**E21 Lab 1: Capacitive touch passcode**
**[ENGR 21 - Fall 2026](index.html)**
**September 10-22, 2026**
**Due on Moodle (one submission per group) one week after your lab meeting**
(#) Objectives
* Investigate capacitive touch sensing.
* Use conditionals (**`if`** statements) to respond to pin inputs on the
Circuit Playground Express (CPX).
!!! Warning
You may be familiar with more advanced Python language
features such as **`for`** and **`while`** loops,
list indexing operations, and functions defined using
using the **`def`** keyword.
**You should not use any of those language features while
completing this lab.** The only core programming skill you will
need today is (potentially nested) **`if`** statements.
# Lab Overview
This week in lab, you will apply what you have learned in class about
programming in CircuitPython on the Circuit Playground Express to
several tasks of increasing complexity, building up a system that
lights up when the correct sequence of capacitive touch inputs is
entered.
## Report template
Use the provided
[lab report template](https://docs.google.com/document/d/1zX4XIjqBkH3_dSBM4zfjCBaGEOoiZbpREtcKvYVq8Ss/copy)
to answer the questions for this lab. For all E21 labs, please make
sure to write up your answers to the questions in each exercise before
you move on to the next task. The questions are designed to help you
complete the labs efficiently.
## Pair Programming
For this lab, I want you to practice a technique callled driver/navigator pair programming. One team member—the driver—uses the keyboard and mouse to write code, run code, test code, etc. The other team member—the navigator—reviews the code the driver writes, checks for errors in syntax or logic, and thinks ahead about the strategic goals and next steps of the programming task. The driver and navigator are in constant communication, thinking aloud and narrating the code as it is written, even though they have two different roles to focus on. Every so often, the driver and navigator switch roles.
Pair programming is used in software development for its benefits of producing higher quality and more organized code as well as facilitating knowledge transfer between programmers. For this lab, you will practice the technique. Think of the analogy of actually driving a car where the driver is reacting to immediate tasks and the navigator is reading the map to alert the driver of upcoming tasks. I will remind you every 20 minutes to switch drivers.
## Lab progress
I expect most groups will complete or at least make substantial
progress on Exercise 4 by the end of lab. You can finish outside of
lab on your own time.
I strongly encourage you to flag me down during lab to check in about your
progress as you complete each exercise during lab.
# Background
## Capacitive touch
![Figure [cpins]: Capacative touch pins on CPX. Image courtesy of Adafruit.](images/circuitpython_cpx_capacitive_touch_pads.jpg width="75%" alt="A close-up image of an Adafruit Circuit Playground Express. Pins A1 to A6 and TX are circled in red.")
The CPX has seven pins capable of serving as capacitive touch inputs,
as described on [this Adafruit
article](https://learn.adafruit.com/adafruit-circuit-playground-express/circuitpython-cap-touch),
which also gives an overview of how access them in CircuitPython.
The basic idea of capacitive touch is that when you connect a
conductive object to the input pin, that object and your own body
create a [capacitor](https://en.wikipedia.org/wiki/Capacitor) that can
store a small amount of charge. The resulting change in charge can be
sensed by a microcontroller and used as an input.
This same principle also governs how phone touch screens work.
For more details about capacitive touch, I recommend reading this
[more in-depth article at All About
Circuits](https://www.allaboutcircuits.com/technical-articles/introduction-to-capacitive-touch-sensing/).
![Figure [vground]: Electrical schematic of a capacative touch system. Image courtesy of All About Circuits.](images/ICTS_diagram4_2.jpg width="75%" alt="The index finger of a hand is nearly toughing an electrical ground. A circuit schematic is superimposed, showing the capacitance between the finger and the ground.")
## **`while True`**
In Python, a **`while True`** loop repeats all of the statements
inside of the loop over and over, forever.
To illustrate this, enter REPL mode (you may need to open the Serial
window in the Thonny editor and press `Ctrl + C` if a program is running) and
paste in the following commands:
~~~ Python
import time
while True:
print('yooooooooo')
time.sleep(0.1)
~~~
You will need to press `Ctrl + C` in the Serial window to terminate
this loop.
All statements in a **`while True`** loop execute in
sequence, no matter how many there are. For example, this loop alternates
between printing two different messages.
~~~ Python
while True:
print('hello')
time.sleep(0.1)
print('goodbye')
time.sleep(0.1)
~~~
Again, you'll need to press `Ctrl + C` in the Serial window to
terminate this loop.
## Nested **`if`**
In Python, **`if`** statements can contain any other statements inside
of them, including other **`if`** statements. For example, the code
snippet
~~~ Python
x = 3
y = 4
if x == 3 and y == 4:
print('x is 3 and y is 4')
~~~
is equivalent to
~~~ Python
if x == 3:
if y == 4:
print('x is 3 and y is 4')
~~~
Nested **`if`** statements are particularly useful when you have a chain of
comparisons you want to make and you want to take some actions
along the way, but only under some circumstances.
