1. EE 242 - Section 02
Final Project: 3D Sensor Box using
Capacitive Sensing
Lab Group #6: Niven Prasad and Willy
Okpobua
Delivery Date: 12/01/15
Introduction
The purpose of this experiment was to build a 3D sensor box made out of capacitive plates
which senses the position of a human hand. The experiment also consisted of measuring the
charge and discharge curves of the capacitive plates in different contexts.
Lab Equipment Used (All sections):
Agilent InfiniiVision MSO-X 2022A
Stanford Research Systems Model SR715 LCR
Innova 3320 Auto-Ranging Digital Multimeter
2 BNC to banana cable
Alligator Clips
Background
There are two main forms of sensing used in electric appliances today, resistive sensing and
capacitive sensing. The change in resistance is the fundamental concept behind how devices
that utilize resistive sensing work . Devices such as 4-wire resistive touch screens work based
off of this principle. The device is made of two layers, the top layer is the resistive layer and
the bottom layer is the conductive layer. The two layers are separated by transparent
microdots known as spacers. A voltage is applied across the conductive surface of the device
so as to maintain a constant flow of current running through the conductive layer at all times.
When a probe such as a pen credit card or finger is used to apply pressure to the top film of
the device, the screen is activated. The pressure applied by the probe to the top layer causes
the film to flex inwards, making the top resistive and bottom conductive layers to make
contact thus causing a drop in voltage. The potential difference is detected by the controller

2. which in turn computes the X and Y coordinates of the user’s touch and executes a command
synonymous to that coordinate on the device. The diagram below briefly describes how
resistive sensing works.
Figure A: Resistive sensing screen
From http://www.dawar.com/touch-screen-tools
Capacitive sensing on the other hand, which is what this lab is mainly about, is a technology
which relies on human capacitance as an input by means of capacitive coupling. Capacitive
sensing is based off of the principles of capacitance which state that when a voltage is applied
to the plates of a conductive object, an electric field is created between the plates which
polarizes the charge in the voltages source thus leading to the collection of positive and
negative charges on either plate of the plates of the conductive object and a reverse in the
polarity will cause the charge to act in the opposite manner. Capacitive sensors make use of
this concept by using alternating current which causes the charge to continuously reverse its
position. The constant motion of the charges creates an alternating current which is detected
by the sensor. The amount of current flow is determined by the capacitance which is in turn
determined by the area and proximity of the conductive objects. This principle is important in
the manufacture of capacitive sensors in that larger and closer objects cause a greater change
in current than smaller more distant objects. The diagrams below briefly describe how
capacitance in capacitive sensors works.
Figure B

3. From http://www.dawar.com/touch-screen-tools
Many devices today utilize the concept of capacitive sensing, A classic example is the iPhone
which has a touch-sensitive screen consists a layer of capacitive material. The device’s
capacitors are arranged according to a coordinate system that has advanced circuitry capable
of detecting changes at each point along the grid. The grid then generates its own signal and
relays it to the device processor for processing. This in turn enables the device to determine
locations and simultaneous touches at different locations on the device. Because of the
reliance on the capacitive material used to build the iPhone screen, it only works through
touch by a fingertip as opposed to a stylus which can not be used since it has no coupling
effect on the screen unlike the the human body that can transfer charge to or away from the
surface of the screen, a concept crucial to the functioning of capacitive sensors. The iPhone
screen fundamentally detects touch through one of two ways; mutual or self capacitance.
Under mutual capacitance the circuitry of the phone contains two distinct layers of material.
The first layer houses driving lines which carry the current and the other layer contains the
sensing layer which sense the amount current and charge being carried by the driving lines.
The lines are oriented perpendicular to each other on the glass substrate. When a probe such
as the human finger is placed on the screen of this device, this changes the amount of charge
in the driving lines and this change in the driving lines is detected by the sensing lines which
relay this information to the processor which strips out the background noise and determines
if you have touched one place, two places or made a swipe across the screen for
example. However, recent advancements have been made to this technology and now we
have on-cell where the touch-screen technology is on the surface of the smartphone screen
and in-cell technology where the touch-screen technology has been embedded into the screen
of the smartphone. The second mechanism through which the iPhone detects touch is self
inductance which uses one layer of individual electrodes connected with iPhone.

