How do active stylii work on touchscreen displays?

Question

I recently bought an Acer laptop. Then I tried to buy an Acer stylus. The salesperson at the online store told me that one model would work. It turns out he was wrong, and that my laptop has no active stylus that works currently.

First question: if the dumb styluses (stylii?) work by touching the screen like your finger would, what does an active stylus do?

Second, is there something hardware specific about them, or is it possible to write a driver to interact with it (assuming one could get any data on it). I ask because it turns out that machines that support active stylii are actually pretty rare.

Here is the url for the actual stylus in question: http://us-store.acer.com/aspire-active-stylus-r13-switch-11-12

The stylus has buttons and supposedly is pressure sensitive, which I really wanted. I can see that the tip moves slightly, which made me think perhaps it is some kind of strain gauge to do the pressure sensing. What kills me is that it does not even work as a simple capacitative touch device. Tom's hardware claims that bluetooth devices will function as capacitative stylii, and transmit the force and button presses.http://www.tomshardware.com/answers/id-1707448/active-passive-stylus-pen.html

The original Acer laptop: http://techforpennies.com/acer-aspire-r14-r3-471t-53la-review/

Now I am using a much higher end Lenovo Y50: http://www.costco.com/Lenovo-Y50-4K-Ultra-HD-Laptop-%7c-Intel-Core-i7-%7c-2GB-Graphics-%7c-Backlit-Keyboard.product.100158117.html

If it's a matter of modifying a linux wireless mouse driver, I'd be willing to work on it, but first, how do these things work?

Answer 1

You can't just write a driver to add active stylus to any touch laptop.

How the active stylus works depends on the touchscreen manufacturer. But in a typical capacitive touchscreen that supports an active pen the stylus transmits a signal from the tip that is detected by the touchscreen controller. The signal is encoded with pressure and button information.

There are some bluetooth active pens that "work" with apple products, but they're flaky. Those devices listen for the touch signals, then drive an inverse signal through the pen tip to make it look like a regular touch. In this way, there's no way for the computer to know which touch is from the pen and which is from a finger.