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4.1.7 Traffic Light

Introduction

In this project, we will use LED lights of three colors to realize the change of traffic lights and a four-digit 7-segment display will be used to display the timing 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
Raphael Kit	337	Raphael Kit

You can also buy them separately from the links below.

COMPONENT INTRODUCTION	PURCHASE LINK
GPIO Extension Board	BUY
Breadboard	BUY
Jumper Wires	BUY
Resistor	BUY
LED	BUY
4-Digit 7-Segment Display	-
74HC595	BUY

Schematic Diagram

T-Board Name	physical	wiringPi	BCM
GPIO17	Pin 11	0	17
GPIO27	Pin 13	2	27
GPIO22	Pin 15	3	22
SPIMOSI	Pin 19	12	10
GPIO18	Pin 12	1	18
GPIO23	Pin 16	4	23
GPIO24	Pin 18	5	24
GPIO25	Pin 22	6	25
SPICE0	Pin 24	10	8
SPICE1	Pin 26	11	7

Experimental Procedures

Step 1: Build the circuit.

Step 2: Change directory.

cd ~/raphael-kit/python-pi5

Step 3: Run.

sudo python3 4.1.12_TrafficLight_zero.py

As the code runs, LEDs will simulate the color changing of traffic lights. Firstly, the red LED lights up for 60s, then the green LED lights up for 30s; next, the yellow LED lights up for 5s. After that, the red LED lights up for 60s once again. In this way, this series of actions will be executed repeatedly. Meanwhile, the 4-digit 7-segment display displays the countdown time continuously.

Code

Note

You can Modify/Reset/Copy/Run/Stop the code below. But before that, you need to go to source code path like raphael-kit/python-pi5. After modifying the code, you can run it directly to see the effect.

#!/usr/bin/env python3

from gpiozero import OutputDevice, LED
import threading

# Setup GPIO pins for 74HC595 shift register
SDI = OutputDevice(24)   # Serial Data Input
RCLK = OutputDevice(23)  # Register Clock
SRCLK = OutputDevice(18) # Shift Register Clock

# Setup GPIO pins for digit selection on 7-segment display
placePin = [OutputDevice(pin) for pin in (10, 22, 27, 17)]

# Segment codes for numbers 0-9 on 7-segment display
number = (0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xf8, 0x80, 0x90)

# Setup GPIO pins for traffic light LEDs
ledPinR = LED(25) # Red LED
ledPinG = LED(8)  # Green LED
ledPinY = LED(7)  # Yellow LED

# Duration settings for traffic lights
greenLight = 30
yellowLight = 5
redLight = 60

# Traffic light color names
lightColor = ("Red", "Green", "Yellow")

# Initialize state variables
colorState = 0
counter = 60
timer1 = None

def setup():
    """ Initialize the traffic light system and start the timer. """
    global timer1
    timer1 = threading.Timer(1.0, timer)
    timer1.start()

def clearDisplay():
    """ Clear the 7-segment display. """
    for _ in range(8):
        SDI.on()
        SRCLK.on()
        SRCLK.off()
    RCLK.on()
    RCLK.off()

def hc595_shift(data):
    """ Shift data to the 74HC595 shift register for digit display. """
    for i in range(8):
        SDI.value = 0x80 & (data << i)
        SRCLK.on()
        SRCLK.off()
    RCLK.on()
    RCLK.off()

def pickDigit(digit):
    """ Select a specific digit to display on the 7-segment display. """
    for pin in placePin:
        pin.off()
    placePin[digit].on()

def timer():
    """ Handle the timing for traffic light changes. """
    global counter, colorState, timer1
    timer1 = threading.Timer(1.0, timer)
    timer1.start()
    counter -= 1
    if counter == 0:
        counter = [greenLight, yellowLight, redLight][colorState]
        colorState = (colorState + 1) % 3
    print(f"counter : {counter}    color: {lightColor[colorState]}")

def lightup():
    """ Update the traffic light LED based on the current state. """
    global colorState
    ledPinR.off()
    ledPinG.off()
    ledPinY.off()
    [ledPinR, ledPinG, ledPinY][colorState].on()

def display():
    """ Display the current counter value on the 7-segment display. """
    global counter

    for i in range(4):
        digit = counter // (10 ** (3 - i)) % 10
        if i == 0 and digit == 0:
            continue
        clearDisplay()
        pickDigit(3 - i)
        hc595_shift(number[digit])

def loop():
    """ Main loop to continuously update display and lights. """
    while True:
        display()
        lightup()

def destroy():
    """ Clean up resources when the script is terminated. """
    global timer1
    timer1.cancel()
    ledPinR.off()
    ledPinG.off()
    ledPinY.off()

try:
    setup()
    loop()
except KeyboardInterrupt:
    destroy()

Code Explanation

1. Imports the OutputDevice and LED classes from the gpiozero library, enabling control of general output devices and specifically LEDs. Imports Python’s threading module, which will be used for creating and handling threads for concurrent execution.

