Buy this sensor or kit.

MINI reed switch module

A reed module is a magnetic sensor that is normally open and get closed when exposed to a magnetic field.

In our example turn off the buildin LED on pin 13 if the sensor is closed. For this you need to keep a magnet near the sensor.

The module includes a 10 K Ohm resistor, so no additional parts needed.

Schematic

• Arduino GND --> Module pin -
• Arduino +5V --> Module PLUS (middle pin)
• Arduino Digital pin 3 --> Module S

Example Code

//
// Example code for sensor KY021
// More info on http://tkkrlab.nl/wiki/Arduino_KY-021_Mini_magnetic_reed_modules
//
int Led = 13 ;// define LED Interface
int buttonpin = 3; // define the Reed sensor interfaces
int val ;// define numeric variables val
void setup ()
{
  pinMode (Led, OUTPUT) ;// define LED as output interface
  pinMode (buttonpin, INPUT) ;// output interface as defined Reed sensor
}
void loop ()
{
  val = digitalRead (buttonpin) ;// digital interface will be assigned a value of 3 to read val
  if (val == HIGH) // When the Reed sensor detects a signal, LED flashes
  {
    digitalWrite (Led, HIGH);
  }
  else
  {
    digitalWrite (Led, LOW);
  }
}