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Project CO2MeterHacking
Status Completed
Contact bertrik
Last Update 2018-02-19

status: it works. We've modified the code to work using an ESP8266 which publishes the measurement data directly on an MQTT stream.

The G, C, D, V signals from the CO2-sensor inside the CO-100 are routed internally to the RJ45-plug, so there's actually no need to open it and solder a connector on the G, C, D, V lines.


This project is about hacking the Voltcraft CO-100 CO2-meter, such that we can read the exact ppm value as displayed on the LCD.

This particular CO2-meter is present in the main space of RevSpace. Having the CO2 ppm value available as a number allows for nice things such as logging the levels over time, announce them on IRC, show them on the LedBanner, fine-grained control of the ventilation system, etc.

The user manual of the Voltcraft CO-100 says "Achtung! Der RJ45-Anschluss (siehe Kapitel 7, Position „K“) darf nicht verwendet werden. Der Anschluss ist nur für den Hersteller vorgesehen." Of course, a claim like that can only be interpreted as a challenge! :)

See also:

Investigation & findings

CO-100 internals

The CO2 sensor inside the CO-100 (in the left of the picture) has a sticker saying ZGw063RY. Googling for this number reveals a CO2 module that looks just like the Voltcraft CO-100, so it appears that the CO-100 is basically a rebranded ZyAura ZGw063RY module.

The CO-100 seems to miss a bunch of components that can be mounted on the PCB, close to the RJ45 connection (most likely an RS232 chip with charge pump capacitors).

components around the RJ45 connector has some information on what looks like a clone/similar device, describing a serial cable for the RJ45, and the protocol used, as well as some info on re-calibration. Commands/Identifiers seem to match.

The CO2 sensor in the CO-100 is a ZyAura ZG-01 module. This sensor uses the ZyAura protocol, which vaguely resembles SPI, see File:ZyAura CO2 Monitor Carbon Dioxide ZG01 Module english manual-1.pdf.

On the bottom left of the PCB is a set of pads that are marked with G, C, D, V, meaning Ground, Clock, Data, Voltage of the ZG01 sensor. The voltage level on the clock and data pins is 3.3V (the voltage on V pin is 3.3V too).

The ZG-01 sends 5-byte frames containing measurement values:

  • byte 0 is an identifier for the measurement item, e.g. whether it is a CO2 ppm value or a temperature.
  • byte 1 and 2 contain the value of the item (byte 1 is the MSB, byte 2 is the LSB)
  • byte 3 is a checksum over bytes 0-2, just the sum modulo 256.
  • byte 4 is always 0x0D

Besides the CO2 ppm value and temperature, it also sends various other (so far unknown) measurement items.

Measurement items encountered so far:

Item Value Remark
0x41 'A' 3290 Relative humidity in units of 0.01%
0x42 'B' 4708 Temperature in Kelvin (unit of 1/16th K)
0x43 'C' 2964 ?
0x46 'F' 6882 Temperature in degrees Fahrenheit (unit of 0.01)?
0x4F 'O' 7754 ?
0x50 'P' 857 CO2 concentration in ppm
0x52 'R' 10438 Barometric pressure?
0x56 'V' 10443 Barometric pressure?
0x57 'W' 7880 ?
0x6D 'm' 2559 Seems to always have same value
0x6E 'n' 17146 ?
0x71 'q' 855 Always close to value of item 0x50

Pin-out of the 8P8C ("RJ45") connector on the side of the CO-100:

T568B Colour Signal
1 orange-white Supply voltage out, measured about 5.7V
2 orange DATA, 3.3V level
3 green-white CLOCK, 3.3V level
4 blue GND


NRF version

The ZG-01 module and NRF24L01+ transceiver are connected to the Arduino as follows:

Arduino Module Remark
D2 ZG-C ZG01 clock signal
D3 ZG-D ZG01 data signal
GND NRF-1, ZG-G NRF ground, ZG01 ground
3V3 NRF-2 NRF power
- 8 NRF IRQ - not connected

ESP version

In the ESP version of the CO2 meter, signals from the ZG-01 module are connected as follows:

Wemos D1-mini ZG01 Remark
D1 ZG-C ZG01 clock signal, 3.3V
D2 ZG-D ZG01 data signal, 3.3V
GND ZG-G ZG01 ground
5V ? Supply voltage from 8P8C, about 6V!


The basic function of the software on the arduino is to monitor the C and D signals, until a CO2 measurement is received from the ZG-01, then send it using the wireless transceiver.

The source code can be found on github. This archive contains two arduino projects:

  • one for an Arduino Pro Mini, using an NRF24L01+ for the wireless connection
  • one for an ESP8266, sending measurement values directly to MQTT over the WiFi.

ZG-01 decoding

The ZG-01 protocol is decoded using a simple state machine. On each falling edge of the clock line, a sample of the data line is taken until a total of 40 bits is received. If the time between a bit and the previous bit is longer than 2 milliseconds, it is assumed that a new 5-byte frame has started.

The technical description for a very similar CO2 module: ZG106 protocol.

Wireless protocol


To control the NRF24L01+ wireless transceiver, we use the gcopeland fork of the RF24 library. This library has several important fixes over the original RF24 library (and is used in the receiver as well).

The wireless message consist of 7 bytes:

  • 0x06 "CO_2" <MSB> <LSB> for the CO2 concentration message (in ppm)
  • 0x06 "HUMI" <MSB> <LSB> for the relative humidity message (in units of 0.01%)


The ESP version of the software uses MQTT to publish the measurement values, on the following topics (retained):

  • revspace/sensors/co2 with the CO2 concentration in ppm, e.g. "400 PPM"
  • revspace/sensors/humidity with the relative humidity in percent, e.g. "43.56 %"
  • revspace/sensors/temperature with the temperature in degrees Celcius, e.g. "23.9 °C"

For example, running

 mosquitto_sub -h -t revspace/sensors/co2 -v

produces output like

 revspace/sensors/co2 530 PPM
 revspace/sensors/co2 530 PPM
 revspace/sensors/co2 532 PPM
 revspace/sensors/co2 533 PPM

Graphs and heat maps

Interesting visualizations: