in to drive the pulse frequency. The "wheel" is just a weird looking knob on what is otherwise bog-standard potentiometer

There is a LED connected across one of the output pins and ground with a resistor in between to set the current

The next part is just my guess, but I think its right.

The part that is hanging (little black square) is probably an optocoupler, the two pins on the left are just connected to an arduino output pin and ground, with a resistor for current limiting for the internal LED's

On the ipad side we have an electrode (a piece of copper foil) taped to the touch screen, the wire goes to the optocoupler (which is basically an isolating switch controlled by the current from the arduino), and out to his hand. The touch screen senses the capacitance of his hand and registers it like a touch just as if he had touch the screen. When he lets go of the wire it is simply grounded to the arduino ground, and the touch screen probably does not register the small stray capacitance of all the wires as a touch, by touching the wire he increases the stray capacitance and the touch screen registers it as a touch.

The optocoupler is basically a switch, that is (most likely) controlled by the same pulsing code as the speaker and the LED.

Here is my (quick and dirty) markup of the video

enter image description here

And a rough schematic for what I'm trying to describe (the numbers on the Digital outs do not correspond to the pins on the arduino as he has them wired up)

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks for the indepth answer - How does the pulse toggle the electrode on/off? \$\endgroup\$ – j03m Aug 4 '13 at 17:36
  • \$\begingroup\$ I'm guessing the pulse to the octocouple completes the circuit to ground creating a touch, when the circuit isn't complete to ground there isn't enough capacitance for a touch? \$\endgroup\$ – j03m Aug 4 '13 at 18:09
  • \$\begingroup\$ The optocoupler or optoisolator is just a switch. Regardless if he's actually using an isolator or something else, it doesn't appear that he is using his own body capactiance to drive the oscillation (which would be more clever, IMO) or some kind of virtual capacitance to trigger the touch screen. Everything seems to be driven from the MCU insoftware \$\endgroup\$ – crasic Aug 4 '13 at 21:07

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