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

A reed switch is an electromechanical device that operates when exposed to a magnetic field. It consists of two ferromagnetic metal contacts (reeds) in a hermetically sealed glass envelope. The contacts are normally open and close when a magnetic field is applied. Reed switches are used in various applications such as proximity sensing, security systems, and switching circuits.

• Magnetic door and window sensors in security systems
• Position and end limit sensing in industrial controls
• Reed relays for switching high voltages or RF signals
• Keyboard and keypad switches
• Flow metering when paired with a magnetic impeller

• Switching Voltage: Typically up to 170 VDC
• Switching Current: Up to 0.5 A
• Carry Current: Up to 1.2 A
• Contact Resistance: Typically 150 milliohms or less
• Operate Time: Approximately 0.5 ms
• Release Time: Approximately 0.1 ms
• Operating Temperature: -40°C to +200°C (varies by model)

Note: NC (Normally Closed), NO (Normally Open)

1. Positioning: Place the reed switch at the desired location where magnetic field presence will be detected.
2. Magnet Placement: Position a magnet so that it will come close to the reed switch when the desired event occurs (e.g., a door opening).
3. Circuit Integration: Connect the reed switch in series with the load and power source. Ensure that the current and voltage ratings are within the specifications of the reed switch.
4. Testing: Test the switch operation by bringing the magnet close to ensure the contacts close.

• Magnetic Field Strength: Ensure the magnet's field is strong enough to actuate the switch but not so strong that it permanently damages the contacts.
• Mounting: Avoid mechanical stress on the glass envelope.
• Switching Load: Use a snubber circuit if the load is inductive to prevent contact arcing.
• Contact Protection: Consider using a contact protection circuit to extend the life of the reed switch if switching a high load or in a high-frequency application.

// Example code for using a reed switch with an Arduino UNO

const int reedPin = 2; // Reed switch connected to digital pin 2
const int ledPin = 13; // Onboard LED connected to digital pin 13

void setup() {
  pinMode(reedPin, INPUT_PULLUP); // Initialize reed switch pin as input with internal pull-up
  pinMode(ledPin, OUTPUT); // Initialize LED pin as an output
}

void loop() {
  if (digitalRead(reedPin) == LOW) { // Check if reed switch is closed by magnet
    digitalWrite(ledPin, HIGH); // Turn on LED
  } else {
    digitalWrite(ledPin, LOW); // Turn off LED
  }
}

Note: The internal pull-up resistor is used to ensure a stable state when the switch is open.

• Switch Does Not Actuate: Verify the magnet's proximity and strength. Check for damage to the reed switch.
• Intermittent Operation: Ensure there is no mechanical vibration affecting the switch. Check for loose connections.
• Shortened Life Span: High switching loads or frequent operation can wear out the contacts. Use appropriate protection circuits.

• Debouncing: Implement software debouncing in the Arduino code to handle mechanical contact bounce.
• Magnetic Interference: Keep other magnetic sources away from the reed switch to prevent false operation.
• Testing: Use a multimeter to check for continuity when the switch is actuated.

Q: Can I use a reed switch with AC voltage? A: Yes, but ensure the specifications are within the limits for AC voltage and current.

Q: How can I extend the life of my reed switch? A: Use contact protection circuits, avoid excessive force on the glass envelope, and select a switch with a higher rating than your application requires.

Q: Is there a polarity to a reed switch? A: No, reed switches do not have polarity and can be connected in any direction.