In the previous section, we already learned how to display four digits on a 7-segment module and wrote the program for displaying a 4-digit number (ch02_04.py). In this section, we continue to build a simple program for a countdown timer using a 4-digit 7-segment module and two 74HC595 shift registers.

Our scenario is to get a number input from the user, for instance, 30. After this, the number is displayed on the module. Then, we decrease the number down to 0.

Let's copy the ch02_04.py file and then modify it as follows:

# ch02_05.py
…
…
print("Running...")
number_s = raw_input("Enter a number (1-999): ")
number = int(number_s)
print("Countdown " + number_s)
try:
    timer = 0

    while 1:
        digit = number

        LED_display(0, digit % 10, 0)
        digit /= 10
        time.sleep(0.01)

        LED_display(1, digit % 10, 0)
        time.sleep(0.01)
        digit /= 10

        LED_display(2, digit % 10, 0)
        time.sleep(0.01)
        digit /= 10

...