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Topic: Trying to trigger an iPhone's touch screen electronically. (Read 2 times) previous topic - next topic

mikeriver

I have an Arduino triggering a relay switch that goes from ground to float every second. The switch is connected to the iPhone's screen by taping a penny to it, and it's supposed to trigger a 'touch' event in the screen every time goes to ground. I got the idea from this blog: http://blog.theultimatelabs.com/2012/06/two-way-serial-interface-using-touch.html and you can see a similar circuit working in an iPhone too here: http://www.youtube.com/watch?v=z4eTjH4zjwE.

My circuit works... some times. with this array, it only works when I put my hand behind the phone (sometimes not even touching it). See https://vine.co/v/bIeWP0pdvuP for this case.

If I connect the iPhone's USB metal shield to ground and remove my hand from the back of the phone it works intermittently, but less predictably.

Here's my circuit:



I'd appreciate any advice, from an error in my circuit to any addition to it that may make it work more reliably.

Thanks,

Miguel

Grumpy_Mike

Are you sure you have those relay contacts right. They are labled correctly but drawn wrong. Have you checked for continuity?

At best it is a very iffey technique. Touch screens like the iPods work on capacitave action so basically you need a bigger ground. Maybe a cradle holding it connected to your ground side of the relay.

laadams85

Why not just use a stylus connected to an actuator?

Grumpy_Mike


Why not just use a stylus connected to an actuator?

Cos iPhons don't work with a stylus.


Retroplayer

A stylus still requires your fingers to be contact with it. It is just conductive. So, the problem the poster is having with the penny would still remain.

Penny = $.01, Stylus = >$1

Of course, tin foil would be even cheaper still.

laadams85

I guess it depends on what side of the penny is in contact with the screen.  Is it the face of the penny or the edge?  Neither of these seem ideal.  If its the face side then you don't really have a good even contact area.  If it's the edge of the penny then your contact area is really small.

You really want something that is about the size and shape of a finger.  A stylus is good for that, and you could make a DIY version using a pencil and tin foil around the eraser end.  Then you would just ground the stylus.  Note that your not grounding the stylus to your electrical ground.  The stylus and electronics should be electrically separated. 

Grumpy_Mike

I think you are missing the point. You hold the stylus, it conducts the body capacatiance of you into the screen. With this coin tapper there is no YOU in the circuit.

AWOL

You can use all sorts of materials; we've used black conductive foam ( the sort of stuff some ICs are shipped pressed into) to avoid scratches, but some styli are tipped with conductive gasket material.
Just make sure it is earthed.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

laadams85

I get the point.  I'm just pointing out that you want to mimic the same change in capacitance that a finger does to the screen.  The shape of a stylus mimics the shape of a finger and connecting it to a ground plane, or as AWOL called it being earthed, mimics the conduction through the human body to ground.

Grumpy_Mike

Quote
mimics the conduction through the human body to ground.

No only to the ground defined by the device.
That is why I said:-
Quote
Maybe a cradle holding it connected to your ground side of the relay.

To make it clear I was suggesting a cradle holding the device and this is electrically connected to the common of the relay contact.

laadams85

I am envisioning having the stylus ground completely separated from the electronics ground.  It could even be connected to the iphone case, or even better a large metalic cradle that is holding the iphone.  It's a completely different concept than what he has here, but might be much easier to implement and get working.

laadams85

Just as a test I sandwiched a stylus inside a book, to give it weight and rigidity, and leaned the stylus onto my tablet.  The tablet had no problem registering a touch event.

jrobertfox

Have you considered using a variable capacitor? How have your solid state experimentations gone?

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