Electronic – Is a human (or other) electrical signal absolutely necessary to activate a capacitive touchscreen

capacitancecapsenseiphonetouchscreen

I know this is the general wisdom, and that someone has asked a similar question here: How can a capacitive touch screen be triggered without human contact?

However, I didn't quite understand the conclusion that by simply grounding e.g. A carrot or stylus, or connecting them to a conductive plate, this would be enough to allow them to activate the touchscreen. Is the idea that the ground or conductive plate pick up enough electricity in the environment that they can act as a substitute for a human?

As a related note, I held a stylus through many layers of folded up, thick woollen sweater, and it was still able to activate the touchscreen. Was my body's electrical signal definitely conducted through the sweater?

Apologies if my question sounds a bit ignorant. I am not an electrician, but I understand the basics of electrical circuitry and how capacitors work. The iPhone's capacitive touchscreen nevertheless seems like a bit of a black box to me.


Thank you all. Based on your responses and my reading of the Q&A referenced by @gbarry, my understanding of this is now almost complete. There remainas these issues that I am unclear on:

  • It seems that it is a very common misconception, then, that it is a body's electrical signal (current) that is necessary to activate the iPhone touchscren in particular. Rather, what is sensed by the electrodes of the touchscreen circuit is either insulation or capacitive coupling, but supposedly not conductivity. Since I always considered insulation and conductivity as belonging on the same scale, could someone perhaps clear this up for me, while illuminating whether or not this is what is meant here by "capacitive coupling"?

  • I have understood that large surface area is often an important determinant of capacitive coupling. Is this the only reason that it is suggested the contacting body be grounded, or is it necessary for another reason, e.g., To "complete the circuit" initiated by the iPhone?

I'll gladly award the bounty to whomever can answer these questions in a way that I can understand! Thank you again.

Best Answer

The reason the stylus worked even through layers of wool is related to the very nature of capacitance: It is not about conduction (except at the electrodes themselves), it is about the insulation between the plates: Wool is about as good a dielectric as dry air, if it is not woven with some metallic fiber.

The way capacitive touch screens typically work is that they sense the change in capacitance that occurs when an earthed body - essentially any large conducting body, such as a human - comes close(r) to the surface electrode. By using a metallic stylus, that "earthed body" is being brought closer, with merely the wool in between.

This capacitance change can be measured in a number of ways. A common approach is to inject a high frequency signal between the sensing electrode and the device's own ground plane, with air (or glass, or some other material) as a dielectric between these two "electrodes". Any other conductor coming into proximity changes the dielectric (i.e. insulation) pattern, thus changing the capacitance, and thereby the pattern of load shown by the capacitor across the native capacitance.