Why does a laptop touchpad not detect a pen?

Touchpad

or Why does  a laptop touchpad (or trackpad) only detect fingers?

This came up in discussion in office and I searched the net a lot to get the right answer. Although, the internet almost always have googles of pages on anything, I could not find the perfect answer.

Ultimately, I have to take the harder path to the answer. First understand how a trackpad works and then find why it only detects fingers.

Wikipedia is the best place to look first.

http://en.wikipedia.org/wiki/Touchpad

Touchpads operate in one of several ways, including capacitance and conductance sensing. The most common technology used today entails sensing the capacitance of a finger, or the capacitance between sensors. Because of the property being sensed, capacitance-based touchpads will not sense the tip of a pencil or other similar implement. Gloved fingers will generally also be problematic (such as in a cleanroom environment). Likewise, moist, sweaty, or callused fingers will also affect capacitance sensing. However, some touchpads, such as have been manufactured by Elographics, sensed electrical conductance. In the case of the Elographics touchpads, there were little nipples that separated a flexible top surface from a lower surface. When the top surface was pushed into contact with the bottom surface, using either a finger or any other device, the point of contact could be sensed due to the signal being conducted. In this case, one could use a finger or a wooden stylus, and performance would not be affected by sweat or calluses, for example.

From this I understand, Touchpads that work by sensing the capacitance of finger are tuned in such a way that they will detect only capacitance of fingers or human body parts.

The rest of the article was not of much help regarding the question I had.

Then, I found another source of some information at : http://www.answerbag.com/q_view/36881

Many people shared there views on this here. And the closes to the perfect answer to my query I got here is :

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 ^_^

This basically almost clears the doubt. I am still trying to understand it further. I will add more to this article soon.

However, think that the touchpad does not move even when you touch it with your nails!

Nowadays Youtube is also a good place to look for information. However, the only video I found there was this one. I dont realize who is it itended for?

I am still googling.