For example, the following code snippet could be the control flow of a
program that plays a (very boring) game that asks the user to
name two even numbers. Doing so results in victory, but providing
any odd numbers results in immediate defeat:
~~~ Python
print('Give me two even numbers, please')
num = int(input('First number? '))
if num % 2:
print('First number odd, you lose!')
else:
num = int(input('Second number? '))
if num % 2:
print('Second number odd, you lose!')
else:
print('Both even, you win!')
~~~
In the above code, the user is only asked for the second number if the
first number is even. Hopefully you can see the utility of nested
**`if`** statements!
## Functions
!!! Tip
**You'll learn more about functions in class later this month.**
This basic intro is just enough to get you going in the lab.
Functions in Python are bits of code that you can execute at a later
time. They are defined using the **`def`** keyword.
For example, we could make a function **`hello()`** that prints
the string `"Hello, world!"`, like this:
~~~ Python
def hello():
print("Hello, world!")
~~~
The code inside of a function does not run until you *call* the
function using parentheses containing zero or more comma-separated
arguments, for example:
~~~ Python
hello()
~~~
...should result in `Hello, world` being printed to the console.
Without the parentheses, you'll just see some information about
the **`hello`** function itself, rather than executing
the code inside of it. Try it:
~~~ Python
hello
~~~
You've already used one very common Python function quite a bit -- the
built-in **`print()`** function.
There are many reasons people use functions including being able to
re-reun the same code over and over without copy-pasting it all;
however, in Lab 1, we are mostly using functions in order for you to
focus on each exercise below, one at a time. Hence, the starter code
comes with functions named **`lab1_ex1()`** through **`lab1_ex5()`**
that you will call, and in some cases modify.
# Lab Exercises
Before you begin, download this [`code.py`](lab1/code.py) file and
copy it over to the root directory of your CPX.
!!! Warning
**If you already have a `code.py` file on your CPX with valuable
code in it, beware of overwriting it!** Consider renaming the one
on your CPX or backing it up to your computer before copying over
the starter code.
When moving from exercise to exercise, you will comment and un-comment
the function calls at the very bottom of Lab 1's `code.py`.
!!! Tip
Everything in a line of Python code including and after a hash
mark **`#`** character is a *comment* and will not be executed. To
comment a line of code, add a single **`#`** to the start of the
line (after indentation). To uncomment,
simply delete the **`#`**.
## Exercise 1: Raw touch input
When you run the code, open the Serial and Plotter windows, and
observe the effects of touching the **`A1`** pad on the CPX. (There
should be nothing electrically connected to the pad.) You should see
the graph change significantly depending whether you are touching the
pad or not.
Write down the reading from the Serial window *before*, *during*, and
*after* touching the pad for each group member. If the readings wander
a lot during a condition (e.g. *during* touch), try to pick the
average value. You should now have a triplet of readings per group member.
Repeat touching the pad twice more per group member, to get three triplets of readings for each group member.
Now use the provided alligator clips to attach a variety of objects to
the **`A1`** pad and repeat the process of recording readings
*before*, *during*, and after *touching*, this time when touching the
object clipped to the pad instead of the pad itself. Do this for at least three
objects total, including some of those you brought, as well as one of the
tinsel balls provided in lab. You should now have twelve before/during/after triplets of readings per group member (three triplets for the pad with nothing clipped to it, plus three each for three different objects).
!!! Warning
**For this and all future E21 labs, please make sure to write up
your answers to the questions in each exercise before you move on
to the next task.** The questions are designed to help you complete the
labs efficiently! Don't forget to use the [provided lab report template](https://docs.google.com/document/d/1zX4XIjqBkH3_dSBM4zfjCBaGEOoiZbpREtcKvYVq8Ss/copy).
* **Q1)** Compile a table of all of the readings you collected. The
columns should include *object*, *person*, *before*, *during*, and *after*.
* **Q2)** Which object has the smallest differences between *before* and
*during* readings? (E.g. the quantity *during* minus *before*) Which
object has the largest differences?
* **Q3)** Do you see any significant difference between *before* and
*after* readings for any object? How large are they compared to the
*before*-vs-*during* differences?
* **Q4)** Does the object being touched significantly affect the
readings in any other ways (e.g. do their graphs look visually
different)? Does one group member tend to have larger or smaller
readings than another?
## Exercise 2: Calibration
Edit the lines at the bottom of `code.py` to comment out
**`lab1_ex1()`** and uncomment **`lab1_ex2()`**.
Run the program with no objects clipped to any capacitive touch input pads,
and look at the results in the Serial window as you touch the pads.
