ZyTemp CO2 Monitor

Home Assistant integration for USB CO₂ sensors built around the ZyAura ZG01 chip (USB 04d9:a052, "Holtek USB-zyTemp"). Sold under many names: Даджет MT8057 and MT8057s, MasterKit, TFA Dostmann AIRCO2NTROL MINI, ZyAura ZGm053U, CO2Mini.

Reports carbon dioxide and temperature, and raises two configurable alarms.

Why this exists

The sensor ships with a Windows-only program that draws a live graph and nothing else — no history, no automations, no place in a smart home. This integration reads the device directly so Home Assistant can record it and act on it.

What you get

Entity	Type	Notes
CO2	sensor	ppm, carbon_dioxide device class
Temperature	sensor	°C, with a configurable offset
CO2 warning	binary_sensor	problem, on above the warning threshold
CO2 critical	binary_sensor	problem, on above the critical threshold
CO2 warning threshold	number	the warning threshold itself, editable
CO2 critical threshold	number	the critical threshold itself, editable

Both alarms clear 50 ppm below their threshold, so a reading sitting on the line does not flap your notifications. While CO₂ is unknown they report unknown rather than "no alarm", which would be a quiet false negative.

Requirements

• Linux. The device is read through /dev/hidraw*.
• No Python dependencies. Everything is done with the standard library, so there is no hidapi to compile on your ARM board.

Permissions

Install a udev rule so the device is readable, then replug it:

# /etc/udev/rules.d/90-zytemp-co2.rules
SUBSYSTEMS=="usb", ATTRS{idVendor}=="04d9", ATTRS{idProduct}=="a052", \
    KERNEL=="hidraw*", GROUP="plugdev", MODE="0660", SYMLINK+="co2mini%n"

sudo udevadm control --reload-rules && sudo udevadm trigger

Home Assistant in a container

Docker takes a snapshot of /dev when the container is created, so a sensor plugged in later is invisible inside it — and privileged: true does not change that. Share the host's /dev instead:

services:
  homeassistant:
    volumes:
      - /dev:/dev

Do not use devices: for this device: it pins one node at start-up and breaks as soon as the sensor is replugged under a different hidraw number.

Installation

HACS → three dots → Custom repositories → this repository, category Integration → install → restart Home Assistant → Add integration → ZyTemp CO2 Monitor. The sensor is found automatically; there is nothing to enter.

Configuration

Settings → Devices & services → ZyTemp CO2 Monitor → Configure

Option	Default	Meaning
CO2 warning threshold	1000 ppm	Warning alarm turns on above this
CO2 critical threshold	1400 ppm	Critical alarm turns on above this
Temperature offset	0 °C	Added to every reading

The offset is there because the sensor sits inside the case next to the electronics and typically reads 1–3 °C high.

Both thresholds are also exposed as number entities, so they can be put on a dashboard or set from an automation — to tighten the limit at night, say. They read and write the same options the settings dialog does, so the two can never drift apart, and the critical threshold is refused if it would drop to or below the warning one.

Changing a threshold takes effect immediately and does not reopen the USB device, so your automations see no gap.

How it works

The device streams frames by itself once it has been handed a key over a HID feature report; nothing polls it. Home Assistant watches the file descriptor and decodes each 8-byte frame as it arrives.

Firmware older than 2.00 obfuscates its frames. The decoder detects which kind it is talking to from the data and latches that decision, so both revisions work.

The hidraw node number changes between replugs, so the device is located afresh on every open by scanning sysfs for its USB id — a replugged sensor is picked up again on its own, without restarting Home Assistant.

Troubleshooting

Download diagnostics from the integration page. They carry the current hidraw path, whether the firmware was detected as obfuscated, frame counters, the age of each reading and how many times the device had to be reopened.

Credits

The ZG01 protocol was reverse engineered by the community — in particular Henryk Plötz's write-up and the dmage/co2mon, heinemml/CO2Meter and vfilimonov/co2meter projects.