This video is from a student preparing for Final project for PHY 12 at Tufts, fall semester 2015.
Have you ever wondered how the
touchscreen on your phone or laptop
We’re here to explain!
These devices use capacitive touch sensors
with property of capacitance to
detect a conductive object like
your finger when it touches the screen.
Before we can explain what a capacitive
touchscreen display is, you need to have a basic
understanding of circuits.
A circuit is a closed loop that charge can travel
through at any point in the circuit.
The charges have a voltage or potential
which corresponds to their electrical
potential energy measured from a
reference point called ground.
A battery creates a potential difference which
allows charge to flow around the circuit
and through any circuit elements or
devices connected in the circuit.
A capacitor is a vice that stores electric
A common type is the parallel
plate capacitor which consists of two
conductive plates and an insulator.
Between them initially, these are
neutrally charged meaning that positive
and negative charges on them.
When you put a battery onto them, the negative
charges accumulate on one side and the
positive accumulate on the other.
The difference in charge creates a voltage
across the capacitor and this voltage
can be used to power another circuit
element like let’s say a light bulb.
Capacitance is the amount of charge that
something can hold. The capacitance of a
parallel plate capacitor is modeled by
C equals epsilon a over D.
Epsilon I will get to that later.
A is the area of the parallel plate and D is the
distance between the two plates so if
you make the area bigger you’re going to
have higher capacitance you make the
distance smaller you get higher
Now epsilon is the
dielectric constant which is a
characteristic of how difficult it is
for charge to move
in the insulator.
Here we’re going to show you how a capacitor can be charged
and then used to light an LED.
First we’re going to check that the LED works
by connecting it to a power source through resistor.
The resistor slows down the flow of charge.
The led doesn’t burn out . Now we’re going to charge the
capacitor by connecting it to the power
source. When we touch the capacitor to
The LED lights up notice how the LED fades out.
This is because the resistor keeps all the charge from the
capacitor from flowing through the LED
at once although it doesn’t look at the
cylinder. The cool capacitor we used in the
demo is actually a parallel plate
It’s made of two conductive
sheets separated by an insulator and
rolled up. You can build a capacitor for
materials you can find at home here.
We’ve built one by separating two
conductive sheets of aluminum foil with
a sheet of plastic.
So here we have our capacitor even
though it’s this big, it actually only
has a capacitance of a couple nano farad.
That is about a thousand times smaller
than what we used to light the LED so we
can’t use this one to light an LED.
We can’t show you that it will hold
charge. This machine is called an
oscilloscope and the line here shows us
the voltage on the capacitor over time.
So what I’m going to do is I’m going to
hook up the capacitor to a voltage
source and I’m going to disconnect it.
Something interesting is going to happen,
you see how this line curves down.
That shows us that the charge when we
disconnected the battery didn’t go away
right away. There was something
holding on to the charge and that
something could only have been our
Now we can look at how capacitive touch screens work.
Let’s consider just a single point on your
phone screen so when your finger comes
in contact with the glass of your phone
screen then your finger forms a
capacitor with an electrode that is
underneath the screen.
For now imagine that your finger is actually connected
to ground it might be some other voltage
but the results are the same so your
phone uses a two step process to figure
out this capacitance.
That will tell how far your finger is from the screen
because capacitance changes with
distance so the first thing it does is
it actually puts a voltage on this
electrode and that puts some charge on.
A fundamental equation of capacitors
is that the voltage across a capacitor
is equal to the charge on that capacitor
over its capacitance so this puts a
fixed amount of charge on because this
is a steady capacitance.
We want to figure out the next thing that happens
is the phone disconnecting this voltage
and actually connects this electrode to
ground through a second capacitor.
This capacitors capacitance is known and
since we now have two capacitors going.
That is to ground the total of capacitance
increases since this increases the
voltage right here. It must decrease and
that decrease will actually be related
to the capacitance here so by measuring
the voltage here after making this
connection the phone can figure out
whether it’s being touched or not so a
two-dimensional touchscreen is made up
of a bunch of rows and columns of these
It figures out where your
finger is in two dimensions by just
looking at all the electrodes together
and seeing which ones are being touched.
This specific type of touchscreen is
called a self capacitive touchscreen.
It can only accurately detect one touch at
a time. When you touch it with two
fingers, the controller knows the rows
and columns that have been touched but
it can’t tell whether the contact points
are here .
Here is a newer form of
touchscreen is called a mutual
capacitive touch screen which can detect
two touches because each row and column
intersection has a capacitor multi-touch
touchscreens that allow you to do things like
zoom in and drag.