# How to use a capacitive touch screen without a human hand

I'm a programmer, and I usually hang out over on stack overflow, but I have a electricity question.

I'm trying to interact with an APP on my phone by indirectly touching the the screen. Modern touchscreens, as I'm sure everyone here knows are capacitive rather than resistive, so that means that I should be able to interact with it as long as I can bring a small current to the screen where I want to touch.

I'm experimenting with using some speaker cable and a couple other wires I have laying around the house to see how far away I can be and still pass enough current through the wire to register a touch on the screen, based on my distance and the size and quality of the wire.

What I'm finding though is that touching wire to the screen seems to register a touch even without me touching the wire. So is there already a current inside wire? Is it the type of wire I am using? I was under the impression that a wire on its own had no current and only passes electricity when there is an outside source.

Is there anything I can do to decharge the wire, or block it somehow?

Thanks for everyone for letting me know how exactly capacitive touchscreens work. I had it wrong in my head.

The problem that I'm trying to solve is that I want to interact with my phone through its touchscreen from a distance. Is there any material I can use to attach to my phones screen that will essentially extend the touchscreen to where I am? I will only be a few feet out of reach, and it doesn't need to be fancy, just basically detect a touch event.

I realize it might be a strange question, so if I don't get any answers to this I will still choose an answer below, I've learned a lot from this question already.

• I personally liked this document to understand a bit more about capacitive touch technology: ti.com/lit/an/slaa363a/slaa363a.pdf – jippie Mar 9 '13 at 19:54
• Thanks. That helps a lot. If I understand it correctly, the device has an electric field around the various sensor pads, and it detect interference. The interference comes when a object moves into the field, and takes some of the charge because its conductive. The device recognized a lower amount of electricity so it knows something is touching it. Is that sort of correct? – Jeff Ryan Mar 9 '13 at 20:38
• No, not really, that is more like how a theremin works. Maybe see youtube.com/watch?v=JVRuDY4X88M&sns=em – Grady Player Mar 9 '13 at 21:44
• Thanks everyone for your help. Every answer was super helpful to me. I went back an forth with Olin the most, so I choosing his answer, but thanks to everyone else as well. – Jeff Ryan Mar 11 '13 at 16:43
• Interacting with computing machinery from a distance is solved by image processing of real-time video to read the position of the human body and limbs. You can control the position of a cursor on the screen with gestures and generate events like mouse clicks. See Microsoft's Kinect for instance. – Kaz Mar 20 '13 at 9:03

so that means that I should be able to interact with it as long as I can bring a small current to the screen where I want to touch.

No. You don't "bring" current to the device. These devices measure capacitance, not current or voltage. The additional capacitance thru your body back to ambient or circuit ground is detected by looking at how the capacitive pad responds to some specifically chosen signals.

• Ah, Ok. My ignorance on these matters is clearly showing. So, instead of the current of electricity going into the device, you seem to suggest that the device detects when something touching it can receive a current. I don't know if Im using the terminology correctly, but the idea is that the direction flows from the device to me, not the other way around. Is that correct? – Jeff Ryan Mar 9 '13 at 20:34
• @Jeff: Yeah, sortof. The logical direction is the device is actively looking for the presence of something. The something is purely passive. – Olin Lathrop Mar 9 '13 at 21:21
• Maybe it would make more sense if the op saw the same circuit work as a proximity detector? – Grady Player Mar 9 '13 at 21:37
• @Jeff: No, it means the dime by itself does not have enough capacitance to elsewhere until you touch it. It is plenty conductive, but that doesn't matter much since we're not sensing conductivity. Your body is less conductive than the dime, but much much larger and therefore more likely to have some minimum level of capacitive coupling back to the circuit's ground reference. It is this capacitive coupling back to ground that the circuit is looking for. – Olin Lathrop Mar 9 '13 at 23:42
• Whoever downvoted this, it would be useful to know what you think is incorrect, misleading, or badly written. – Olin Lathrop Aug 24 '16 at 1:16

I generally resist analogies, but since you're a software-only guy, and since everyone else insists on talking to you about capacitance, I'm going to go for the story.

Imagine you are firing paint balls at a wall. Everywhere you hit the wall, there's a splat of paint. Except there's one place you fire at, and when the paint hits it, it makes a little mark but otherwise the paint just kind of disappears. When you investigate, you find someone has attached a suction device to the other side of the wall, and since the wallboard is paper, it's able to draw the paint right through it.

The mechanism in the phone is doing a similar thing. It drives a bunch of electrons onto the backside of the glass. Normally this can then be observed as a rise in voltage. But at the spot your finger is touching, the charge is absorbed by your finger, and the voltage at that location will be significantly reduced. How it works through the glass has to do with the fact that charges attract and repel each other, but you didn't come here to learn electronics, so we'll just be happy with this much explanation.

Perhaps you can see that a dime placed on the screen doesn't draw off enough charge to be registered by the touch circuits, but if that dime is connected to something larger, like yourself, now enough charge is drawn off to be detected. This is rather more like the behavior of static electricity, rather than the classic circuits involving batteries, wires, resistors, and light bulbs.

[Edit, to address, "What can I do..."]

I do not know if something like this can be made to work, but the goal is to be able to simulate a touch by coupling the wire over the screen back into the ground system of the phone. The grounding (done by laying the phone on a metal plate or sheet) is also capacitive and is the equivalent of holding the phone in your hand. The idea is that the transistor switch can disconnect the short lead over the screen, and that there wouldn't be enough capacitive coupling back to the controlling circuit (the computer you were going to try to generate touch events from) to trip it when it was supposed to be off. Switching the transistor on would connect the lead to the ground plate and should then generate a touch event in the phone.

