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I've seen some DIY capacitive touch sensor projects where the sensors themselves are strips of aluminum or copper soldered to a non-grounded wire, where the user touches these strips directly with their finger to be sensed.

I'm a bit unclear as to why this design is considered "capacitive". My understanding of stuff like glass touchscreens is that there is a cathode on one end of the glass, and when a user touches the glass it forms the anode, with the glass serving as the dielectric.

But if the aluminum or copper strip is the cathode, and the finger touches it directly - wouldn't this be a resistive phenomenon? I thought capacitance implied a separation of two charged surfaces.

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    \$\begingroup\$ Can you add a sample schematic you have found. Or a link ?. If the user is touching only one piece of conductor and they are insulated from chassis or ground, there is no (return) path for a continuous current flow. So resistance measurement will not be practical. However, the insulated user will still have some capacitance to chassis / ground. A sample circuit diagram will help clarify if the circuit is measuring capacitance or resistance. \$\endgroup\$ – AJN Aug 1 at 16:52
  • \$\begingroup\$ I don't have a great schematic in mind, the question was more of a conceptual one. I guess I had misinterpreted the role the user takes (I thought of a user as ground, but sounds like they can be considered a new dielectric) in a capacitive touch design. The example I had in mind was this Arduino design (mathworks.com/matlabcentral/mlc-downloads/downloads/submissions/…) which times how long it takes for two pin's states to match and gives a rough approximation of capacitance using the RC factor. \$\endgroup\$ – Nick Nagy Aug 1 at 17:08
  • \$\begingroup\$ Following a link in the link you posted, they say "a more useful range of values is obtained if the sensor is covered with paper, plastic, or another insulating material, so that users do not actually touch the metal foil.". So the system does work better when not corrupted by a conducting user. \$\endgroup\$ – AJN Aug 1 at 17:59
  • \$\begingroup\$ I think that the user can be modelled by a capacitor in parallel with a resistor. When the pin suddenly changes state, the user capacitance acts as a short and user resistance is irrelevant till the time it takes the capacitor to charge. \$\endgroup\$ – AJN Aug 1 at 18:03
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OP's Mathworks link has a link to Ardunio playground Capacitive sensing library link. This link mentions a few points (emphasis mine)

The capacitive Sensor library turns two or more Arduino pins into a capacitive sensor, which can sense the electrical capacitance of the human body.

At its most sensitive, the sensor will start to sense a hand or body inches away from the sensor.

For many applications, a more useful range of values is obtained if the sensor is covered with paper, plastic, or another insulating material, so that users do not actually touch the metal foil.

When the send pin changes state, it will eventually change the state of the receive pin. The delay between the send pin changing and the receive pin changing is determined by an RC time constant, defined by R * C, where R is the value of the resistor and C is the capacitance at the receive pin, plus any other capacitance (e.g. human body interaction) present at the sensor (receive) pin.

In fact, according to them, resistance is bad.

Also if the sensor is exposed metal, it is possible that the send pin will never be able to force a change in the receive (sensor) pin, and the sensor will timeout.

The page gives even more helpful guidelines.

In short, the answer to your question is that, it is dependent on the specific circuit. In the link you provided, the circuit still depends on capacitance and resistance is detrimental to its working.

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