(###) How capacitive touch works on the CPX
As you saw in Exercise 1, a capacative touch input produces a
continously varying value, but in order to be used as a button, that
continous range needs to be reduced to *touched* or *not touched*.
According to the
[Adafruit CircuitPython documentation](https://docs.circuitpython.org/en/latest/shared-bindings/touchio/index.html#touchio.TouchIn.threshold),
when a **`TouchIn`** object is initialized, the initial reading $r_0$
of the touch sensor is recorded. Then, later at time $t$ if the
touch sensor reports a reading $r_t$ more than a threshold value
$\tau$ away from $r_0$, the state of the input becomes *touched*,
otherwise it is *not touched*.
Or to put it mathematically, the binary or *Boolean* variable $v_t$
can be described as a function of $r_t$ as follows:
$$
v_t = \left\{ \begin{array}{ll}
1 & \text{if}\ r_t > r_0 + \tau \\
0 & \text{otherwise} \\
\end{array} \right.
$$
The default threshold value is $\tau = 100$.
The
[**`cp.adjust_touch_threshold()`**](https://docs.circuitpython.org/projects/circuitplayground/en/latest/api.html#adafruit_circuitplayground.circuit_playground_base.CircuitPlaygroundBase.adjust_touch_threshold)
function allows you to adjust the threshold used to detect touches. An
adjustment of zero corresponds to setting the default value of $\tau =
100$, whereas an adjustment of 900 corresponds to setting $\tau =
1000$. **Higher adjustments make the touch sensor less sensitive.**
(###) Calibrating the touch sensor to your objects
With no object clipped, the default adjustment of zero should work
just fine. However, you may find that with some objects, a larger
adjustment is required to prevent spurious touch detections. On the
other hand, if the threshold is too large, it becomes more likely for
the program to fail to recognize valid touches.
Use alligator clip leads to once again connect the three objects you
used in Exercise 1 to the touch inputs. Then restart your code.
!!! Warning
**Always restart your code when connecting or disconnecting
objects to touch inputs!** The program only records the initial
sensor reading $r_0$ when it begins, and different objects may
want different $r_0$ readings.
You can restart the program by clicking the ***Save*** button in
the Thonny Editor, or by using the Serial window and pressing `Ctrl + C`
then `Ctrl + D` on your keyboard to exit the program and then
reload it.
Use trial and error to change the argument passed into
**`cp.adjust_touch_threshold()`** inside of **`lab1_ex2()`** in order
to to ensure that the program successfully detects touches on all
objects without any false detections.
!!! Tip
Some objects result in false touch detections when your hand is
near, but not touching them. If that happens you'll want to
increase your threshold.
Remember, if your adjustment is too high, you will end up missing
valid touches.
* **Q5)** What argument passed to **`cp.adjust_touch_threshold()`**
gave you the best result? How did your work in Exercise 1 inform
the process of adjusting the threshold?
## Exercise 3: Reacting to a capacitive touch event with lights
Edit the lines at the bottom of `code.py` to comment out
**`lab1_ex2()`** and uncomment **`lab1_ex3()`**. Then run the code.
!!! Tip
Before you continue, modify the argument to
**`cp.adjust_touch_threshold()`** inside of **`lab1_ex3()`** to
the value you successfully identified in Exercise 2.
!!! Warning
When running the code for this and the remaining exercises,
you will be prompted to press button *A* or button *B* on
the microcontroller. **Make sure not to press the small *Reset*
button by mistake, or you will temporarily eject the CIRCUITPY drive
and restart the microcontroller.**
This program waits for a touch input, then fills the CPX board's
NeoPixel lights with white color until a button is pressed.
Modify this program by adding an **`if`** statement to check whether
input **`A1`** (and only **`A1`**) was touched, and only filling the
NeoPixels with white if it was. If any other input is touched the
NeoPixels should stay dim.
!!! Tip
If you are stuck figuring this out, try dropping into REPL mode
(press `Ctrl + C` in the Serial window if a program is running)
and pasting in the following lines of code. Pay close attention to
the results printed after each comparison operation using the
**`==`** operator:
~~~ Python
import board
pin = board.A1 # assignment, not comparison
pin == board.A1 # first comparison, should print True or False
pin == board.A2 # second comparison
pins = [board.A1, board.A2] # assignment, not comparison
pins == [board.A1, board.A2] # third comparison
pins == [] # fourth comparison
~~~
Finally, modify your program to fill the NeoPixels with white if
**`A1`** is touched, and red if any other input is touched.
The program should still wait for a button press after each touch,
and reset the NeoPixels to black after each button press.
!!! Tip
**Hint:** you don't need a whole new **`if`** statement to accomplish
this second task. Instead, try adding an **`else`** to the **`if`**
statement.
