Python in the physical world · Day 2

Make it decide

Turn inputs into decisions, then build a reaction-time challenge with a button, a potentiometer, and clear Python choices.

Mission: turn numbers and button signals into behaviour you can predict, test, and explain.

Press S for notesPress F for fullscreen

Learning goals

By the end, you can…

Sort values

Tell the difference between whole-number, decimal, and Boolean values.

Cast a value

Use int(...) to turn a calculated decimal into a whole number.

Read a question

Explain how a comparison becomes True or False.

Choose a path

Use if, elif, and else for different outcomes.

Remember results

Store several reaction times in a list with append, len, and min.

Stop a loop on purpose

Use while conditions that end when the state changes.

Today’s route

Numbers → questions → choices → circuits → game

Types and
casting
→
Boolean values
and comparisons
→
not and
active-low input
→
Decision code
and two builds
→
Lists and loops
for the game

Each circuit appears right before you need to build or test it.

01

Part 1 · Start with values

What kind of value is Python holding?

Before a program can decide, it needs values in forms Python understands.

Kinds of values

Whole numbers and decimal numbers do different jobs

Watch and discuss · Say what each line stores


raw_value = 32768
fraction = raw_value / 65535

print(type(raw_value))
print(type(fraction))

int

A whole number such as 32768. Useful for counts, pin numbers, and many sensor readings.

float

A number with a decimal part, such as the result of dividing one number by another.

Casting

Sometimes you want a new value of a different type

Try in Thonny · Run the lines and inspect each result


raw_value = 32768
fraction = raw_value / 65535
percent = int(fraction * 100)

print(fraction)
print(percent)
print(type(percent))

Casting means asking Python to make a new value in a different form. Here int(...) drops the decimal part.

02

Part 2 · Ask a question

Python can decide only after a question becomes true or false

Comparisons produce Boolean values, and Boolean values guide later choices in the code.

Boolean values

A comparison asks a question with one of two answers

Try in Thonny · Predict before pressing Run


score = 347

print(score < 350)
print(score >= 350)
print(score == 347)

True

The question is correct.

False

The question is not correct.

Comparison

<, >=, and == compare values and produce a Boolean answer.

A common mix-up

= stores a value; == asks whether two values match

Watch and discuss · Read each line aloud in words

CodeRead it as…Job
reaction_ms = 412“Store 412 in reaction_ms.”Assignment
reaction_ms == 412“Is reaction_ms equal to 412?”Comparison
reaction_ms != 412“Is reaction_ms different from 412?”Comparison

Use = to store. Use == to ask “equal?” and != to ask “different?”

not and active-low input

The button event makes the electrical value low

On paper · Fill in the last column before checking

Physical statebutton.value()not button.value()Meaning we want
Released1Falsenot pressed
Pressed0Truepressed

Active-low means the important event makes the signal low. not turns that electrical reading into a natural Python answer.

Choosing a branch

if, elif, and else choose one path

Watch and discuss · Which branch runs at 10%, 45%, and 92%?


if percent < 20:
    led.duty_u16(0)
elif percent <= 70:
    led.duty_u16(20000)
else:
    led.duty_u16(65535)

Python checks from top to bottom and runs only the first branch whose condition is True.

03

Part 3 · Build an active-low input

Use a button to create a real Boolean question

Now the code and the circuit meet: GP13 reads the question, and GP15 shows the answer.

Build while unplugged · Task 1

Button input and LED output

Exact diagram + table
Full button-and-LED breadboard diagram with Pico USB on the left, button across the centre gap, a 10 kilo-ohm pull-up to 3.3 volts on GP13, and an LED on GP15 through a 220 ohm resistor to ground.

Run the button program

Test the question with light

Test the circuit · Reconnect only after the partner check

Open and run

01_button_decisions.py


pressed = not button.value()

if pressed:
    led.on()
else:
    led.off()

What you should observe

  1. Released button: LED off.
  2. Press and hold: LED on and the Shell prints Pressed: True.
  3. Press Stop or Ctrl+C: the LED turns off.

If the program behaves as if the button is always pressed, unplug USB and check the button orientation and the 10 kΩ pull-up before changing code.

04

Part 4 · Use one number to choose a brightness

Use one knob to create many possible values

A potentiometer does not answer yes or no; it gives a range of numbers you can classify.

Build while unplugged · Task 2

Potentiometer on GP26, LED on GP15

Exact diagram + table
Full potentiometer-and-LED breadboard diagram with the potentiometer connected to 3.3 volts, GP26 ADC0, and ground, and a red LED on GP15 through a 220 ohm resistor to ground.