   #!/usr/bin/env python3
   from gpiozero import OutputDevice, LED
   import threading
   

2. Initializes GPIO pins connected to the shift register’s Serial Data Input (SDI), Register Clock Input (RCLK), and Shift Register Clock Input (SRCLK).

   # Setup GPIO pins for 74HC595 shift register
   SDI = OutputDevice(24)   # Serial Data Input
   RCLK = OutputDevice(23)  # Register Clock
   SRCLK = OutputDevice(18) # Shift Register Clock
   

3. Initializes the pins for each digit of the 7-segment display and defines the binary codes for displaying numbers 0-9.

   # Setup GPIO pins for digit selection on 7-segment display
   placePin = [OutputDevice(pin) for pin in (10, 22, 27, 17)]
   
   # Segment codes for numbers 0-9 on 7-segment display
   number = (0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xf8, 0x80, 0x90)
   

4. Initializes GPIO pins for the Red, Green, and Yellow LEDs used in the traffic light simulation. Sets the duration (in seconds) for each color state in the traffic light system. Defines the names of the traffic light colors for reference.

   # Setup GPIO pins for traffic light LEDs
   ledPinR = LED(25) # Red LED
   ledPinG = LED(8)  # Green LED
   ledPinY = LED(7)  # Yellow LED
   
   # Duration settings for traffic lights
   greenLight = 30
   yellowLight = 5
   redLight = 60
   
   # Traffic light color names
   lightColor = ("Red", "Green", "Yellow")
   

5. Initializes variables for tracking the current color state, a counter for timing, and a placeholder for a timer object.

   # Initialize state variables
   colorState = 0
   counter = 60
   timer1 = None
   

6. Initializes the traffic light system and starts the timer thread.

   def setup():
       """ Initialize the traffic light system and start the timer. """
       global timer1
       timer1 = threading.Timer(1.0, timer)
       timer1.start()
   

7. Functions to control the 7-segment display. clearDisplay turns off all segments, hc595_shift shifts data into the shift register, and pickDigit activates a specific digit on the display.

   def clearDisplay():
       """ Clear the 7-segment display. """
       for _ in range(8):
           SDI.on()
           SRCLK.on()
           SRCLK.off()
       RCLK.on()
       RCLK.off()
   
   def hc595_shift(data):
       """ Shift data to the 74HC595 shift register for digit display. """
       for i in range(8):
           SDI.value = 0x80 & (data << i)
           SRCLK.on()
           SRCLK.off()
       RCLK.on()
       RCLK.off()
   
   def pickDigit(digit):
       """ Select a specific digit to display on the 7-segment display. """
       for pin in placePin:
           pin.off()
       placePin[digit].on()
   

8. Manages the timing for traffic light changes and updates the counter and color state.

   def timer():
       """ Handle the timing for traffic light changes. """
       global counter, colorState, timer1
       timer1 = threading.Timer(1.0, timer)
       timer1.start()
       counter -= 1
       if counter == 0:
           counter = [greenLight, yellowLight, redLight][colorState]
           colorState = (colorState + 1) % 3
       print(f"counter : {counter}    color: {lightColor[colorState]}")
   

9. Updates the state of the traffic light LEDs based on the current color state.

   def lightup():
       """ Update the traffic light LED based on the current state. """
       global colorState
       ledPinR.off()
       ledPinG.off()
       ledPinY.off()
       [ledPinR, ledPinG, ledPinY][colorState].on()
   

10. Calculates the digit to be displayed on each segment of the 7-segment display and updates it accordingly.

    def display():
        """ Display the current counter value on the 7-segment display. """
        global counter
    
        for i in range(4):
            digit = counter // (10 ** (3 - i)) % 10
            if i == 0 and digit == 0:
                continue
            clearDisplay()
            pickDigit(3 - i)
            hc595_shift(number[digit])
    

11. The main loop that continuously updates the display and the traffic light LEDs.

    def loop():
        """ Main loop to continuously update display and lights. """
        while True:
            display()
            lightup()
    

12. Cleans up resources when the script is terminated, such as turning off LEDs and stopping the timer thread.

    def destroy():
        """ Clean up resources when the script is terminated. """
        global timer1
        timer1.cancel()
        ledPinR.off()
        ledPinG.off()
        ledPinY.off()