There are active styli out there, and they take 3A or 4A batteries and don't require any wireless connection. How do those emulate human touch?

EDIT: I am specifically interested in iPad touch screens. After some research, I have realized that there are many types of capacitive sensor tech, but most people seem to be in agreement that Apple uses mutual capacitive projected capacitance sensor arrays. These are set up with a grid of capacitors. A voltage is applied to one axis, and the other axis reports change in voltage when the local electric field of a finger alters mutual capacitance of the capacitors directly underneath it.

EDIT 2: what I really want is something with a fine tip like a ballpoint pen to project about a 5mm electric field the same way that a human finger does. There are styli on the market that try to work with a fine tip, but they all fail and I do not want one. I want to make my own because DIY projects are way more fun than buying an overpriced failure of a product.

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    \$\begingroup\$ "don't require any wireless connection"? what does this mean? Does it mean they have a wired connection? \$\endgroup\$ – Andy aka May 16 '14 at 15:48
  • \$\begingroup\$ As far as I can tell, an active stylus uses the "active" part to interact with a digitiser in the screen. The capacitance interaction is passive. \$\endgroup\$ – Andrew Morton May 16 '14 at 19:16
  • \$\begingroup\$ @Andy, there are more expensive active styli that use Bluetooth and send motion tracking information to the device, so as to help it decide where the tip is, but require devs of apps to put the SDK of that stylus in their app. iOS doesn't natively support anything that's smaller than whatever Apple decides a finger is. \$\endgroup\$ – Raiden Dark May 17 '14 at 19:50

How do those emulate human touch?

Capacitive touch screens don't work from the supposed "aura" that some folk believe emanates from humans - they'll work with dogs, cats, medium sized rodents, pieces of metal and probably wet wood. Maybe they'll also work with high dielectric plastics too.

Capacitance is not given by gods to humans but is firmly routed in the physics of objects and the "styli" or pen has to be held in the hand and therefore a significant capacitive connection to the body is made (whether wireless or otherwise).

I suspect that wireless styli that use batteries may work by sensing the capacitive sensor's AC field and re-injecting some form of antiphase signal - the disruption is likely to be enough to "register" a keypress.

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  • \$\begingroup\$ I'm pretty sure the snark response to op's use of the phrase "human touch" isn't warranted -1 \$\endgroup\$ – Passerby May 16 '14 at 17:38
  • \$\begingroup\$ @Passerby Aw... just a little humour can't be much out of place now and then can it? \$\endgroup\$ – Andy aka May 16 '14 at 17:42
  • \$\begingroup\$ No, but yours falls on the wrong side of snide \$\endgroup\$ – Passerby May 16 '14 at 19:51
  • \$\begingroup\$ @Andy:I said "human touch" for a reason. The ipad touchscreen is looking for something that gives off a similar signal and is similar in size, which is why passive styli are at least 6mm in width. Any smaller, and the sensor array refuses to treat whatever it senses as "input". \$\endgroup\$ – Raiden Dark May 17 '14 at 19:46
  • \$\begingroup\$ @Andy, I believe that is how one of them works, but its competition uses other methods. One uses ultrasound I believe. At least it claims to. Why is an anti-phase signal necessary though? The human body doesn't do that, right? \$\endgroup\$ – Raiden Dark May 17 '14 at 20:14

A capacitive touch screen is one half of a capacitor. To make it work, the equivalent of the remaining capacitor plate is needed. This will be a conductive element that is capable of absorbing charge. Your finger is connected to you and your body is large enough to absorb the small amount of charge to be sensed by the screen.

Interestingly, if you held a battery cell in your hand and touched the button end to the screen, it would sense that, too. It's just a piece of conductive metal, after all.

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  • \$\begingroup\$ I am specifically talking about an iPad screen. Sorry, should have specified. See edit. \$\endgroup\$ – Raiden Dark May 17 '14 at 19:53
  • \$\begingroup\$ I see Wikipedia talks a bit about this technology. But the gist of the answer is the same: it's the effect on the capacitance caused by bringing another conductive surface near the screen. Although I don't have an iPad to try it on, I bet the battery trick still works. \$\endgroup\$ – gbarry May 18 '14 at 2:30
  • \$\begingroup\$ Battery is too fat and I've know about that since high school. See edit. \$\endgroup\$ – Raiden Dark May 18 '14 at 3:46

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