A Home Assistant Community member posted a teardown of a cheap battery-powered Zigbee 24 GHz ceiling radar. The sensor is sold as Z3-24G under several brand names. Zigbee2MQTT identifies it as model MS01 from vendor zbeacon, and Home Assistant labels it “Motion sensor (SNZB-03) by SONOFF”. The original thread is here. This is one person’s report, not a manufacturer statement.
What was found
Over Zigbee you get occupied or clear, plus battery level. There is no sensitivity, range or hold time. The poster had six of these around the house and found them wrong in both directions: one fired with nobody present, another kept dropping a person sitting still right underneath it.
Inside, they found a Zigbee board and a separate radar daughterboard (a SKYRELAY BC2412FR). The radar board talks over a plain-text serial connection, and the Zigbee chip only reads a single occupancy signal from it. The Zigbee side never sends the radar anything, so settings written to the radar stay put, even with the batteries removed.
The radar divides its view into three distance zones (0 to 2 m, 2 to 5 m, 5 to 10 m). Each has its own motion and still-person threshold, and there are also a hold time and a trigger count. The poster says they could change all of these, and showed a close target disappearing from the readings after switching off the nearest zone. They could not find a way to change the zone distances themselves.
Why factory settings suit a ceiling badly
The seller quotes a beam of about 110 degrees. From a 2.4 m ceiling, the poster calculates the farthest point inside that beam is about 4.2 m away, so the 5 to 10 m zone sits entirely outside it and anything reported there is a reflection. The factory profile also makes the zone directly under the sensor the least sensitive for still people. In the poster’s words, that is a wall profile on a ceiling product.
How the tuning went
Honestly, mixed. On the entry unit, an aggressive first tune produced 122 false triggers between 3 and 6 am and turned living area lights on about 40 times, while a home-made LD2412 radar beside it saw 2. A gentler tune fixed that, left it about three times quicker than factory to notice a person, and matched the reference radar on 103 of 104 triggers.
But the hallway unit kept firing on its own whatever was set, and later the kitchen and lounge units began ghosting too. Five of the six are now out of the poster’s automations. Their conclusion: “I wouldn’t trust one of these alone to turn lights on at night.”
What it means for your install
- If you already own these: they work as plain Zigbee occupancy sensors. Nothing here says they stop working, and working Zigbee gear does not need replacing. Just don’t let one be the only thing switching lights at night.
- Don’t open one casually. The modification means taking the case apart and connecting a serial adapter. We haven’t done it, the settings are undocumented by the maker, and wiring details come from a hobbyist, so we are not repeating them here. Read the full write-up the poster links before trying anything.
- If you want to try it: change one setting at a time and run a second radar nearby as a referee, as the poster did. Overnight logs matter more than daytime tests.
- Buying for a ceiling: a radar that exposes its sensitivity and zone settings in Home Assistant is a safer bet than a sensor you need a soldering iron to adjust. Check the settings are available before you buy.
Where this leaves Zigbee presence
As of today Zigbee is the more mature option in Home Assistant: a big proven catalogue, fully local, very low power. Matter is the industry’s direction and wins on working across ecosystems, but it is younger. This story is a reminder that a protocol being mature does not make every cheap device on it good. The weak link here is the radar module and its factory profile, not Zigbee.
Open questions remain from the thread: whether anyone has found a command for the zone distances, and whether the BC2412 module turns up in other Zigbee or Tuya radars. Nobody has answered yet.