CO2MeterHacking

Project CO2MeterHacking
Status	Initializing
Contact	bertrik
Last Update	2014-03-25

Introduction

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

This particular CO2-sensor is present in the klusbunker at RevSpace and is currently used to control the ventilation in a crude manner (by monitoring the warning LEDs on the display module). 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 , etc.

The user manual of the Voltcraft CO-100 says "Attention! The RJ45 connection (see chapter 7, item „K“) must not be used. The connection is only intended for the manufacturer". Of course, a claim like that can only be interpreted as a challenge! :)

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 just 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

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 these pins is 3.3V.

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 2 is a checksum over bytes 0-2, just the sum modulo 256.
• byte 3 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	3290	?
0x42	4708	Temperature in hexi-degrees Kelvin
0x43	2964	?
0x46	6882	?
0x4F	7754	?
0x50	857	CO2 ppm value
0x52	10438	Pressure?
0x56	10443	Pressure?
0x57	7880	?
0x6D	2559	?
0x6E	17146	?
0x71	855	Always close to value of item 0x50

Software

Software has been written to interface an arduino to the clock and data lines of the ZG-01. It consists of an finite state machine (FSM) that processes the data line on each falling edge of the clock line. The FSM assumes that a new frame is starting when the time between bits is more than 3 milliseconds. Measurements of the CO2 ppm level are sent over the air using a 2.4 GHz NRF24L01+ module.

Arduino pin mapping:

• digital pin 2 (PD2): ZG01 clock signal
• digital pin 3 (PD3): ZG01 data signal

• TODO: pin mapping of the NRF24L01+ module.

The source code can be found on github.

Integration

The plan to integrate the CO2 sensor into the space infrastructure is as follows:

• Attach an arduino to the G, C, D, V lines. The arduino takes power from the V-line and decodes the SPI-like signals from the ZG-01 on the C-line and D-line.
• Connect an NRF24L01+ module to the arduino SPI port and send the CO2/temperature frames over the air to a central NRF24L01+ receiver (e.g. the one which handles the SkipButton, Nomz Bell). The unique 4-character id for the CO2 sensor is "CO_2".
• From there on, the CO2/temperature data can be further processed (to be logged, sent to the ledbanner, announced on IRC, etc.)

This requires only minimal modifications to the CO-100 modules itself and makes uses of the existing wireless infrastructure.

Future work

Future investigation:

• consider powering the Arduino from the 3.3V line coming from the CO2 sensor
• fix the wireless RF24 based protocol, it seems to work sometimes, but not all the time
• investigate further into the unknown measurement items sent by the ZG01