Sonoff SNZB-04 Zigbee Door and Window Sensor: Open/Close Alerts

The Sonoff SNZB-04 is a compact Zigbee contact sensor that tells you the moment a door or window opens. It runs on a CR2450 coin cell and works with eWeLink, Home Assistant, and most Zigbee hubs. At around $9, it's one of the most affordable ways to add door alerts to your home.

This sensor doesn't need its own hub when you pair it with a Zigbee USB dongle or the SONOFF Zigbee Bridge Pro. It joins your Zigbee mesh in seconds and sends an alert within 0.5 seconds of opening. That's fast enough to catch an open back door before you leave the house.

What's in the Box

The SNZB-04 ships with the sensor body, a small magnet piece, a CR2450 battery, and a 3M adhesive strip. No tools needed. You press the two pieces together on a door frame - sensor on the frame, magnet on the door itself. Setup takes under two minutes.

Technical Specs

The SNZB-04 uses Zigbee 3.0, which means it works with any Zigbee 3.0 coordinator. It reports open and closed states only - no temperature, no humidity. That keeps the firmware lean and the battery life long.

Key specs at a glance:

• Protocol: Zigbee 3.0
• Battery: CR2450 (included), rated for up to 2 years
• Alert speed: 0.5 seconds from trigger to notification
• Dimensions: 47 x 27 x 13.5 mm (sensor body)
• Mounting: 3M adhesive or two small screws

The magnet gap can be up to about 15 mm before the sensor stops reading correctly. That's enough for most standard doors and windows, but thicker frames or older casements may need a test fit first.

How It Works With Home Assistant

Home Assistant users get the most out of the SNZB-04. You can add it through ZHA (Zigbee Home Automation) or Zigbee2MQTT - both work without any cloud connection. Once paired, it shows up as a binary sensor that triggers on open or closed.

From there you can build automations like turning on a light when the back door opens at night, or sending an alert if a door stays open for more than five minutes. The sensor also exposes battery level, so you won't be caught off guard when it drops.

The Zigbee2MQTT project officially supports the SNZB-04. That means you get MQTT topics for state changes and battery - useful if you run Node-RED or other automation tools alongside Home Assistant.

Zigbee Hub Options

You need one of these to use the SNZB-04:

• SONOFF Zigbee Bridge Pro (cloud or local mode)
• Any Zigbee USB dongle (CC2652, SkyConnect, etc.) with ZHA or Zigbee2MQTT
• Third-party hubs like Aeotec Smart Home Hub or SmartThings with Zigbee support

The SNZB-04 does not support Wi-Fi, Thread, or Matter. It's Zigbee only.

eWeLink App Experience

If you use the eWeLink app, the SNZB-04 pairs through a SONOFF Zigbee bridge. The app shows open/closed history and lets you set push alerts. You can also link eWeLink to Alexa or Google Home so the sensor shows up in those ecosystems.

The eWeLink experience is straightforward. You get a log of events, alert settings, and the option to share access with family members. It's not as flexible as Home Assistant, but it's much simpler to set up. For someone who just wants door alerts on their phone, eWeLink does the job well.

Who Should Buy the SNZB-04

This sensor is a good fit for three types of buyers.

First, Home Assistant users who want cheap, reliable Zigbee sensors to fill gaps in their setup. At $9 each, you can cover every door and window in a small home for under $100.

Second, people building a SONOFF Zigbee ecosystem. If you already have a Zigbee Bridge Pro or other SONOFF Zigbee devices, the SNZB-04 drops right in without any extra config.

Third, anyone who wants basic door alerts without a subscription. The SNZB-04 has no monthly fees and no account required if you run it fully local through Zigbee2MQTT.

It's not ideal if you need Apple HomeKit support (no native HomeKit), or if your home runs entirely on Wi-Fi devices with no Zigbee coordinator. You'd need to add hardware to make it work.

Final Thoughts

The SNZB-04 does one thing - it tells you when a door or window opens - and it does it well. The 0.5-second alert time is fast, the two-year battery life means you won't be swapping cells every few months, and the Zigbee 3.0 protocol keeps it compatible with a wide range of hubs and automations.

It's not a glamorous device. There's no display, no temperature reading, no fancy app. But if you need a reliable, cheap door sensor that works with Home Assistant or most Zigbee setups, the SNZB-04 is hard to beat at its price point. Buy a few and put them on every exterior door in your home. You'll notice the peace of mind right away.

Setup and Reliability in Practice

This sensor is about as simple as Zigbee gets. Pair it to a SONOFF Zigbee bridge or any Zigbee2MQTT coordinator, peel the adhesive, and stick the two halves on a door and its frame. The whole process takes under five minutes, and the CR2032 battery lasts well over a year in normal use.

What makes it worth the few dollars is reliability. Open and close events register in well under a second, and because it's Zigbee rather than Wi-Fi, it sips power and extends your mesh's reach as a quiet endpoint. I run several on doors and one on a medicine cabinet tied to a Home Assistant alert. The only real limit is alignment: if the door settles over a season, nudge the magnet a few millimeters closer so a closed door always reads closed. Get that right and it just works, year after year.