Pico W traffic light with a countdown display

# Traffic light: green on GP7, yellow on GP8 and red on GP9, each through
# 220 ohm. A Timer ticks once a second: green 30 s, yellow 5 s, red 30 s,
# and the four-digit display counts down what is left.
from machine import Pin, Timer
import time

DIGITS = [0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F]
ds = Pin(18, Pin.OUT)
latch = Pin(19, Pin.OUT)
clock = Pin(20, Pin.OUT)
selects = [Pin(n, Pin.OUT, value=1) for n in (10, 11, 12, 13)]   # digit 1 to 4

def segments(byte):
    for i in range(7, -1, -1):
        ds.value((byte >> i) & 1)
        clock.value(1)
        clock.value(0)
    latch.value(1)
    latch.value(0)

def show(text):
    # text is four characters, digits or spaces, with an optional "." after
    # one of them. Each digit is lit for a moment in turn, fast enough
    # that the eye sees all four.
    cells = []
    for ch in text:
        if ch == "." and cells:
            cells[-1] |= 0x80
        else:
            cells.append(DIGITS[int(ch)] if ch.isdigit() else 0)
    for n, byte in enumerate(cells[-4:]):
        segments(byte)
        selects[n].value(0)
        time.sleep_ms(2)
        selects[n].value(1)

lights = [Pin(n, Pin.OUT, value=0) for n in (7, 8, 9)]   # green, yellow, red
TIMES = [30, 5, 30]
NAMES = ["green", "yellow", "red"]
state = 0
left = TIMES[0]

def tick(timer):
    global state, left
    left -= 1
    if left == 0:
        state = (state + 1) % 3
        left = TIMES[state]
        print(NAMES[state])

Timer(period=1000, mode=Timer.PERIODIC, callback=tick)
print(NAMES[state])
while True:
    for n, light in enumerate(lights):
        light.value(1 if n == state else 0)
    show("%4d" % left)