I have had this idea about how touch pad or the touch screen on my cell phone works. There are no physical buttons per say. Nor do I hear a mechanical click, nor feel one when I use my touchpad or phone screen. From what I have heard and read, that is because cellphone screens and touchpads use a series of capacitance sensing switches. In this article I will talk about my experience making a capacitance sensor / switch using a working circuit as example. So what is a capacitance sensor? If I am correct, a capacitance sensor / switch allows one to control the electricity passing through it using capacitance.

Now whenever one is in close proximity to a conductor, one supposedly forms a capacitor.


The closer you are, the capacitance changes. In my article on negative resistance, I show a capacitor can control the amplitude of an output voltage. The secret - the capacitor is used as a resistor in a voltage divider circuit. We will use a modification of this circuit with a very special electronic component :)

The speical component is you touching your touch pad or your cellphone screen. Notice, one complete the circuit by standing on "Virtual ground"!

So, back to the voltage divider. Since the voltage output depends on the circuit's resistances, and one of its resistances just happens to be caused by this capacitor our body forms, by moving my hand near to the conductor, I can vary the output voltage.
So demonstration time! The series circuit below shows the voltage source, the inductor and a piece of aluminum foil which serves as the conductor side of the capacitor. (Of course me bringing my finger near the conductor will create the actual capacitor!)


Below is the graph with the circuit running in the state shown above. Notice the output is 16.68V.


I will now place my finger over the aluminum foil but I would not touch it. In fact, I use a paper folded to the thickness of an American 25 cents (quarter) - to ensure my finger never touches the conductor!


Here you can see the output voltage varying. The output has now dropped to 15.61V.


Obviously there is more to making a working touch screen, but maybe you can find some clever use for this very simple but interesting concept. :)

Inductor Capacitor Oscillator Caculator Applet




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