Project: a home security door/window sensor with alerts

What you will build and what you need

You will build a contact sensor that sends open and closed events to your MQTT broker, plus an hourly heartbeat with the battery voltage. A home automation system such as Home Assistant or Node-RED turns events into phone notifications. This is a learning project; it is not a replacement for a certified, professionally monitored alarm.

Design

Fit the reed switch to the frame and the magnet to the door. Wire the switch between an RTC-capable GPIO (for example GPIO 33) and ground, with the pull-up resistor to 3.3 V. With the door closed, the magnet closes the switch and the pin reads low. When the door opens, the switch opens and the pin goes high, which wakes the ESP32 through wake_on_ext0. A timer wake every hour sends a heartbeat so the system knows the sensor is alive.

Build steps

1. Prototype on a breadboard and confirm the pin value changes as you move the magnet.
2. Reuse the Wi-Fi, TLS and secrets.py approach from the temperature monitor.
3. Upload the code below, then test open, close and heartbeat messages with mosquitto_sub.
4. Create an automation: notify when open arrives while the house is in "away" mode, and notify when no heartbeat arrives for 90 minutes.
5. Measure sleep current with a multimeter and estimate battery life before final installation.

import machine, esp32, time
from machine import Pin, ADC
from net import publish          # your Wi-Fi + MQTT/TLS helper from the earlier project

door = Pin(33, Pin.IN)            # external pull-up fitted
batt = ADC(Pin(35)); batt.atten(ADC.ATTN_11DB)
BASE = "home/hall/door01/"

def battery_volts():
    return round(batt.read_uv() * 2 / 1_000_000, 2)   # divider halves the voltage

if machine.wake_reason() == machine.EXT0_WAKE:
    publish(BASE + "state", "open", retain=True)
    while door.value() == 1:      # wait for the door to close again
        time.sleep(1)
    publish(BASE + "state", "closed", retain=True)
else:
    state = "open" if door.value() else "closed"
    publish(BASE + "heartbeat", str(battery_volts()))
    publish(BASE + "state", state, retain=True)

esp32.wake_on_ext0(pin=door, level=esp32.WAKEUP_ANY_HIGH)
machine.deepsleep(3_600_000)      # wake again in one hour for a heartbeat

Connecting to Wi-Fi from deep sleep takes a few seconds, so expect a short delay before the alert. A retained state message lets a dashboard show the current position immediately.

Threats and security checklist

• Supervise the sensor: alert on a missing heartbeat and on low battery, not only on "open".
• Encrypt and authenticate with TLS and per-device MQTT credentials so nobody on the network can publish a fake "closed".
• Restrict topics: the sensor can write only to home/hall/door01/#.
• Tamper resistance: hide wiring, and consider a second switch on the enclosure lid.
• Avoid single points of failure: keep the broker and router on a small UPS.
• Protect privacy: door events reveal when you are home; keep them local rather than in a public cloud if you can.