# Affecting Mutual Capacitance w/ an External Field

by pin0ut
Tags: affecting, capacitance, external, field, mutual, w or
 P: 2 I've been playing around with projected capacitive touchscreens - specifically, the iPhone's. I've attached a white paper that outlines the details, but the basic idea is as follows: An iPhone's touchscreen has a dielectric layer sandwiched between a layer of conductors arranged in rows and a layer of conductors arranged in columns. The result is a grid of capacitors that can be independently driven. When a finger (or anything capable of sapping enough charge) comes close enough to these pseudo-capacitors, the field between them is altered, and a touch is registered. ("Close enough" has been tuned to exactly the point at which a finger touches the iPhone's glass screen.) What I'm wondering is - is it possible to alter these fields in the same way, but from further away than originally intended? Perhaps with a generated E-field? In general, can you alter the mutual capacitance of two conductors with an external field? Thanks in advance!
Mentor
P: 37,602
 Quote by pin0ut I've been playing around with projected capacitive touchscreens - specifically, the iPhone's. I've attached a white paper that outlines the details, but the basic idea is as follows: An iPhone's touchscreen has a dielectric layer sandwiched between a layer of conductors arranged in rows and a layer of conductors arranged in columns. The result is a grid of capacitors that can be independently driven. When a finger (or anything capable of sapping enough charge) comes close enough to these pseudo-capacitors, the field between them is altered, and a touch is registered. ("Close enough" has been tuned to exactly the point at which a finger touches the iPhone's glass screen.) What I'm wondering is - is it possible to alter these fields in the same way, but from further away than originally intended? Perhaps with a generated E-field? In general, can you alter the mutual capacitance of two conductors with an external field? Thanks in advance!
Welcome to the PF.

You could pick up an external field with the conductor matrix, but it would be hard to make the external field focused enough to give good resolution.

You might be able to use an alternate dielectric material between the row and column electrodes, and use a laser pointer to alter the capacitance at the laser spot...
 P: 2 Thanks! Resolution isn't actually a huge issue. The goal isn't to alter the field in a single pseudo-capacitor in the grid, but rather several of them. The iPhone's touchscreen controller has to deal with EMI and false positive touches, so it basically scans for "blobs" or small regions of change on the grid. If the blob is large enough and the changes large enough, a touch is registered in the rough center of the blob. All this is done in hardware, and is probably a close-guarded secret, so the specifics are hard to come by.

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