How to Use Reed Switch: Examples, Pinouts, and Specs

A reed switch is an electromagnetic switch that opens and closes in response to a magnetic field. It consists of two ferromagnetic contacts sealed within a glass tube. When a magnetic field is applied, the contacts close, completing the circuit. Once the magnetic field is removed, the contacts return to their open state.

Reed switches are widely used in applications requiring non-contact switching. Common use cases include:

• Door and window sensors in security systems
• Position and proximity sensing
• Speed sensing in bicycles and treadmills
• Liquid level detection in tanks
• Automotive applications, such as brake or gear position sensing

Below are the key technical details of a typical reed switch. Note that specifications may vary depending on the manufacturer and model.

Reed switches are simple two-terminal devices. The terminals are the two ferromagnetic contacts sealed within the glass tube. Below is a description of the pins:

1. Basic Circuit Connection:

   • Connect one terminal of the reed switch to the positive side of the power supply.
   • Connect the other terminal to the load (e.g., an LED with a current-limiting resistor).
   • Complete the circuit by connecting the load to the negative side of the power supply.
   • When a magnetic field is applied near the reed switch, the circuit will close, and the load will activate.

2. Interfacing with a Microcontroller (e.g., Arduino UNO):

   • Connect one terminal of the reed switch to a digital input pin on the Arduino.
   • Connect the other terminal to the ground (GND) pin.
   • Use a pull-up resistor (10kΩ) between the digital input pin and the 5V pin to ensure a stable signal.

// Reed Switch Example with Arduino UNO
// This code reads the state of a reed switch and turns on an LED when the switch is closed.

const int reedSwitchPin = 2; // Pin connected to the reed switch
const int ledPin = 13;       // Pin connected to the onboard LED

void setup() {
  pinMode(reedSwitchPin, INPUT_PULLUP); // Set reed switch pin as input with pull-up
  pinMode(ledPin, OUTPUT);             // Set LED pin as output
  Serial.begin(9600);                  // Initialize serial communication
}

void loop() {
  int reedState = digitalRead(reedSwitchPin); // Read the reed switch state

  if (reedState == LOW) { // LOW means the reed switch is closed
    digitalWrite(ledPin, HIGH); // Turn on the LED
    Serial.println("Magnetic field detected!"); // Print message to serial monitor
  } else {
    digitalWrite(ledPin, LOW); // Turn off the LED
    Serial.println("No magnetic field."); // Print message to serial monitor
  }

  delay(100); // Small delay for stability
}

• Magnet Placement: Ensure the magnet is positioned close enough to the reed switch to reliably activate it. The activation distance depends on the strength of the magnet and the sensitivity of the reed switch.
• Debouncing: Reed switches may produce noise or "bouncing" when switching states. Use a software debounce routine or a capacitor across the terminals to filter out noise.
• Current Limiting: Avoid exceeding the maximum current rating of the reed switch to prevent damage to the contacts.
• Mounting: Handle the glass tube carefully to avoid breakage. Use a secure mounting method to protect the switch from mechanical stress.

1. Reed Switch Not Activating:

   • Cause: The magnet is too far from the reed switch.
   • Solution: Move the magnet closer or use a stronger magnet.

2. Switch Stuck in Closed State:

   • Cause: The contacts may have welded together due to excessive current.
   • Solution: Replace the reed switch and ensure the current does not exceed the rated value.

3. Intermittent Operation:

   • Cause: Electrical noise or poor connections.
   • Solution: Add a pull-up or pull-down resistor, and check all connections for stability.

4. No Response in Arduino Circuit:

   • Cause: Incorrect wiring or missing pull-up resistor.
   • Solution: Verify the wiring and ensure a pull-up resistor is used.

Q: Can a reed switch detect non-magnetic materials?
A: No, reed switches only respond to magnetic fields and cannot detect non-magnetic materials.

Q: Can I use a reed switch in high-vibration environments?
A: Reed switches are sensitive to vibration, which may cause false triggering. Consider using a solid-state alternative like a Hall effect sensor in such cases.

Q: How do I protect the reed switch from voltage spikes?
A: Use a flyback diode across inductive loads (e.g., relays) to suppress voltage spikes and protect the reed switch contacts.

Q: Can I use a reed switch for AC applications?
A: Yes, reed switches can handle both AC and DC currents, provided the voltage and current ratings are not exceeded.