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1.5 Relay Control

Introduction

In this project, we will learn how to control a relay module using a Raspberry Pi. Relays are electrically operated switches that allow low-power circuits to control high-power devices safely. This project demonstrates how to toggle a relay on and off at regular intervals.

What You’ll Need

To complete this project, you will need the following components:

COMPONENT	PURCHASE LINK
Breadboard	BUY
Jumper Wires	BUY
Resistor	BUY
LED	BUY
Transistor	BUY
Relay	BUY
Diode	BUY
Fusion HAT+	-
Raspberry Pi	-

Circuit Diagram

The GPIO signal passes through a 1 kΩ resistor to the transistor base, allowing the transistor to switch the relay coil on and off. When GPIO17 outputs HIGH, the transistor turns on, current flows through the relay coil, the relay activates, and the LED (through the 220 Ω resistor) lights up. When GPIO17 outputs LOW, the transistor turns off, the relay coil is de-energized, the relay releases, and the LED turns off. A flyback diode is placed across the relay coil to protect the circuit from voltage spikes.

Wiring Diagram

Follow these steps to build the circuit:

Running the Example

All example code used in this tutorial is available in the ai-lab-kit directory. Follow these steps to run the example:

cd ~/ai-lab-kit/python/
sudo python3 1.5_Relay.py

After running the script, the relay connected to GPIO 17 repeatedly switches on and off every second. When the relay is turned on, the console displays “Relay open…”, and when it is turned off, “…Relay close” is shown. This cycle continues until you press Ctrl + C, at which point the program stops and the relay is safely turned off.

Code

The following Python code controls a relay module, toggling it on and off:

#!/usr/bin/env python3
from fusion_hat.pin import Pin, Mode
from time import sleep  # Import the sleep function for delay

# Initialize the relay connected to GPIO pin 17
relay = Pin(17,mode=Mode.OUT)

try:
    # Loop to continuously toggle the relay's state every second
    while True:
        print('Relay open...')  # Inform that the relay is being activated
        relay.high()  # Turn on the relay
        sleep(1)   # Maintain the relay in the on state for 1 second

        print('...Relay close')  # Inform that the relay is being deactivated
        relay.low()  # Turn off the relay
        sleep(1)   # Maintain the relay in the off state for 1 second

except KeyboardInterrupt:
    # Handle a keyboard interrupt (Ctrl+C) to exit the loop
    relay.off()  # Ensure the relay is turned off before exiting
    pass

This Python script controls a relay module connected to GPIO pin 17. When executed:

The script enters an infinite loop that toggles the relay state every second
When the relay is activated (HIGH), “Relay open…” is printed to the console
When the relay is deactivated (LOW), “…Relay close” is printed to the console
The program can be interrupted gracefully using Ctrl+C, which ensures the relay is turned off before exiting

Understanding the Code

Library Import

The Pin class from the fusion_hat library is used to control GPIO pins, and time.sleep introduces delays between state changes.
```
from fusion_hat.pin import Pin, Mode
from time import sleep  # Import the sleep function for delay
```
Relay Initialization

The relay is initialized on GPIO pin 17 and set to output mode.
```
# Initialize the relay connected to GPIO pin 17
relay = Pin(17,mode=Mode.OUT)
```

Main Control Loop

The while True loop continuously toggles the relay state with 1-second intervals.

while True:
    print('Relay open...')  # Inform that the relay is being activated
    relay.high()  # Turn on the relay
    sleep(1)   # Maintain the relay in the on state for 1 second

    print('...Relay close')  # Inform that the relay is being deactivated
    relay.low()  # Turn off the relay
    sleep(1)   # Maintain the relay in the off state for 1 second

Keyboard Interrupt Handling

The try-except block ensures the program stops gracefully when interrupted (e.g., Ctrl+C), and turns off the relay before exiting.

except KeyboardInterrupt:
    # Handle a keyboard interrupt (Ctrl+C) to exit the loop
    relay.off()  # Ensure the relay is turned off before exiting
    pass

Troubleshooting

Relay Not Responding
- Cause: Incorrect wiring or wrong GPIO pin configuration.
- Solution: Verify all connections match the wiring diagram and confirm GPIO 17 is correctly specified.
Relay Stays On or Off
- Cause: Logic level mismatch or power supply issues.
- Solution: Ensure the relay module is compatible with 3.3V logic levels and check that the power supply is adequate.
Program Doesn’t Exit Cleanly
- Cause: Keyboard interrupt not properly handled.
- Solution: Make sure to use Ctrl+C to terminate the program and check that the exception handling code is present.

Extendable Ideas

Scheduled Control

Modify the code to control the relay based on a schedule, turning it on and off at specific times:

import datetime

# Turn relay on during specific hours
current_time = datetime.datetime.now().time()
if datetime.time(9, 0) <= current_time <= datetime.time(17, 0):
    relay.high()
else:
    relay.low()

Sensor-Activated Control

Connect a sensor (like a temperature or motion sensor) and control the relay based on sensor readings:

# Pseudocode for sensor-controlled relay
if temperature_sensor.read() > 25:  # If temperature exceeds 25°C
    relay.high()  # Turn on cooling device
else:
    relay.low()   # Turn off cooling device

Multiple Relay Control

Expand the project to control multiple relays simultaneously:

# Initialize multiple relays
relays = {
    'light': Pin(17, Pin.OUT),
    'fan': Pin(18, Pin.OUT),
    'pump': Pin(19, Pin.OUT)
}

# Control individual relays
relays['light'].high()
relays['fan'].low()

Conclusion

This project demonstrates how to control a relay module using a Raspberry Pi. Relays are fundamental components in home automation, industrial control systems, and IoT applications. By mastering relay control, you can interface low-voltage circuits with high-power devices safely and effectively.