How does a laptop touch pad work?

My tongue works on it - just wanted to toss that out there...

I was wondering how it works and I found this site.

I made some simple tests with the touch pad.

Sorry but this theory of the grid of wires can not be right, because when you touch the touch pad with a pen or something else that does not conduct electricity it does not work.

I imagine that this is a electromagnetic device, it should works with induction. Like ... under the touch pad should be positives and negatives charges, when you touch it with your finger, which one is electrical charged, the positives and negatives charges move and then it create a magnetic field that can be easily detected.

I really not sure about it but... I used my imagination. If anyone know the right answer please email me.

The mouse pad is touch sensitive and has coordinates (like a street map that has an index to find a street). It produces events and transmits coordinates for those events. A driver is installed in the operating system that translates events and coordinates into actions in the operating system. It is totally up to the driver as to what actions are performed for events. There are however some generally accepted event / action. For example, contact with the pad at any coordinate for far less than a second followed by no contact with the pad followed again by contact witht he pad at any coordinate for far less than a second is translated to a double click. Touching the pad at a coordinate and moving to another coordinate is considered to be a mouse movement. How far to move the mouse and the direction to move the mouse is up to the driver.

The grid of wires is how stylus touch pads work, like a PDA or and iPod Touch, laptop mouse touchpads usually use heat to detect touch, which is why it doesn't work as well (i.e. skips a bit) when your hands are very cold. The difference in usage is that the heat touchpads cannot easily be made transparent, and are very low resolution, but cheaper to make. The low resolution means that you need to use a larger object to contact them in order to get a good result, such as a finger rather than a stylus. The conductivity pads are much higher resolution, and can be two transparent layers (one flexible film and one the hard glass) that can detect a much smaller and precise contact. You can tell the difference between the two by tapping it gently. If you hear a normal hard surface tap, it's a heat sensitive pad. If you hear a noise almost like two distinct clicks, it's a conductivity sensitive pad.

On the subject of how does a touchpad know when you touched it. There are different methods and materials that can be used. If you want the technical answer then you go to http://www.patentstorm.us/patents/5521336.html for a specific patent. Other patents exist for touchpad technology.

If you don't have an electronics background, then it is probably much more than you really wanted to know. The following is incorrect but it does give you the basic idea of how it works. If you want to know how it really works then you should read the patent for the touchpad you want to know about..

Think of a touchpad as a grid of wires that cross each other but are not touching each other. There is a low conductivity material between the wire intersections so that electricity does not flow between the wires. When you press on it, the material get's thinner between the wires which when thin enough will allow electricity to flow. It knows where you touched by the wires where the electricity is flowing.

I just tried all above options with the same result but the weird one is I used the bottom of a pen and it never worked but then tried the side of a pen and it works fine (I even tried again with a rubber glove on to make sure it wasn't because my hand was warm triggering the movement)

It works based on heat.
I took a paper towel and rolled it up tight and held it under the faucet with hot water.
Then I ringed it out so it is not dripping. I was able to operate the touch pad with the fake rolled up paper towel finger.

I Think Anartech Systems has made the most convincing argument here, if you can get your head round it, I think he is saying that basically your finger interferes with the reception of the electrical receiver/transmitter AKA touch pad to a degree of precision that can be translated into co-ordinates IE where your finger is on the touch pad, and if the item on the pad does not interfere with the reception enough or maybe even too much it will not register. Simple eh?

based on my "research" on a macbook it's not heat, it's conductivity. i tested a cold sterling silver ring, it works perfectly even when not in contact with my body. a 50 euro cent coin works, but a stainless steel ikea fork does not - even if i warm it up and/or hold the fork.

Hey folks,

Most of the above answers are correct in terms of touchpads through history, they have all worked on gird array's of wires, thermal pickups etc throughout the evolution of the devices, but duffmean has come the closest to it so far.