4. Below are diagrams which best describe how capacitive sensing works in modern day
devices such as the iPhone.
Figure C: A diagram illustrating the different layers of the iPhone screen and its components.
From: http://electronics.howstuffworks.com/iphone1.htm
Figure D: A diagram illustrating how touch sensing works on an iPhone.
From: http://electronics.howstuffworks.com/iphone1.htm
The 3D Sensor Box Build and Theory:

5. Materials and equipment used:
RadioShackⓇ 6’ Shielded Audio Cable
Arduino Uno Microcontroller
Alligator Clips
10㏀ and 220㏀ resistors
Cardboard and Reynolds aluminum foil
Short wires
The sensor box was built according to the schematic below
Figure 1
The plates and wires are connected through 3 of the shielded audio cables. These shielded
cables are used to minimize any antenna effects. All connections are soldered the 60/40 lead-
based solder. The plates are simply made of aluminum foil glued onto three 11in. x 11in. x
11in. cardboard. Also the three 10㏀ resistors are connected to digital I/O pins on the
Arduino. The finished product is shown below.
Figure 2
Note: left plate = X plate, bottom plate = Y plate, right plate = Z plate

6. This 3D box uses a simpler form of capacitive sensing to detect the position of a hand inside
the box. The general idea of this box is to use the charge time of the plates to detect the
position of the hand on a digital representation of the box on the computer screen (Figure 3).
In the Arduino code, the digital pins output a voltage and charge the three plates and then
reads the state of each of these plates. The program starts a timer when the digital pin outputs.
While the plates are not fully charged, the digital pins read a “0”. and when the plates are
fully charged, the “0” turns into a “1” and the program stops the timer and records it. The
three plates take a certain time to charge when the hand is not near it. When a hand is near the
plates, the charge time of the plates increases because as the hand approaches the plate, the
body electrically couples with the plate to increase the capacitance of the plate. A diagram of
a simple circuit with a human hand affecting capacitance is shown in Figure E below:
Figure E
From: http://playground.arduino.cc/Main/CapacitiveSensor?from=Main.CapSense
So when the capacitance increases, the charge time increases since the capacitor charging
equation is:
VC(t) =Vo(1-e-t/RC)
Charge time depends on the time constant, τ = RC. So when the capacitance is increased, the
time constant increases, therefore making the capacitor take longer to charge.
Another coding program on the computer uses all this information from the Arduino code to
track the hand. This software is called, Processing. It essentially uses the time difference
from the three plates to charge to triangulate the position of the hand. So if a hand is near the
X and Y plates, those plates will take longer to charge while the Z plate will charge quicker,
so the program can track that your hand is near the X and Y plates. The tracking graphic is
drawn by the code is shown in Figure 3 below,
Figure 3

7. The yellow tracking ball represents where the hand is and the little cubes are just points on
the sensor box. The tracking ball follows where the hand goes as the person moves the hand
inside the sensor cube.
Note: The code for both the Arduino and Processing programs were written by the designer
of this 3D box and is open source. It can be found at: https://github.com/Make-
Magazine/3DInterface
Procedure and Data Section 1: Measuring resistor values
The values of the resistors used in our circuit were measured and are shown below:
Table 1
Nominal Actual % diff
10㏀ 9.81㏀ -1.9%
10㏀ 9.62㏀ -3.8%
10㏀ 9.94㏀ -0.6%
220㏀ 0.223㏁ 1.36%
220㏀ 0.221㏁ 0.45%
220㏀ 0.223㏁ 1.36%
Procedure Section 2: Measuring curves
First, Channel 1 of the oscilloscope was connected to the X plate of the sensor box. The
positive end made contact with the plate and the negative to the digital ground on the
Arduino. The resulting waveform on the oscilloscope was recorded without the presence of a
hand near that plate. Then a hand was placed on the plate, making contact, and the resulting
waveform was recorded. This was done for the Y plate as well.
Data Section 2:
Measuring the charging curves:

8. The graphs below show the voltage taken from the plate and the arduino ground pin. The
voltages are not representative of the actual voltages across the capacitor themselves since it
was difficult to make this measurement, but these curves do give us the main idea of how the
capacitor behaves in this sensor box.
Graph 1 below shows the curve obtained for the X capacitive plate when there was no hand
present near the plate.
Graph 1
Graph 2 below shows the curve obtained for the X capacitive plate when there was a hand
touching the plate.
Graph 2

9. Graph 3 below shows the curve obtained for the the Y capacitive plate without the presence
of a hand in the box.
Graph 3
Graph 4 below shows the curve obtained for the the Y capacitive plate when a hand was
touching this plate.
Graph 4

