Simulating touch on a capacitive multitouch with a micro controller

[victor]

Hello guys,

I'm trying to simulate a capacitive touchscreen programmatically, I tried many ways of doing it, none seems to work properly. I taped down a piece of aluminum foil on the screen and attached a wire to a pin o an arduino board, and I toggled the pin LOW and floating. It kinda works, but is very touchy and hard to get it working.

I'm very intrigued about this, because these multitouch are very very reliable, and it works flawless without never ever missing a single touch, works no matter what conditions, and I'm sure a human finger may have the eletrical characteristics dramatically changed, from person to person, works with your fingers wet or dry, etc

But I can't get it working nearly as good as a human finger, I'm pretty sure it can be done, but how? Any ideas?

[Bored@Work]

You could try to head over to eBay and look for specific capacitive touch screen styluses. There are a multitude of them, e.g. eBay auction: #271211093543,eBay auction: #121110821125. People use them if they e.g. wear gloves. Once you have them you could take a few apart, measure them, etc. to figure out what's the magic.

[Paul Price]

A touch switch should be "very touchy".

I made a touch control for a night lamp next to my bed for easy turn on in the dark. I used a PIC MCU analog input pin and was easily able to detect the presence of a finger by measuring the analog voltage changes (house mains body pickup) using  a software scheme that only allowed a verifiable press if the touch persisted for a 2-second or more interval and then it did not allow a second press until the press changed voltage was no longer detected and a software debounce timer(1-sec) had run out, so only then it was ready for another touch.

I used a 10-meg resistor on the analog input pin and a Schottky diode to VDD and ground to protect the input pin and the analog voltages were all over the place when a finger was applied, yet max and min volts were small enough to detect that there existed a non-touched condition.

[amyk]

First of all your touch contact is way too big - it needs to be around the size of a fingertip.

Secondly, it's sensing touches due to changes in capacitance, and these changes are *very* small - the length of wire between the MCU output and the screen is probably more than enough to cause stray electromagnetic fields to induce voltage and affect it.