Read the knob

Test the circuit and collect real values

Test the circuit · Turn the shaft slowly from one end to the other

Run

02_potentiometer_led.py


raw_value = knob.read_u16()
percent = int(raw_value / 65535 * 100)

Record

Find readings close to 0%, 25%, 50%, 75%, and 100%. The LED should fade smoothly as the numbers change.

Your measured values do not need to be exact. The important pattern is low knob position → small number, high knob position → large number.

Choose brightness zones

Write three brightness zones

Try in Thonny · Edit the same file and test each zone

Add this branch


if percent < 20:
    led.duty_u16(0)
elif percent <= 70:
    led.duty_u16(20000)
else:
    led.duty_u16(65535)

Concrete test order

  1. Turn the knob near 0% and confirm the LED is off.
  2. Turn near the middle and confirm the LED is dim.
  3. Turn near the top and confirm the LED is bright.
05

Part 5 · Remember and repeat

Use lists and exit conditions for a game

A reaction game needs several results, and every loop must know when to stop.

Lists

One list can keep several reaction times

Try in Thonny · Run each line and explain the result


times = [410, 375, 522]
print(times[0])
times.append(330)
print(len(times))
print(min(times))
print(times[-1])

Index

times[0] asks for the first item.

Append

append adds one new result at the end.

len

len(times) counts how many results are stored.

min

The workshop adds this built-in tool today to find the fastest score.

Conditional while

Each loop needs an exit condition you can name

On paper · Match each loop to the moment it stops

LoopRead it as…Stops when…
while len(scores) < ROUNDS:repeat while we still need more scoresthe list already holds ROUNDS results
while not button.value():wait while the button is pressedthe button is released
while button.value():wait while the button is releasedthe button is pressed

These loops stop because the condition changes. You do not need an endless loop for every job.

Reaction game circuit reminder

Reuse the same button-and-LED circuit

Build while unplugged · Rebuild only if you took the circuit apart

Reminder of the button-and-LED circuit used again for the reaction game.

Keep the same wiring

  • GP13 reads the button.
  • 10 kΩ pull-up connects GP13 to 3.3 V.
  • GP15 drives the LED through 220 Ω.
  • Use the exact diagram and table.

Plan the game first

Put one round in order before you type it

On paper · Number the steps from first to last

Open

03_reaction_game_starter.py

The imports, pin setup, scores = [], and LED cleanup are already supplied.

One round, in order

  1. Wait until the button is released.
  2. Wait a random number of milliseconds.
  3. Turn on the LED and record the start time.
  4. Wait until the button is pressed.
  5. Calculate reaction_ms and turn the LED off.
  6. Add the result to scores.
  7. Print a message for the result.

Build the starter

Replace pass with working game logic

Try in Thonny · Get one round working before you add all three

Write these parts

  1. Inside try:, delete pass.
  2. Write one working round using the two button waits.
  3. Add scores.append(reaction_ms).
  4. Add if reaction_ms < 350 → Quick!, else Keep practising!.
  5. Wrap the round in while len(scores) < ROUNDS:.

Use the supplied tools as written


wait_ms = randint(1000, 3000)
sleep_ms(wait_ms)
start_ms = ticks_ms()
end_ms = ticks_ms()
reaction_ms = ticks_diff(end_ms, start_ms)

The timing tools and the try/except/finally cleanup are already provided. Today you focus on the game logic inside them.

Check the result

Test the game with evidence you can show

Test the circuit · Run the full three-round version

Your program should show

  1. The LED waits, then turns on.
  2. A button press creates one whole-number reaction time.
  3. Each round prints either Quick! or Keep practising!.
  4. After three rounds, the Shell prints the full list and the fastest score with min(scores).

Useful checks if something looks wrong

  1. If the LED never turns on, print wait_ms and confirm the program is still waiting.
  2. If the game never notices a press, test the button again with 01_button_decisions.py.
  3. If a result looks impossible, print start_ms and reaction_ms for one round.

Exit ticket

Write one exact condition for the middle range

On paper · Answer before packing away the hardware

Write a condition that is true when reaction_ms is at least 200 and below 500. Then explain what happens exactly at 200 and exactly at 500.

Day 2 completeStop programs · unplug USB · leave parts sorted

Source reference

Where today’s sequence comes from

TEALS Unit 2 ideas

Types and casting, Boolean values, comparison operators, = versus ==, not, if/elif/else, lists, and while loops with clear stopping conditions.

Workshop-specific hardware tasks

Active-low button input on GP13, PWM LED on GP15, potentiometer reading on GP26 / ADC0, and the student-built reaction game starter using supplied timing and cleanup code.

Workshop links used in this deck: ../code/day-2/01_button_decisions.py, ../code/day-2/02_potentiometer_led.py, ../code/day-2/03_reaction_game_starter.py, ../diagrams/button-and-led.html, and ../diagrams/potentiometer-and-led.html.