Skip to content

Lab 09: A Digital Clock

The screen shows Sunday in yellow at the top, the time 10 09 in tall white digits made of straight bars, AM in cyan under the digits, and October 11, 2026 in yellow at the bottom. The colon between the 10 and the 09 is dark, in the off half of its blink.

Now the kit becomes a real clock. This program shows the hours and minutes in digits big enough to read from across the room. A colon blinks between them once a second. The day of the week goes above the time, and the date goes below it.

Welcome to Lab 09

Chrono waving welcome This is the lab where your kit stops being a gadget and becomes a clock. You will build its big digits yourself, the same way a calculator builds them. Let's make time tick!

What You Will Learn

  • where the Pico gets the time from
  • how one line of math turns hour 13 into 1 PM
  • how to build any digit out of seven bars
  • how the clock redraws only the digits that change

Run It

  1. Open 09-digital-clock.py in Thonny and click Run.
  2. Check the time against the clock on your computer. They should match.
  3. Watch the colon. It is on for one second, then off for the next.

Where Does the Time Come From?

The Pico has a built-in clock called the RTC, short for real-time clock. The program asks it for the time with time.localtime(), which answers with eight numbers at once:

1
year, month, day, hour, minute, second, weekday, _ = time.localtime()

Each name on the left catches one number. The last name, _, catches a number this clock does not use.

Who set the RTC? Thonny did. Each time Thonny connects to the Pico, it quietly copies the time from your computer.

The Pico Forgets the Time

Chrono warning The Pico has no battery for its clock, so unplugging it sends the RTC back to midnight on January 1st. For now, always start this lab from Thonny. Lab 10 fixes this by asking the internet for the time.

From Hour 13 to 1 PM

The RTC counts hours from 0 to 23. Most clocks show 1 to 12, with AM or PM. One line does the change:

1
hour12 = hour % 12 or 12            # 0 -> 12, 13 -> 1, ...
  • hour % 12 is the first step. The % sign gives the remainder after dividing. 13 รท 12 is 1 with 1 left over, so 13 % 12 is 1.
  • or 12 is the second step. Hours 0 and 12 both leave a remainder of 0, but no clock shows a zero hour. Python treats 0 as nothing at all. So 0 or 12 means: there is nothing here, use 12 instead.
hour hour % 12 after or 12 The clock shows
0 0 12 12 AM (midnight)
9 9 9 9 AM
12 0 12 12 PM (noon)
13 1 1 1 PM
23 11 11 11 PM

AM or PM is a simpler test: every hour below 12 is AM.

Digits Made of Seven Bars

Even the kit's biggest font is small for a clock across the room. So this lab builds its own digits, the way a calculator does. Each digit is made of seven bars called segments, named a to g:

1
2
3
4
5
6
7
 aaaa
f    b
f    b
 gggg
e    c
e    c
 dddd

This part of the program lists the bars to light for every digit:

1
2
3
# Which bars are lit for each digit, 0 to 9.
DIGITS = ("abcdef", "bc", "abdeg", "abcdg", "bcfg",
          "acdfg", "acdefg", "abc", "abcdefg", "abcdfg")

Count from zero. DIGITS[1] is "bc", so a 1 lights b and c, the two bars on the right. DIGITS[8] is "abcdefg", so an 8 lights all seven.

A dictionary called SEGMENTS says where each bar sits inside a digit. You met dictionaries in chapter 21 of Learning Python. This function puts the two together:

1
2
3
4
5
6
7
8
9
def draw_digit(position, digit):
    """Draw a digit, 0 to 9, in one of the four positions. None is a blank."""
    x = DIGIT_X[position]
    display.fill_rect(x, TOP, DIGIT_WIDTH, DIGIT_HEIGHT, config.BLACK)
    if digit is None:
        return
    for name in DIGITS[digit]:
        dx, dy, width, height = SEGMENTS[name]
        display.fill_rect(x + dx, TOP + dy, width, height, DIGIT_COLOR)
  • x = DIGIT_X[position] looks up where this digit starts. Position 0 is the digit on the left.
  • The first fill_rect paints a black box over the old digit.
  • if digit is None: return stops there and leaves a blank. The clock uses that for the first digit of a time like 9:05.
  • The for loop takes the letters for this digit one at a time. It looks up each bar in SEGMENTS and paints it as one filled rectangle.

Only the Digit That Changed

A list called shown remembers the digit in each of the four positions. A digit is drawn again only when it differs from the one on the screen:

1
2
3
4
for position in range(4):
    if digits[position] != shown[position]:
        shown[position] = digits[position]
        draw_digit(position, digits[position])

This is the rule from lab 07: redraw only what changed. The last digit changes once a minute. Most seconds, nothing moves but the colon.

Try This

  1. Change DIGIT_COLOR = config.WHITE to config.GREEN.
  2. Change THICKNESS = 6 to 10 for chunky digits. Then try 2.
  3. Pick a digit that is on your screen right now. In DIGITS, take one letter out of its pattern. Which bar goes dark? Put the letter back when you finish.
  4. Make a 24-hour clock. Change hour12 = hour % 12 or 12 to hour12 = hour. In the afternoon, 3:42 PM shows as 15 42.

If It Doesn't Work

  • The shell says The Pico's clock has not been set. Thonny sets the clock each time it connects. Click Stop to connect again, then Run. If the message comes back, set the clock with lab 10.
  • A digit has a bar missing. Check the DIGITS lines for a typing slip. Each pattern needs its quote marks and its comma.
  • The screen stays blank. Go back to lab 03 and get the display working first.

It's a Real Clock!

Chrono celebrating You turned 24-hour time into AM and PM with % and or. You built ten digits out of just seven bars. Next, your clock learns to ask the internet for the time.

Next: Lab 10: Set the Clock from the Internet