Lab 06: The Red Second Hand

The most famous part of a Swiss station clock is its red second hand: a thin rod that ends in a round disc, like the signal paddle a train dispatcher used to hold up. This lab adds it, and your face is done, frozen at 10:10:25.

Welcome to Lab 06

This is the finished picture, the face you'll animate for the rest of the kit. Only one idea is new here, and it decides what lies on top of what. Let's make time tick!

What You Will Learn

• why drawing order matters
• how a hand can be built from two shapes
• how the red disc is sized so it just touches the bars

Run It

Open 06-red-second-hand.py in Thonny and click Run. The dial and black hands appear, and the red hand arrives last, lying on top of the black ones where they cross near the center.

Two Shapes Make One Hand

The second hand is a thin stem and a disc:

Part	Size in a
Stem width	1.4
Stem reaches back (the tail)	16.5 behind the center pin
Disc diameter	10.5
Disc center	31.25 out from the center

def second_hand(display, angle, color):
    bar(display, angle, -16.5 * A, 31.25 * A, 0.7 * A, color)
    x, y = point(angle, 31.25 * A)
    shapes.circle(display, x, y, round(5.25 * A), color, config.FILL)

The stem is a bar() from lab 03, with a negative start so it reaches behind the center. The disc is a filled circle at the point 31.25 a along the hand.

A Tidy Detail

The disc's outer edge is 31.25 + 5.25 = 36.5 a from the center. That is exactly where the heavy five-minute bars begin. So when the red hand points straight at an hour, the disc just touches the bar, as if the clock had been designed so that the two fit like puzzle pieces. They were.

Draw Order

The program draws things in this order, and the order is the whole point:

1. the dial and its 60 bars
2. the black hour and minute hands
3. the red second hand

second = 25
...
second_hand(display, second * 6, config.RAIL_RED)    # last, so it is on top

Whatever is drawn last lies on top, like laying paper cutouts on a table. If you drew the red hand first, the black hands would cover it near the center. A display with no memory of what is under each shape, like this one, has no way to fix that afterward.

The Red Hand Disappeared!

If the red hand vanishes under the black ones, you moved its drawing to before the hour and minute hands. Each new shape paints over whatever came before, so put second_hand(...) back at the bottom of the file.

Try This

1. Change second = 25 to second = 45. Where does the disc land?
2. Move the second_hand(...) line above the black hands. What changes near the center?
3. Set second = 0, then second = 15. Notice the disc just touching a big bar each time.

The Face Is Finished!

You built a Swiss-railway face from design units, loops, functions, and draw order. Next, you'll stop typing the time into the program and read it from the Pico instead.

Next: Lab 07: Show the Real Time