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In a video at Kickstarter a project called Tapcaps is mimicing human capacitance for triggering touches to a capacitive touch screen (the iPhone). At 0:45 in the video we can see a glimpse of the device. It kind of resembles an RFID tag.

enter image description here

Can anyone explain how this device works?

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Okay, it seems to be simpler than I originally thought (I should not have read the marketing first) I now realise most touch screens use mutual capacitance between a grid of electrodes (I was thinking of the case of a single touch pad initially)
I think it's just a piece of foil. With mutual capacitance any conductive object should disturb the field, however given the algorithms implemented to avoid false presses, etc, using just anything doesn't work You can try with a small piece of Al foil wrapped around the finger tip of a glove, I just tested this and it works fine.

For a "human free" version, a conductive object with a surface about the same size as a finger works best (I tried various things, anything the wrong size will not work or just give the odd random touch, but nothing consistent)

Here is a photo of my technical setup:

Cap Touch

There are two main types of capacitive sensors, "self capacitance" sensors and "mutual capacitance" sensors.
The self capacitance sensor is a single electrode (conductive plate) and the finger forms the other "plate" to create a capacitor. The capacitive load is measured with reference to circuit ground.

With a mutual capacitance sensor, there are two electrodes and the finger (or other conductive object) disturbs the electrostatic field between them causing the capacitance to change. So in this scenario circuit ground is not used.

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  • \$\begingroup\$ Thank you, excellent answer! Why is the pad on the non-contacting lead necessary? \$\endgroup\$ – runeb Sep 13 '12 at 6:59
  • \$\begingroup\$ The second pad probably isn't necessary, I just made it so I could experiment a bit with placement of the second pad to see where the coupling was most effective. Also they were initially different sizes to see which was most effective for touching. \$\endgroup\$ – Oli Glaser Sep 13 '12 at 10:23
  • \$\begingroup\$ After experimenting with this I cannot replicate your results if I isolate myself properly. Perhaps your pliers was enough to trigger touches? When using a piece of conductive foam, and a wire going into it, it will trigger touches if the wire is touching me, or even the table. If the wire is soldered to an 100uF electrolytic and isolated properly from me, the touches will not trigger. \$\endgroup\$ – runeb Sep 20 '12 at 12:38
  • \$\begingroup\$ Okay, I may have made an error here, thanks for pulling this up - I forgot my pliers have metal that goes down inside the plastic handles which could provide capacitance from the hand still (even through a glove I was also using) I think holding the capacitor case with the pliers was enough to detect the touch (using a totally plastic clip does not work) I have another idea, just trying now and will update (or delete :-)) \$\endgroup\$ – Oli Glaser Sep 20 '12 at 12:59
  • \$\begingroup\$ Sorry for the confusion, I was assuming a different type of system before. Doing a bit of research shows that all that is needed is a conductive element to alter the capacitance between two electrodes, rather than a single electrode and added capacitance to somewhere like circuit ground. Have a go and see if you can replicate the above tests, I will try and add a bit on the theory. \$\endgroup\$ – Oli Glaser Sep 20 '12 at 13:55

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