I am curious on how capacitive sensing work. How does it sense hand gesture, or measure water level when the distance between the capacitance plates are large (5-10cm)?
I thought that capacitance plates need to be very close to each other to have inducing of electromagnetic waves? For example, a normal capacitor consists of a nonconductive layer sandwiched by two conductive layers, so the distance is very small (< 1mm).
Capacitive sensing can employ a parametric amplification scheme. That means, the developed signal depends not on an input voltage or current, but on a parameter of the apparatus (in this case, the capacitance being sensed).
One can balance an AC bridge containing a pair of matched capacitors. The excitation of the bridge with an AC reference will cause zero developed signal across the output, because of the matched condition, but a nonzero signal if there is a capacitance change.
simulate this circuit – Schematic created using CircuitLab
Here, for example, note that the speaker will give minimum sound output only when the capacitors match; any change in the sensor capacitance results in an audible indication of the imbalance. More elaborate schemes are possible, and there's almost no limit to the sensitivity of such an apparatus (your signal drive can be increased, or frequency modified, or output signal amplified).
Adjust a calibrated trim capacitor for minimum sound (signal output), and you have measured the sensor capacitance. Alternately, use synchronous rectification and differential amplification instead of a speaker, and read out the capacitance difference on a voltmeter.
Even a very small capacitance change can be sensed reliably.