10. Section 2 Discussion:
As seen from the graphs above, the two plates behave almost exactly the same which is
important for the sensor box to work properly. Also, the graphs obtained are what we
generally expected. As seen from Graphs 1 and 3, the curves have a familiar capacitor
charging and curves when the hand is not near the plate. When the hand does touch the plate,
the curves stretch out (as seen in Graphs 2 and 4) because the capacitance of the plate
increases and therefore the time constant increases since τ=RC.
Procedure Section 3: Measuring the time constant
Then, the cursor function on the oscilloscope was used to measure the time constant of the Y-
plate curve by placing a horizontal cursor at 63% of the peak voltage and vertical cursors to
measure that time. This time was recorded when there was not a hand making contact with
the plate and with the hand making contact with the plate.
Section 3 Data:
The graph below shows the time constant measurement of the Y plate capacitor with no
contact made on the plate.
Graph 5

11. As seen from the graph, the time constant is about 32µs for the Y plate without a hand near it.
Now a hand was placed and the time constant was measured again as shown below in Graph
6
Graph 6
With the hand touching the plate, the time constant changed to about 55µs.
Section 3 Discussion:
The time constant measurements further confirm how the time constant increases as the hand
makes contact with the plate. The time constant was 32µs without the hand and 55µs with the
hand contact.
Procedure and Data Section 4: Measuring the Capacitance
The LRC bridge was used to measure the capacitance of the plates. The positive lead was
connected to the plate and the human hand was used as the ground by holding the negative

12. lead in hand. The capacitance was measured when the hand was not making contact with the
plate and then with contact. This was done for all the plates. Note that these measurements
are fairly crude and rounded since the capacitance fluctuated a bit depending on how close an
object or person was as well as other interference. The table below summarizes the measured
capacitances:
Table 2
Plate No hand contact Hand Contact
X 15pF 40nF
Y 18pF 46nF
Z 12pF 50nF
Section 4 Discussion:
As seen from Table 2, the capacitance when there was no hand contact is in pF and when
there was contact from the hand, the capacitance went into the nF range. So the hand had a
significant effect on the capacitance of the plate. Once again, these values fluctuated a bit
when measuring it and how much of the contact the hand made on the plate varied from plate
to plate, however, the difference between no contact and contact is clear in the measurements
for all three plates.
Procedure and Data Section 5: 3D box performance
The box was tested by placing the hand at various locations inside the sensor box and
recording the corresponding tracking on the computer.
Figure 4 shows some of the tracking snapshots.
Figure 4.1 Figure 4.2
Figure 4.3 Figure 4.4

13. Figure 4.4 Figure 4.5
Section 5 Discussion:
So as seen from the figures above, the box can sense the position fairly well at these corner
points, however, sometimes when the hand is in the middle of the box, the sensor box is not
very accurate. The reason for this may be because the capacitive plates are not big and
sensitive enough for the capacitance to change when the hand is in the middle. The more
likely reason is due to electrical interference since the large surface area of the plates can
make it act like an antenna, so other electrical interference can change the capacitance of the
plates and reduce the effect of the hand on the plates. When the sensor box is placed in a
more electrically isolated location, though, the tracking ball remains pretty accurate in all
parts of the box and the plates do not need to be touched for the tracking ball to move.
Conclusion
Niven Prasad:
In our project, we built a sensor box out of capacitive plates which sensed the position of a
hand based on the time to charge. It used the time difference from three capacitive plates to
track the position of the hand inside the cube. We also saw how the capacitor charging curve
changed when there was no hand making contact with the plates versus a hand making
contact. We saw this change when measuring the time constant as well. Finally, we saw how
the 3D box was able to perform. We found that it worked significantly less accurately in a

14. location with a lot of electrical devices since there was an antenna effect. Overall, though, the
box performed well.
Willy Okpobua:
In this project we learned about the fundamentals of capacitive sensing and how it is
applicable in appliances that we use on a day to day basis such as smartphones and
thermostats. Using this knowledge we were able to devise a sensor box that identified the
position of a human hand next to it based off the amount of time it took the charge. We were
also able to learn that for any form of capacitive sensing to take place, there has to be direct
contact with the human body which acts as a medium through which charge is channeled to
ground through human coupling. This was evident in the change in the curve of a charging
capacitor which exponentially increased as a result of transference of external charge by
human contact to the electric network.