Nowdays, 99.9% of laptops use capacitance pads. This is a system where wires are overlaid across each other, but the change in capacitance when your finger moves over is what triggers the pad. A capacitor consists of 2x main elements, the plates and the dialectric (being some sort of material that interferes with current flow between the plates, electrolyte in an electrolytic capacitor for example, tantilum in a tantilum capacitor etc). Imagine 2 metal plates about an inch apart, with power flowing through them. If you place an object between the plates, the capacitance will change as you are effectivly modifying the distance between the plates by adding mass (those who have ever pulled the guts out of a valve radio will be familiar with the rotary plate tuners they use. basically a series of metal plates that are inserted and removed to adjust capacitance, and therefore tuning, see http://www.eham.net/articles/5217). You dont nessicarily need to go between the plates, you can just go near them, which is how theramins work, and why TV's with indoor antenna's sometimes go off tune when you approach the antenna.

So, with this in mind, we return to the touchpad. The grid of wires are set up as individual capacitors, and a finger moving across them works to change the capacitance at a given intersection of wires. Trying to proove myself wrong, I actually went and got some ice water and chilled my finger to the point where it went blue (I love science!) and tried this, my touchpad still worked. I heated a pencil eraser with a cigarette lighter to approximately human body temp, this failed to trigger the touch pad. Based on all this its fair to say that the touchpads are "tuned" in terms of capacitance to only allow objects of similar density to the human body to trigger them, which is why pens and other objects fail to trigger them. Short of wiping a piece of ham or something over my touchpad (which I would if I had it at hand) thats about as much as I can test, but if someone out there wants to wipe some meat over their laptop to check this out, please let us know! in the meantime thats my theory, and im sticking to it ^_^

there is a historic way that the touch pad works that is it uses a grid of wires with are arranged in the form of a matrix where the spaces are filled with insulating material let this grid be layer 1 now there is another grid layer where the same assumptions are made as in the layer1 the entire gaps in the layer2 are filled with insulators and the two layers are orthogonal to each other and they are also seperated by insulating material.(as known the insulating material does not provide a path between two conducting wires.now the grid of wires are conductors).so now a sheet is placed over it now on it being pressed the wires are pressed ,and they come closer to each other they fianly touch each other thus conducting electricity which can be collected by the digital banks and the postion can be proscesed and displayed


Moost of todays computer are using capacitive sensing touch pad . the capacitive sensing touch pad are divided into two parts ,grnd-sensor-grnd ,so when the finger is brought near the touch pad the capacitance of the system increases and thus the more nearer the more capacitance by catching the varying capacitance and proscesing the signals using counters for the charging and discharging of the capacitors with and with out finger the position can be displayed.

actually the conducting touchpad can be operated by pen ,finger, and laptop freindly objects.as it depends and the pressure and not on the material used.where as on the other hand the capacitance pad depends on the capacitance of the finger and are designed for the use by finger only as they depend on the fingers capacitance.

The touchpad, as I believe it, is comprised of overlaying wires, as someone had said before. But as the wired touch, they need a magnetic source to tell the laptop that something magnetic is touching it. I don't know... Why doesn't someone try to put a magnet next to their laptop and see if it works. There is only one weird thing with that theory. I tried the touchpad with my fingernail, and it didn't work. As far as I know, bone doesn't stop a magnetic field.

I have one question. I have tried this one. Cover the laptop touch pad with a single layer of cloth or paper and then touch it. It works exactly fine.
Does the heat get transferred from our hand to the cloth and then to the touch pad so fast.
I dont think the heat theory is perfect.

if i breathe on the touchpad it shows activity on the pressure graph

OK folks! It is an interesting thing to see how people imagine things like this working! Actually all answers here are partially right and partially wrong.

The most difficult detail to comprehend is the process of "scanning" coordinates. Most have guessed correctly there are two layers separated by something. This two layers are composed of lines; one set horizontal, in rows, and another vertical or in columns.

A scan controller chip produces a set of two pulse sequences which "crawl" from left to right and at the same time from top to boom, feeding one by one the columns while the lines (horizontal) are held also one by one into activity. The pulses feed either, wires, capacitor electrodes with the shape of fine strips or resistive lines as well.
The idea is when a pressure point gets the two electrodes (vertical and horizontal) crossing or nearly crossing (depending on the system), the TIME when this happens is detected and it is the equivalent to the corresponding coordinate.

An output is registered at that time and translated into two numbers, one for the horizontal position and another for the vertical one. (The row and the column)

So, in few words; it is a sequential system which explores the whole touchpad surface, measuring the time when the disturbance occurs. NOT the time when the pad is touched... No! The time when the two electrodes, being forced near together, are feed by the scan controller!

