A reed switch is one of the simplest and most reliable ways to detect when a door or window opens. When combined with an ESP32, it becomes a fast, local, battery-free, and cloud-free door sensor that integrates perfectly with Home Assistant.

This guide covers both ESPHome and MQTT methods, along with wiring diagrams and automations.

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1. What Is a Reed Switch?

A reed switch contains two tiny metal contacts sealed inside a small glass tube.
When a magnet comes close, the contacts pull together (closed).
When the magnet moves away, the contacts separate (open).

Perfect for:

• Doors
• Windows
• Cabinets
• Garage doors
• Mailboxes
• Safes

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2. Hardware Required

• ESP32 DevKit
• Reed switch module OR bare magnetic reed switch
• Small magnet
• Jumper wires
• Resistor (10k pull-up if using bare reed switch)
• USB cable

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3. Wiring Diagram

Using a reed switch module (recommended)

Most modules include a pull-up resistor.

ESP32 → Reed Module
3.3V → VCC
GND  → GND
GPIO27 → OUT

Bare reed switch wiring (requires resistor)

ESP32 → Reed Switch
GPIO27 → Reed Switch → GND
GPIO27 → 10k → 3.3V

GPIO27 is used as an example. Any GPIO except 34–39 can be used.

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METHOD 1 — ESPHome Integration

ESPHome provides a simple binary_sensor configuration.

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4. ESPHome Reed Switch YAML

esphome:
  name: esp32-reed
  platform: ESP32
  board: esp32dev

wifi:
  ssid: "YOUR_WIFI"
  password: "YOUR_PASSWORD"

logger:
api:
ota:

binary_sensor:
  - platform: gpio
    name: "Front Door"
    pin:
      number: 27
      mode:
        input: true
        pullup: true
    device_class: door
    filters:
      - delayed_off: 50ms

Notes

• pullup: true ensures stable readings with bare reed switches
• device_class: door gives correct icons in Home Assistant
• delayed_off prevents bouncing

ESPHome devices appear automatically in Home Assistant.

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METHOD 2 — MQTT Integration (Matches Your Style)

This method uses Home Assistant’s MQTT binary_sensor block, just like your PIR and relay articles.

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5. Home Assistant configuration.yaml — MQTT Door Sensor

Add under:

mqtt:
  binary_sensor:

MQTT YAML Block

mqtt:
  binary_sensor:
    - name: "Front Door"
      state_topic: "home/door/front"
      payload_on: "OPEN"
      payload_off: "CLOSED"
      device_class: door

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6. ESP32 MQTT Code for Reed Switch

#include <WiFi.h>
#include <PubSubClient.h>

#define REED_PIN 27
#define WIFI_SSID "YOUR_WIFI"
#define WIFI_PASS "YOUR_PASSWORD"
#define MQTT_SERVER "192.168.0.10"

WiFiClient espClient;
PubSubClient client(espClient);

int lastState = -1;

void setup() {
  Serial.begin(115200);

  pinMode(REED_PIN, INPUT_PULLUP);

  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) delay(500);

  client.setServer(MQTT_SERVER, 1883);
}

void loop() {
  if (!client.connected()) {
    while (!client.connected()) client.connect("ESP32_Door");
  }

  int state = digitalRead(REED_PIN);

  if (state != lastState) {
    lastState = state;

    if (state == LOW)
      client.publish("home/door/front", "OPEN");
    else
      client.publish("home/door/front", "CLOSED");
  }

  client.loop();
  delay(50);
}

The ESP32 now sends "OPEN" and "CLOSED" events to Home Assistant instantly.

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7. Example Home Assistant Dashboard

type: entities
entities:
  - entity: binary_sensor.front_door

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8. Useful Automations

Notify when front door opens

automation:
  - alias: "Front Door Open Alert"
    trigger:
      - platform: state
        entity_id: binary_sensor.front_door
        to: "on"
    action:
      - service: notify.mobile_app
        data:
          message: "Front door just opened."

Turn on lights when door opens at night

  - alias: "Hallway Lights on Door Open"
    trigger:
      - platform: state
        entity_id: binary_sensor.front_door
        to: "on"
    condition:
      - condition: sun
        after: sunset
    action:
      - service: light.turn_on
        target:
          entity_id: light.hallway

Send an alert if the door stays open for too long

  - alias: "Door Left Open Warning"
    trigger:
      - platform: state
        entity_id: binary_sensor.front_door
        to: "on"
        for: "00:02:00"
    action:
      - service: notify.mobile_app
        data:
          message: "Front door has been open for 2 minutes!"

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9. Troubleshooting

Sensor reads OPEN all the time

• Magnet placed too far away
• Reed switch wired backwards (use NC/NO pins correctly on modules)

Sensor flickers

• Add delayed_off: 50ms filter in ESPHome
• Use INPUT_PULLUP in Arduino code

Door is detected as CLOSED instead of OPEN

• Reverse logic:

ESPHome:

inverted: true

MQTT (Arduino code):

if (state == LOW) → CLOSED  
if (state == HIGH) → OPEN

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10. Tips for Reliable Installation

• Mount magnet and reed switch parallel
• Keep gap < 1 cm
• Use 3M tape + screws for stability
• Keep ESP32 away from metal door frames if WiFi is weak
• For high-security doors, use two reed switches for redundancy

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