Four Digit LED Display

Welcome to the 4-Digit Display Lab

In this lab, you will build a working clock using a 4-digit LED display! You will learn how to read the time and show it with a flashing colon. Let's build something amazing!

What Is a 4-Digit LED Display?

A 4-digit LED display is a row of four 7-segment digit displays. Each digit is made of seven LED segments that light up in different combinations to show numbers from 0 to 9. Many of these displays also have a colon (:) between the second and third digits — perfect for showing hours and minutes on a clock.

You can buy these displays for about $2 each.

These displays use the TM1637 chip to communicate with the Pico. You will use Mike Causer's tm1637 library to talk to the chip. Download this file and copy it to your Pico before running the examples below.

Connections

The display has four pins:

1. GND — ground (negative)
2. VCC — power (3.3 V or 5 V)
3. DIO — data signal (connects to a GPIO pin)
4. CLK — clock signal (connects to a GPIO pin)

Wiring Steps

1. Connect GND on the display to any GND pin on the Pico.
2. Connect VCC on the display to the 3.3 V pin on the Pico.
3. Connect DIO on the display to GP0 on the Pico.
4. Connect CLK on the display to GP1 on the Pico.

Quick Start: Show a Number

This example writes the number 1234 to the display.

from machine import Pin    # import Pin to control GPIO pins
from time import sleep     # import sleep for delays
import tm1637              # import the TM1637 display driver

# Pin numbers for data and clock
DIO_PIN = 0   # data signal on GP0
CLK_PIN = 1   # clock signal on GP1

# Set up the display using the TM1637 library
tm = tm1637.TM1637(clk=Pin(CLK_PIN), dio=Pin(DIO_PIN))

# Display the digits 1, 2, 3, 4
tm.write([1, 2, 3, 4])

What Each Line Does

Line	Purpose
import tm1637	Loads the TM1637 driver library
tm1637.TM1637(clk=Pin(CLK_PIN), dio=Pin(DIO_PIN))	Creates the display object using GP0 and GP1
tm.write([1, 2, 3, 4])	Sends the digits 1, 2, 3, 4 to the display

The tm.write() function takes a list of numbers and shifts them in from right to left.

Key Idea

The TM1637 chip handles all the hard work. You just tell it which digits to show and it figures out which LED segments to light up. The tm.numbers() function makes showing hours and minutes even easier!

Clock

You can build a simple clock using the localtime() function. It reads the current time from the Pico's internal clock. Call it once when the program starts, then count the seconds yourself with sleep().

localtime() returns a list of numbers. The numbers at positions 3, 4, and 5 hold the hour, minute, and second.

import tm1637                        # import the TM1637 display driver
from machine import Pin              # import Pin to control GPIO pins
from utime import sleep, localtime   # import sleep and localtime

# Set up the display with clock on GP1 and data on GP0
tm = tm1637.TM1637(clk=Pin(1), dio=Pin(0))

# Read the current time once at startup
now    = localtime()
hour   = now[3]   # the hour is at position 3 in the localtime list
minute = now[4]   # the minute is at position 4
sec    = now[5]   # the second is at position 5

# Convert from 24-hour to 12-hour format
if hour > 12:
    hour = hour - 12

print(hour, ':', minute, ' ', sec, sep='')   # print the time to the console

# Main loop — update the display every half-second to flash the colon
while True:
    tm.numbers(hour, minute, colon=True)   # show time with the colon lit
    sleep(0.5)                             # wait half a second

    tm.numbers(hour, minute, colon=False)  # show time with the colon off
    sleep(0.5)                             # wait half a second

    sec = sec + 1         # add one second to the counter
    if sec == 60:         # if we reach 60 seconds, reset and add a minute
        minute = minute + 1
        sec = 0
        if minute == 60:  # if we reach 60 minutes, reset and add an hour
            hour = hour + 1
            minute = 0
            if hour == 24:  # if we reach 24 hours, reset to midnight
                hour = 0

What Each Line Does

Line	Purpose
localtime()	Returns a list of numbers describing the current date and time
now[3]	Gets the hour (position 3 in the localtime list)
now[4]	Gets the minute (position 4)
now[5]	Gets the second (position 5)
tm.numbers(hour, minute, colon=True)	Shows the hour and minute with the colon lit
sleep(0.5)	Waits half a second — the colon flashes once per second
sec = sec + 1	Counts up one second each loop

Watch Out!

The Pico's internal clock resets every time you unplug the power. The time shown will be wrong after a restart unless you set the clock or use a Pico W to fetch the time from the internet.

Accurate Clock

This version reads the real time from the system once every minute. That keeps the display more accurate over time.

import tm1637                        # import the TM1637 driver
from machine import Pin              # import Pin for GPIO
from utime import sleep, localtime   # import sleep and localtime

hour   = 0   # start with hour 0
minute = 0   # start with minute 0
sec    = 0   # start with second 0

def update_time():
    """Read the current time from the Pico's internal clock."""
    global hour, minute, sec    # tell Python to update the global variables
    now    = localtime()
    hour   = now[3]             # get the current hour
    if hour > 12:
        hour = hour - 12        # convert to 12-hour format
    minute = now[4]             # get the current minute
    sec    = now[5]             # get the current second

# Set up the display with clock on GP1 and data on GP0
tm = tm1637.TM1637(clk=Pin(1), dio=Pin(0))

update_time()   # set the time before the loop starts

while True:
    tm.numbers(hour, minute, colon=True)    # show time, colon on
    sleep(0.5)
    tm.numbers(hour, minute, colon=False)   # show time, colon off
    sleep(0.5)
    sec = sec + 1       # count one second
    if sec == 60:
        update_time()   # re-read the real time every 60 seconds
        print(hour, ':', minute, ' ', sec, sep='')
        minute = minute + 1
        sec = 0
        if minute == 60:
            hour = hour + 1
            minute = 0
            if hour == 24:
                hour = 0

What Each Line Does

Line	Purpose
def update_time():	Defines a function that reads the real clock
global hour, minute, sec	Allows the function to change these variables outside itself
if sec == 60: update_time()	Refreshes the time from the real clock each minute

Monty's Tip

Try using a Pico W and a Wi-Fi connection to fetch the correct time from the internet. Search for "MicroPython NTP" (Network Time Protocol) to learn how!

References

• Nerd Cave YouTube Tutorial
• Mike Causer's GitHub Repo with TM-1637 driver

Great Work!

You built a working clock with a flashing colon! Next, try the Character LCD lab to display full sentences on a 16×2 text screen.