How does a touch pad on a laptop work?

And why does it only work when a finger touches it and NOTHING else?

[Lissa]

Originally Posted by teriyakimonkey

And why does it only work when a finger touches it and NOTHING else?

What else have you been touching it with?

I'm guessing it's electrical conductivity? A small leccy current can be passed through skin sweat etc and the voltage drop measured? In which case... a gherkin might work too? And a finger in marigolds possibly wouldn't?

I feel an experiment coming on....

However, as an IT manager by trade, I'd encourage everyone to first make sure that you don't work at my firm first before trying....

Have you been trying with your naughty bits?

Yes it's electrical.

if its electrical is it using the body as earthing point and detecting the PD drop caused by the earthing?

It does indeed work by "electric"...

There are rows of horizontal and vertical sensors that pick up the charge from your finger, and by examining which of these sensors has what charge levels allows the computer to work out the motion.

There's some really techie articles about the different types of touchpad, but they seem to have the same basic principals behind.

TouchPad sensors work by using an electrical phenomenon called capacitance. Whenever two electrically conductive objects come near each other without actually touching, their electric fields interact to form a capacitance. The surface of a TouchPad is a grid of conductive metal wires covered by a plastic insulator, usually a facesheet made of Mylar. The human finger is also a good electrical conductor. When you place your finger on a TouchPad, a capacitance forms between your finger and the metal wires in the TouchPad.
The Mylar insulator keeps your finger from actually touching the wires and is also textured to help your finger move smoothly across the surface. By measuring the change in capacitance, the application specific integrated circuit (ASIC) can tell when your finger is touching, to an accuracy of more than 1/1000th of an inch.

I imagine if you held something metal or a good conductor then the touch pad would still work as it would flow through to your fingers.

Originally Posted by Grinch

I imagine if you held something metal or a good conductor then the touch pad would still work as it would flow through to your fingers.

Holding a pen (espescially without the ink part extended - metal part on the end) to the touchpad does indeed work. I've done that many a time.

Originally Posted by Grinch

TouchPad sensors work by using an electrical phenomenon called capacitance. Whenever two electrically conductive objects come near each other without actually touching, their electric fields interact to form a capacitance. The surface of a TouchPad is a grid of conductive metal wires covered by a plastic insulator, usually a facesheet made of Mylar. The human finger is also a good electrical conductor. When you place your finger on a TouchPad, a capacitance forms between your finger and the metal wires in the TouchPad.
The Mylar insulator keeps your finger from actually touching the wires and is also textured to help your finger move smoothly across the surface. By measuring the change in capacitance, the application specific integrated circuit (ASIC) can tell when your finger is touching, to an accuracy of more than 1/1000th of an inch.

I imagine if you held something metal or a good conductor then the touch pad would still work as it would flow through to your fingers.

Spot on...... With the help of "Wiki", there is actually another (alternative) way, other than the wires...

There are two principal means by which touchpads work. In the ‘matrix approach’, a series of conductors are arranged in an array of parallel lines in two layers, separated by an insulator. The conductors in these layers are oriented orthogonally to one another. A high frequency signal is applied sequentially between pairs in the two-dimensional matrix created by the conductor array. The current that passes between the nodes is proportional to the capacitance. When a virtual ground, such as the finger, is placed over one of the intersections between the conductive layer some of the electrical field lines are shunted to this ground point, resulting in a change in the apparent capacitance at that location. This method received U.S. Patent 5,305,017 awarded to George Gerpheide in April 1994.
The ‘capacitive shunt method’, described well in an application note by Analog Devices[4], senses the change in capacitance between a transmitter and receiver that are on opposite sides of the sensor. The transmitter creates an electric field which oscillates at 200-300 kHz. If a ground point, such as the finger, is placed between the transmitter and receiver, some of the field lines are shunted away, decreasing the apparent capacitance.