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2.6 Reed Switch

Introduction

In this project, we will learn how to use a reed switch as a magnetic field sensor to control LEDs. A reed switch is an electrical switch operated by an applied magnetic field, commonly used in security systems, door/window sensors, and position detection. This project demonstrates how to detect the state of a reed switch and provide visual feedback using two LEDs.

What You’ll Need

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

COMPONENT	PURCHASE LINK
Jumper Wires	BUY
Reed Switch Module	BUY
LED	BUY
Resistor	BUY
Fusion HAT+	-
Raspberry Pi	-

Circuit Diagram

The circuit uses a reed switch connected to a GPIO input pin with a pull-down resistor. Two LEDs are connected to separate GPIO output pins, each with a current-limiting resistor. When a magnet approaches the reed switch, it closes the circuit, triggering the LED indicators to change states.

Wiring Diagram

Follow these steps to build the circuit:

Connect the GND & VCC pin of the reed switch to GND & 3.3V
Connect the D0 Pin of the reed switch to GPIO 17
Connect LED1 cathode to GPIO 22 through a 220Ω resistor
Connect LED1 anode to GND
Connect LED2 cathode to GPIO 27 through a 220Ω resistor
Connect LED2 anode to GND

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 2.6_ReedSwitch.py

This Python script creates a magnetic field detection system using a reed switch and two status LEDs. When executed:

The script continuously monitors the state of the reed switch
When a magnet is detected (reed switch closed), LED1 turns off and LED2 turns on
When no magnet is detected (reed switch open), LED1 turns on and LED2 turns off
This creates a clear visual indication of the magnetic field presence
The program can be interrupted gracefully using Ctrl+C

Code

The following Python code monitors a reed switch and controls two LEDs based on its state:

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

# Initialize reed switch (Button) on GPIO pin 17
reed = Pin(17, mode=Mode.IN, pull=Pull.DOWN)

# Initialize LED1 connected to GPIO pin 22
led1 = Pin(22,mode=Mode.OUT)
# Initialize LED2 connected to GPIO pin 27
led2 = Pin(27,mode=Mode.OUT)

try:
    # Continuously monitor the state of the reed switch and control LEDs accordingly
    while True:
        if reed.value() == 0:  # Check if the reed switch is activated
            led1.off()  # Turn off LED1
            led2.on()   # Turn on LED2
        else:  # If the sensor is not activated
            led1.on()   # Turn on LED1
            led2.off()  # Turn off LED2

except KeyboardInterrupt:
    # Handle a keyboard interrupt (Ctrl+C) for a clean exit from the loop
    pass

Understanding the Code

Library Import

The Pin class from the fusion_hat library is used to control GPIO pins, and time.sleep is imported for potential delay operations.
```
from fusion_hat.pin import Pin, Mode, Pull
from time import sleep  # Import sleep for delay
```
Reed Switch Initialization

The reed switch is initialized on GPIO pin 17 as an input with a pull-down resistor to ensure a stable LOW state when the switch is open.
```
# Initialize reed switch (Button) on GPIO pin 17
reed = Pin(17, mode=Mode.IN, pull=Pull.DOWN)
```

LED Initialization

Two LEDs are initialized on GPIO pins 22 and 27 as outputs to provide visual feedback.

# Initialize LED1 connected to GPIO pin 22
led1 = Pin(22,mode=Mode.OUT)
# Initialize LED2 connected to GPIO pin 27
led2 = Pin(27,mode=Mode.OUT)

Main Monitoring Loop

The while True loop continuously checks the reed switch state and controls the LEDs accordingly, creating a clear status indication system.

while True:
    if reed.value() == 0:  # Check if the reed switch is activated
        led1.off()  # Turn off LED1
        led2.on()   # Turn on LED2
    else:  # If the sensor is not activated
        led1.on()   # Turn on LED1
        led2.off()  # Turn off LED2

Keyboard Interrupt Handling

The try-except block ensures the program stops gracefully when interrupted.

except KeyboardInterrupt:
    # Handle a keyboard interrupt (Ctrl+C) for a clean exit from the loop
    pass

Troubleshooting

Reed Switch Not Detecting Magnet
- Cause: Magnet polarity issues, insufficient magnetic strength, or wiring problems.
- Solution: Try flipping the magnet, use a stronger magnet, or verify all connections.
LEDs Not Lighting
- Cause: Incorrect GPIO pin assignments, LED polarity reversed, or insufficient current.
- Solution: Check pin assignments, ensure LEDs are connected with correct polarity (anode to GPIO), and verify resistor values.
Unstable Readings
- Cause: Electrical noise or floating input when reed switch is open.
- Solution: The built-in pull-down resistor should stabilize the input, but ensure proper grounding and check for nearby sources of electromagnetic interference.

Extendable Ideas

Security System

Create a simple door/window security alarm that triggers when a magnet moves away from the reed switch:

# Add a buzzer for audible alarm
from fusion_hat import Buzzer, PWM
buzzer = Buzzer(PWM('P0'))

while True:
    if reed.value() == 1:  # Door opened (magnet moved away)
        buzzer.play('C6', 0.5)  # Sound alarm
        led1.on()  # Red alert LED
    else:
        led2.on()  # Green safe LED
    sleep(0.1)

Position Counter

Count how many times a magnet passes by the reed switch, useful for rotational speed measurement:

counter = 0
last_state = 0

while True:
    current_state = reed.value()
    if last_state == 0 and current_state == 1:  # Detect rising edge
        counter += 1
        print(f"Count: {counter}")
    last_state = current_state
    sleep(0.01)  # Short delay for debouncing

Two-Sensor Detection

Use two reed switches to detect direction of movement:

reed1 = Pin(17, Pin.IN, pull=Pin.PULL_DOWN)
reed2 = Pin(18, Pin.IN, pull=Pin.PULL_DOWN)

# Determine direction based on which sensor triggers first

Conclusion

This project demonstrates how to use a reed switch as a magnetic field sensor to control LED indicators. Reed switches are reliable, simple components widely used in security systems, position sensing, and proximity detection applications. By combining magnetic sensing with visual feedback, you can create intuitive monitoring systems for various applications.