[Edit: An associate says the stray capacitance of the FET would still be too much to let this work. Doesn't cost anything to try, though.]

[Edit: This part of the answer is in response to the comment found below]

It's not about the mass; it's about the surface area. A sheet of foil can hold more charge than a dime will, even though it weighs less. For a piece of wire, it's more about the length. Equally important is whether that wire is attached to anything. If you are holding it in your hand, then the wire plus your body is in the equation. Your body is as good as a piece of metal, in this experiment. And any metal, be it copper, nickel, aluminum, or steel will work. The only requirement is that it conducts electricity.

Incidentally, I just tried a few metal objects on my phone, and I'm surprised you are getting so much response from the wire. A dime does nothing unless I touch it with my finger. If I hold a quarter in my hand and touch the edge to the screen, I get nothing. Two doesn't do it either. Three, stacked sideways provides enough contact to affect the screen. Next, I tried a paperclip, held in my hand. It would not respond if the rounded end was touched to the screen. I had to hold it the long way and touch the side to get a reaction. A metal bar stood on end did not affect the screen, but touching the opposite end with my finger caused it to react.

Voltage is the difference between two potentials. The thing that is measuring the voltage on the screen (and ultimately looking for the effect of your finger) has one connection to that screen, and the other to a reference that will almost certainly be the ground inside the phone. This "ground" is a common connection throughout the phone and probably to its metal parts. The idea is that when you hold the phone in your hand, there is capacitive coupling between your hand and the ground inside the phone. As to various conjectures oabout Earth ground, they may work because everything will ultimately couple capacitively to the phone to some extent. And as for "an ungrounded, capacitive object with sufficient surface area", well that is exactly what is shown in the illustration as a plate underneath the phone.

The charger or USB connection might well provide a ground connection into the phone, but that's not guaranteed. Opening the phone to get at it would work, but that would have limited practicality when it came to any end-user application.

• Hi. You answer is very helpful. If you feel like continuing to explain this phenomena, can you explain why a dime needs a larger ground to be able to draw off enough charge to register a click, but thin speaker wire draws off enough charge on its own? I would imagine total mass of wire is the same. Since dimes are mostly made of nickel, shouldn't I be able to use nickel wire to attach to the phone face and touch with my hand from a distance and essentially get the same effect as touching the dime on the screen? – Jeff Ryan Mar 19 '13 at 6:04
• This is an amazing answer. – j03m Aug 4 '13 at 3:47
• This analogy helped bridge the gap in my layman's understanding. – toolbear Feb 10 '14 at 16:03
• Would you elaborate more on the circuit's "ground system?" You suggest grounding to a plate under the device, but one can ground to "ground" which I interpret as the ground beneath my feet. Also, is ground necessary or does it just help? Would an ungrounded, capacitive object with sufficient surface area also work? – toolbear Feb 10 '14 at 16:14
• Very interesting. Let's say I attach a 10µF capacitor to the coin to increase the capacity and I do ground both sides via the transistor, would that work better? Alternatively could I place the coin onto the touch screen, add a relay between the coin and the capacitor and then trigger the capacity via the relay? I guess it's time for some experiments :-) – Thomas Weller Jan 27 '16 at 15:33

They basically aren't interested in a flow of current, at least not from your actual body, that is why it works through a plastic or glass insulator. It measures the capacitance on several sensors, generally by charging them with a constant current and time, then measuring the resulting voltage.

It registers a capacitance change because your body works to act as half of a capacitor, or more simply by adding capacitance... Because it can work in direct contact, ie not as an ideal capacitor...

In a typical setup it only needs to add a few pF.

• Actually a few nF would be huge capacitance. These things usually measure a few pF, sometimes 100s of fF. You are off by at least three orders of magnitude. – Olin Lathrop Mar 9 '13 at 23:45
• @Olin Lathrop, Perhaps I am... I will have to go back to the ctmu documentation from microchip, that is where I thought I got my figures from. – Grady Player Mar 10 '13 at 3:59
• Yeah the example I found gave a capacitance of 7pF, I was way off. – Grady Player Mar 10 '13 at 4:12

I realize I'm late to this, but perhaps this answer may be useful to future viewers. This will also be off topic because it is more Stack-Overflow or Super User related and less EE related. It also sounds like you care mostly about having an interface to the touch screen and you are not dedicated to your current method.

I'm also going to assume you have and Android device.

Conveniently for you (seeing that you are a programmer), a significantly better interface is using the Android Debug Bridge to simulate touch screen presses, swipes, back and home key presses.

Basically your main goal is to get ADB functioning. There's plenty of resources online so I won't explain it here. You will need to download the Android SDK to get it.

After this, use a command prompt to send ADB commands to your device.

Commands like:

adb shell input tap x y


With 'x' and 'y' as pixel location will press the screen.

• This is a really interesting idea.. You are right, its off topic, but does offer a way to achieve my end goal... Incidentally, I ended up using headphones with volume controls, and then making my application react to the volume up/volume down/play/pause key events.. Gave me a few feet of distance between me and the device, and gave my more options than just a single click... – Jeff Ryan Mar 17 '15 at 19:36
• input tap seems not available with ADB on Android Studio 6. – Thomas Weller Jan 28 '16 at 11:03

Actually, capacitive touch screens do not require to be touched by a body that can source current, rather, they sense when contacted by any conductor (see wikipedia article on capacitive sensing). Since wire is a prototypical conductor, you do not need to touch it to use it as a makeshift stylus.