But this happens so, so, so fast, we never wonder the touchpad is being explored linearly, column by column, row by row, until the crossing point is detected!

Wow! Clever technology eh?

Mike Aparicio. Galileo university, Medialab Director. Guatemala

History
When laptop computers first arrived on the market, most of them had a rollerball, basically an inverted mouse, to control movement on the screen. Mice had a system of directional rollers inside that were in contact with the main rollerball underneath. which told the sensors what direction to move the cursor. The trackball reversed this by having the ball exposed, with a button or two where your thumb would rest. The problem was that the ball wasn't that accurate. Something had to be done. The touchpad was created by George E. Gerpheide in 1988. Computer designers at Apple Computer decided to use the design and licensed it from Gerpheide. The first touchpad appeared on the Macintosh Powerbook 190 in 1995.

Basics
A touchpad allows users to drag their fingers along in the direction they want the mouse pointer to go. It does this by sensing the pressure of the finger along a vertical and horizontal electrode grid just beneath the surface "skin." AC travels through the electrodes to the circuits. These circuits check the capacitance of both grid layers against each other. This determines where the pointer is and where it should be moved to. It's sort of an electronic version of triangulation in that the touchpad has to take your three-dimensional movements, figure out where your finger is and where it's going, and then translate this into two-dimensional movement of the pointer on the screen.

Innovations
Since the original touchpad was introduced to the public by Apple, many innovations have been implemented. The touchpad has gotten more sensitive over the years, but until recently, placing more than one finger on the touchpad at a time has confused the circuits and prevented the pointer from moving. Apple introduced a new touchpad that allows the user to click right on it, plus gives the user the ability to scroll quickly, rotate or zoom photos or use three fingers to flip through a library.

it may be embedded with array of pressure sensors. Not sure of it although :)

Here is a easy explanation... The touch pad is powered by Jesus there I have just solved everything

In my experience, pens don't work but nor do fingernails, or metal keys. The pad knows how much of it is being touched, and a soft finger touches a large area whereas a hard pen touches only one point. That falls below the threshold.

You can test this if you have the system tray icon for pad pressure -- that really measures area, which is easy to test.

I would love to know how it works, my theory is, it has something to do with the heat in your skin. My touchpad doesn't work when my hands are very cold, yet if they are very hot I don't actually have to physically touch it to make it move the cursor. Anybody out there got the answer, which they can write in simplistic form?

In most newer touchpads:

The bottom layer has thin wires in parallel rows horizontally.... Then there is an insulator... Then the top layer (where you touch your finger) has thin wires in parallel rows vertically.

Looking through the touchpad, you'd see a grid of wires, basically.

There is a high frequency signal sent through the wire conductors, and when the capacitance from your finger interferes with the signal, it reads the horizontal and vertical location of the disturbance and outputs it to your mouse controller.

easy. it works on infra red. heat sensors that sense the heat from your hand. same as an ipod.

I think your inductance theory has some ground in this question, I was searching Google because i was playing with the pressure graph of my touch pad and noticed it was sensing me touching it even when it was a couple millimeters above. I verified that it wasn't just erroring, and only picked up these signals close, and touching the pad. Inductance requires a metal to be present, though the human body contains many metals in the blood and skin, it is not enough, it could sense electromagnetic fields created by static buildup though.
I read somewhere that it works with capacitance, but they never elaborated on this. I agree, the grid of wires thing cannot be correct, they are way too accurate to be reliant on such an obsolete technology.
Edit: the answers of other people did not show up until after i posted, i now know what the capacitance thing means, and that makes perfect sense, since heat does not trigger it, and it works without actually touching, and with objects in between.
There is however, one inconsistency. On synaptics touchpads, you are able to adjust the sensitivity, useful for high humidity enviornments and reducing the chance of tap to click while typing and screwing things up. How can this be read in terms of capacitance? Wouldnt it have to mechanically move the array of wires farther from the surface? Or would this be software controlled like the equalizer of itunes, you can add or subtract 12db(i think, maybe more, maybe less) without changing the equalizer things, similar to window's microphone amplification ability available in audio device properties menu?