Lesson 11 Reed Switch

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A reed switch (as shown below) is used to detect the magnetic field. Hall sensors are generally used to measure the speed of intelligent vehicles and count in assembly lines, while reed switches are often used to detect the existence of a magnetic field.


– 1 * Raspberry Pi

– 1 * Breadboard

– 1 * Network cable (or USB wireless network adapter)

– 1 * Reed switch module

– 1 * Dual-color LED module

– 2 * 3-Pin anti-reverse cable

– 1 * Magnet (Self provided)

Experimental Principle

A reed switch is a type of line switch component that realizes control by magnetic signals. It induces by a magnet. The “switch” here means dry reed pipe, which is a kind of contact passive electronic switch component with the advantage of simple structure, small size, and convenient control. The shell of a reed switch is commonly a sealed glass pipe in which two iron elastic reed electroplates are equipped and inert gases are filled. Normally, the two reeds made of special materials in the glass tube are separated. However, when a magnetic substance approaches the glass tube, the two reeds in the glass tube are magnetized to attract each other and contact under the function of magnetic field lines. As a result, the two reeds will pull together to connect the circuit connected with the nodes.

After external magnetic force disappears, the two reeds will be separated with each other because they have the same magnetism, so the circuit is also disconnected. Therefore, as a line switch component controlling by magnetic signals, the dry reed pipe can be used as a sensor to count, limit positions and so on. At the same time, it is widely used in a variety of communication devices.

The schematic diagram:

Experimental Procedures

Step 1: Build the circuit

Raspberry PiReed Switch Raspberry PiDual-color LED
GPIO0                  SIG GPIO1R

For C language users:

Step 2: Change directory

 cd /home/pi/SunFounder_SensorKit_for_RPi2/C/11_reed_switch/

Step 3: Compile

gcc reed_switch.c –lwiringPi

Step 4: Run

sudo ./a.out

For Python users:

Step 2: Change directory

 cd /home/pi/SunFounder_SensorKit_for_RPi2/Python/

Step 3: Run                                    

sudo python 11_reed_switch.py

Then the LED will emit green light. Place a magnet near the reed switch, “Detected Magnetic Material!” will be printed on the screen and the LED will emit red light. Move away the magnet, the LED will emit green light again.

 C Code

#include <wiringPi.h>
#include <stdio.h>

#define ReedPin		0
#define Gpin		1
#define Rpin		2

void LED(char* color)
	pinMode(Gpin, OUTPUT);
	pinMode(Rpin, OUTPUT);
	if (color == "RED")
		digitalWrite(Rpin, HIGH);
		digitalWrite(Gpin, LOW);
	else if (color == "GREEN")
		digitalWrite(Rpin, LOW);
		digitalWrite(Gpin, HIGH);
		printf("LED Error");

int main(void)
	if(wiringPiSetup() == -1){ //when initialize wiring failed,print messageto screen
		printf("setup wiringPi failed !");
		return 1; 

	pinMode(ReedPin, INPUT);
		if(0 == digitalRead(ReedPin)){
			if(0 == digitalRead(ReedPin)){
				printf("Detected Magnetic Material!\n");	
		else if(1 == digitalRead(ReedPin)){
			if(1 == digitalRead(ReedPin)){
	return 0;

Python Code

#!/usr/bin/env python
import RPi.GPIO as GPIO

ReedPin = 11
Gpin    = 12
Rpin    = 13

def setup():
	GPIO.setmode(GPIO.BOARD)       # Numbers GPIOs by physical location
	GPIO.setup(Gpin, GPIO.OUT)     # Set Green Led Pin mode to output
	GPIO.setup(Rpin, GPIO.OUT)     # Set Red Led Pin mode to output
	GPIO.setup(ReedPin, GPIO.IN, pull_up_down=GPIO.PUD_UP)    # Set BtnPin's mode is input, and pull up to high level(3.3V)
	GPIO.add_event_detect(ReedPin, GPIO.BOTH, callback=detect, bouncetime=200)

def Led(x):
	if x == 0:
		GPIO.output(Rpin, 1)
		GPIO.output(Gpin, 0)
	if x == 1:
		GPIO.output(Rpin, 0)
		GPIO.output(Gpin, 1)

def Print(x):
	if x == 0:
		print '    ***********************************'
		print '    *   Detected Magnetic Material!   *'
		print '    ***********************************'

def detect(chn):

def loop():
	while True:

def destroy():
	GPIO.output(Gpin, GPIO.HIGH)       # Green led off
	GPIO.output(Rpin, GPIO.HIGH)       # Red led off
	GPIO.cleanup()                     # Release resource

if __name__ == '__main__':     # Program start from here
	except KeyboardInterrupt:  # When 'Ctrl+C' is pressed, the child program destroy() will be  executed.