## Exercise 4: Capacitive touch passcode
Edit the lines at the bottom of `code.py` to comment out
**`lab1_ex3()`** and uncomment **`lab1_ex4()`**. Then run the code.
!!! Tip
Before you continue, modify the argument to
**`cp.adjust_touch_threshold()`** inside of **`lab1_ex4()`** to
the value you successfully identified in Exercise 2.
This code starts out with the NeoPixels all black, then lights up
one, then two, then three NeoPixels to be green before ending with
the NeoPixels being all white. After that, the sequence repeats.
In case you want to refer to them later, the relevant lines for
controlling the NeoPixels are:
~~~ Python
cp.pixels.fill(BLACK)
cp.pixels[0] = GREEN
cp.pixels[1] = GREEN
cp.pixels[2] = GREEN
cp.pixels.fill(WHITE)
~~~
Comment out or delete the lines inside the loop, then paste in the
interior of the loop from **`lab1_ex3()`** as a starting point for the
remainder of this exercise.
Now choose a combination of four touch inputs (e.g. **`A1`**,
**`A2`**, **`A3`**, **`A4`**) and modify your loop to satisfy the
following specification:
* Initially the NeoPixels are all black.
* When each of the first three inputs in the combination are touched
in the correct order, the next NeoPixel is turned to
green. E.g. first **`cp.pixels[0]`** then **`cp.pixels[1]`** then
**`cp.pixels[2]`**.
* When the fourth and final input is touched correctly,
the NeoPixels should be filled with white, and stay that way
until a button is pressed.
* If any input is touched in the wrong order, or any input not in
the combination is touched, the NeoPixels should be filled with
red, and stay that way until a button is pressed.
* When a button is pressed, the NeoPixels should return to
black and the combination can be attempted again.
Here is a video of a successful solution to this exercise:
The video shows one successful combination entry, multiple incorrect
entries, and finishes with one more successful entry.
When you are finished with this task, shoot a similar video. Your video must be clearly shot and begin with one correct
combination entry, then have at least two incorrect entries, and end
with another correct entry.
* **Q6)** What is the URL to the video you recorded for exercise 4? Make sure it is shared with my Swarthmore email address (mzucker1@swarthmore.edu). Please be nice to my inbox and uncheck “Notify people” when you share it.
* **Q7)** Describe your approach to solving this exercise. Was
there a "eureka" moment? Did completing this task in lab lead
you to think about **`if`** statements in a different way
than you thought about them in class?
!!! Tip
If your lights aren't showing up well on camera, you can adjust
the NeoPixel brightness that is set towards top of the
`lab1_ex4.py` program (or shoot in a darker room).
## Exercise 5: Going further
Edit the lines at the bottom of `code.py` to comment out
**`lab1_ex4()`** and uncomment **`lab1_ex5()`**. Then, copy and paste
the implementation of your **`lab1_ex4()`** function into
**`lab1_ex5()`** to make a starting point for this exercise.
Choose one of these activities to further develop and exhibit your coding skills:
* **Make a cooler "ending" to the program.** Instead of just filling
the NeoPixels with white, make them animate, have the CPX play a
sound, or do some other behavior that is more dramatic or
entertaining than the original specification. You can always refer
to the
[AdaFruit CircuitPython Made Easy articles](https://learn.adafruit.com/circuitpython-made-easy-on-circuit-playground-express)
for inspiration and tutorials.
* **Improve the program.** If you know how to make the program
shorter and easier to maintain by using functions, list indexing
operations, and/or loops, do so. The improved version should be
substantially shorter and more readable than the result after
Exercise 4. (If you don't know how to accomplish this, don't
worry, just choose a different activity.)
* **Propose your own enhancement.** If neither of the above options suits,
consult with me in person or over the [Ed discussion forum](https://edstem.org/) about another
enhancement of your group's choosing.
In your writeup, please answer the following:
* **Q8)** What "Going further" activity did you choose? Briefly
describe how you went about completing this exercise. If the
behavior of the CPX is different from exercise 4, please shoot
another video, share it with me (no notifications please) and
include the video URL in your answer.
# Lab Report
Within one week after your lab, submit the following files to the lab assignment on Moodle:
* Your completed `code.py` file.
* A PDF writeup with your names, the assignment name, answers to
all questions **Q1**-**Q8** above as well as **Q9** below:
* **Q9)** Did all group members contribute equally to all tasks?
If not, who did what for each exercise?
!!! Warning
Don't forget to use the [provided lab report template](https://docs.google.com/document/d/1zX4XIjqBkH3_dSBM4zfjCBaGEOoiZbpREtcKvYVq8Ss/copy) for your PDF writeup.
## Grading
Points will be assigned approximately according to the following breakdown:
Item
Grade value
Answers to **Q1**-**Q9** and general writeup quality