Reed Switch: Magnetic Contact Sensor

Working Principle

A reed switch is a magnetically operated electromechanical switch consisting of two thin ferromagnetic reeds sealed in a glass envelope filled with inert gas. When a magnet comes within range (typically 10–30 mm), the magnetic field magnetises the reeds, pulling them together to close the circuit. Remove the magnet and the reeds spring apart, opening the circuit.

Reed switches come in two types: Normally Open (NO) — closed when magnet present; and Normally Closed (NC) — opened when magnet present. For security applications, NO is standard (alarm triggers when magnet is removed/door opens).

Electrical Characteristics

Parameter	Typical Value
Contact Configuration	SPST-NO (Normally Open, most common)
Operating Range	10–30 mm (depends on magnet strength)
Max Switching Voltage	100–200 V DC
Max Switching Current	0.5–1.0 A
Contact Resistance	<100 mΩ (closed)
Operate Time	~0.5 ms
Release Time	~0.1 ms
Bounce Time	~1 ms (needs debouncing)
Lifecycle	>107 operations (mechanical endurance)

Interfacing with an MCU

Connect one reed switch terminal to a GPIO pin and the other to GND. Enable the MCU’s internal pull-up resistor (or use an external 10 kΩ pull-up to VCC). When the magnet is away (NO switch open), the pin reads HIGH; when the magnet is close (switch closed), the pin reads LOW. Use a 100 nF capacitor across the switch for hardware debouncing.

Calibration

• Range testing: Determine the operate distance and release distance (hysteresis) for your specific magnet/switch combination
• Debouncing: Implement software debounce (10–50 ms delay) or use a hardware RC filter to eliminate contact bounce false triggers
• Orientation: The magnet must approach along the switch’s sensitive axis (lengthwise); perpendicular approach reduces effective range by ~50%

Code Example

/*
 * Reed Switch — Door/Window Contact Sensor
 * Wiring: Pin 1 → D3 (with internal pull-up), Pin 2 → GND
 *         Magnet mounted on door/window
 */
#define REED_PIN 3
#define DEBOUNCE_MS 50

bool lastState = HIGH;   /* Open (magnet away) */
unsigned long lastChange = 0;

void setup() {
    Serial.begin(9600);
    pinMode(REED_PIN, INPUT_PULLUP);
    Serial.println("Reed Switch Door Sensor Ready");
}

void loop() {
    bool current = digitalRead(REED_PIN);

    if (current != lastState &&
        (millis() - lastChange) > DEBOUNCE_MS) {
        lastChange = millis();
        lastState = current;

        if (current == LOW) {
            Serial.println("CLOSED — door/window shut");
        } else {
            Serial.println("OPEN — door/window opened!");
            /* Trigger alarm, send notification, etc. */
        }
    }
    delay(10);
}

Real-World Applications

Limitations

• Mechanical wear: Contact surfaces degrade after 10⁷+ operations; high-current switching accelerates wear.
• Contact bounce: ~1 ms bounce produces spurious pulses; must debounce in software or hardware.
• Magnetic interference: Strong external magnetic fields (motors, speakers) can trigger false activations.
• Fixed range: Detection range is fixed by the magnet strength and switch sensitivity; cannot be adjusted electronically.
• Glass fragility: The glass envelope is delicate; handle carefully during mounting.