Reed Switch

A reed switch is a magnetic sensor consisting of two ferromagnetic reed contacts sealed inside a glass tube. When a permanent magnet is brought near, the reeds magnetize and attract each other, closing the circuit. It is widely used in door/window alarms, proximity detection, flow meters, and RPM sensors.

For this interfacing you need the following components:

• Arduino board (Uno, Nano, Mega, etc.)
• Reed switch (Normally Open, e.g., MKA-10110, MKA-14102)
• 10k ohm resistor (pull-down or pull-up)
• Magnet (permanent magnet or electromagnet)
• Breadboard and jumper wires
• USB cable to connect Arduino to your computer

Schematic

Connect the reed switch to the Arduino as follows:

Reed Switch           Arduino
-----------           -------
Pin 1         -->     Digital Pin 2
Pin 2         -->     GND

Connect a 10k ohm pull-down resistor from Digital Pin 2 to GND so the pin reads LOW when the switch is open. Alternatively, use the Arduino’s internal pull-up for an inverted output:

With internal pull-up:
Reed Switch           Arduino
-----------           -------
Pin 1         -->     Digital Pin 2
Pin 2         -->     GND
// No external resistor needed - use pinMode(pin, INPUT_PULLUP)

Pin Map

Reed switches are non-polarized — connect either way.

Install necessary Library

No external library is required. A reed switch reads like a digital switch using standard digitalRead().

Code with complete explanation

This sketch reads the reed switch state and prints a message when a magnet is detected (switch closed).

const int reedPin = 2;

void setup()
{
  Serial.begin(9600);
  pinMode(reedPin, INPUT_PULLUP); // Use internal pull-up
}

void loop()
{
  int sensorValue = digitalRead(reedPin);

  // With INPUT_PULLUP: LOW = magnet near (switch closed to GND)
  // HIGH = no magnet (switch open, pulled HIGH internally)
  if (sensorValue == LOW)
  {
    Serial.println("Magnet detected!");
    delay(200); // Debounce
  }
  else
  {
    Serial.println("No magnet");
  }

  delay(100);
}

Code breakdown

• pinMode(reedPin, INPUT_PULLUP) — enables the internal pull-up resistor. The pin reads HIGH when the switch is open and LOW when a magnet closes the switch to GND.
• digitalRead(reedPin) — returns LOW when a magnet is near enough to actuate the switch, HIGH otherwise.
• The delay(200) provides debouncing — reed switches can mechanically bounce when closing or opening.

Door/window alarm example

This example uses a reed switch as a door sensor with an LED indicator:

const int reedPin = 2;
const int ledPin  = 13;
const int buzzerPin = 8;

void setup()
{
  Serial.begin(9600);
  pinMode(reedPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop()
{
  if (digitalRead(reedPin) == HIGH) // Door open (magnet moved away)
  {
    digitalWrite(ledPin, HIGH);
    tone(buzzerPin, 1000);
    Serial.println("Door OPEN!");
  }
  else // Door closed (magnet near)
  {
    digitalWrite(ledPin, LOW);
    noTone(buzzerPin);
    Serial.println("Door closed");
  }

  delay(200);
}

RPM counter example

To measure revolutions per minute using a magnet on a rotating shaft:

const int reedPin = 2;

volatile unsigned int pulseCount = 0;
unsigned long lastTime = 0;

void countPulse()
{
  pulseCount++;
}

void setup()
{
  Serial.begin(9600);
  pinMode(reedPin, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(reedPin), countPulse, FALLING);
}

void loop()
{
  unsigned long now = millis();

  if (now - lastTime >= 1000) // Every second
  {
    unsigned int rpm = pulseCount * 60; // pulses per second * 60
    Serial.print("RPM: ");
    Serial.println(rpm / 2); // Divide by 2 if one rotation = 2 pulses

    pulseCount = 0;
    lastTime = now;
  }
}

Steps to perform this interfacing

1. Connect the reed switch to the Arduino with a pull-down resistor (or use INPUT_PULLUP).
2. Copy the code into the Arduino IDE.
3. Select the correct board and port (Tools > Board and Tools > Port).
4. Upload the sketch to the Arduino.
5. Open the Serial Monitor (Tools > Serial Monitor, set baud rate to 9600).
6. Bring a magnet close to the reed switch and observe the state change.
7. Remove the magnet and verify the switch opens.

Caution

• Reed switches contain fragile glass encapsulation. Avoid bending the leads at the glass body — use a tool to grip the lead near the bend point. Dropping or applying mechanical stress can break the glass and destroy the switch.
• The glass tube is not waterproof. Use a potted or encapsulated reed switch for outdoor or wet environments.
• Reed switches have a limited current rating (typically 10-500 mA). Do not drive loads directly — use a transistor or MOSFET for switching higher currents.
• The switch can bounce mechanically, especially when a magnet is moved slowly. Always include debounce logic (hardware or software) for reliable triggering.
• The actuation distance depends on the magnet’s strength and the reed switch’s sensitivity. Test the effective range with your specific magnet before final mounting.
• Strong external magnetic fields (motors, transformers, induction coils) can cause false triggering. Shield the sensor or relocate it away from such sources.
• For high-speed applications (RPM counting above 1000 Hz), reed switches may miss pulses due to mechanical inertia. Use a Hall effect sensor instead.