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7.6 Traffic Light

Traffic Light is a signal device located at roadway intersections, crosswalks and other locations to control the flow of traffic.

Traffic signals are standardized by the Vienna Convention on Road Signs and Signals. Provides users with the right-of-way by alternating LEDs in three standard colors.

• Red light: Traffic should stop if it sees a flashing red light, equivalent to a stop sign.

• Yellow light: A warning signal is about to turn red. Yellow lights are interpreted differently in different countries (regions).

• Green light: Allows traffic to move in the indicated direction.

In this project, we will use three colors of LEDs to implement traffic light changes and a 4-digit 7-segment display to show the time of each traffic state.

Required Components

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Kepler Kit	450+	Kepler Ultimate Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico W	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Resistor	7(220Ω)	BUY
6	4-Digit 7-Segment Display	1
7	74HC595	1	BUY
8	LED	1	BUY

Schematic

• This circuit is based on the 5.3 Time Counter with the addition of 3 LEDs.

• The 3 red, yellow and green LEDs are connected to GP7~GP9 respectively.

Wiring

Code

Note

• Open the 7.6_traffic_light.py file under the path of kepler-kit-main/micropython or copy this code into Thonny, then click “Run Current Script” or simply press F5 to run it.

• Don’t forget to click on the “MicroPython (Raspberry Pi Pico)” interpreter in the bottom right corner.

• For detailed tutorials, please refer to Open and Run Code Directly.

import machine
import time
from machine import Timer

# Define the duration for each traffic light color in seconds [Green, Yellow, Red]
lightTime = [30, 5, 30]

# 7-segment display codes for digits 0-9, using hexadecimal to represent LED segments
SEGCODE = [0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f]

# Initialize pins for shift register communication (74HC595)
sdi = machine.Pin(18, machine.Pin.OUT)   # Serial Data Input
rclk = machine.Pin(19, machine.Pin.OUT)  # Register Clock (Latch)
srclk = machine.Pin(20, machine.Pin.OUT) # Shift Register Clock

# Initialize list to store 4 digit control pins for the 7-segment display
placePin = []
pin = [10, 13, 12, 11]  # Pin numbers for the 4-digit display
for i in range(4):
    placePin.append(None)  # Reserve space in list
    placePin[i] = machine.Pin(pin[i], machine.Pin.OUT)  # Initialize pins as output

# Function to select which digit (0-3) to display by controlling the common anode pins
def pickDigit(digit):
    for i in range(4):
        placePin[i].value(1)  # Turn off all digits
    placePin[digit].value(0)  # Turn on the selected digit

# Function to clear the display by sending '0x00' to the shift register
def clearDisplay():
    hc595_shift(0x00)

# Function to send data to the shift register (74HC595)
def hc595_shift(dat):
    rclk.low()  # Pull latch low to prepare for data shifting
    time.sleep_us(200)  # Small delay for timing stability
    for bit in range(7, -1, -1):  # Loop through each bit (MSB first)
        srclk.low()  # Prepare to send the next bit
        time.sleep_us(200)
        value = 1 & (dat >> bit)  # Extract the current bit from the data
        sdi.value(value)  # Set the data line to the current bit value
        time.sleep_us(200)
        srclk.high()  # Pulse the shift clock to store the bit in the register
        time.sleep_us(200)
    time.sleep_us(200)
    rclk.high()  # Pulse the register clock to move the data to the output

# Function to display a number on the 7-segment display
# This function breaks down the number into its individual digits and displays them
def display(num):
    pickDigit(0)  # Select the units place
    hc595_shift(SEGCODE[num % 10])  # Display units

    pickDigit(1)  # Select the tens place
    hc595_shift(SEGCODE[num % 100 // 10])  # Display tens

    pickDigit(2)  # Select the hundreds place
    hc595_shift(SEGCODE[num % 1000 // 100])  # Display hundreds

    pickDigit(3)  # Select the thousands place
    hc595_shift(SEGCODE[num % 10000 // 1000])  # Display thousands

# Setup for traffic light LEDs (Red, Yellow, Green)
# LEDs are connected to pins 9 (Green), 8 (Yellow), and 7 (Red)
pin = [7, 8, 9]  # LED pin numbers
led = []
for i in range(3):
    led.append(None)  # Reserve space in list
    led[i] = machine.Pin(pin[i], machine.Pin.OUT)  # Initialize each pin as output for LEDs

# Function to turn on the correct LED based on the current state
# 0 = Green, 1 = Yellow, 2 = Red
def lightup(state):
    for i in range(3):
        led[i].value(0)  # Turn off all LEDs
    led[state].value(1)  # Turn on the selected LED (Green, Yellow, or Red)

# Timer-related variables
counter = 0  # Counter for the remaining time
color_state = 0  # Current state of the traffic light (0 = Green, 1 = Yellow, 2 = Red)

# Timer interrupt callback to update the traffic light state and counter
def time_count(ev):
    global counter, color_state
    counter -= 1  # Decrease the counter by 1 second
    if counter <= 0:  # If the counter reaches zero, switch to the next light color
        color_state = (color_state + 1) % 3  # Cycle through Green, Yellow, and Red
        counter = lightTime[color_state]  # Reset counter based on the new color's duration

# Initialize a timer to call the time_count function every 1 second (1000ms)
tim = Timer(period=1000, mode=Timer.PERIODIC, callback=time_count)

# Main loop to update the 7-segment display and traffic light LEDs
while True:
    display(counter)  # Update the display with the remaining time
    lightup(color_state)  # Update the traffic light LEDs based on the current color

When the code runs, the green LED stays on for 30 seconds, the yellow LED stays on for 5 seconds, and the green LED stays on for 30 seconds.