MH-Z19B

Project MH-Z19B
Some research into the MH-Z19B CO2 sensor
Status	Initializing
Contact	Disasm
Last Update	2019-10-01

Introduction

MH-Z19B is an updated version of the MH-Z19 sensor.

Datasheet: https://www.winsen-sensor.com/d/files/infrared-gas-sensor/mh-z19b-co2-ver1_0.pdf

Hardware

• STM32F051K86 MCU
• Unknown analog light+temperature sensor
• Two 3.3V regulators
• 1.5V voltage reference
• Lamp

Teardown

Back side of the MH-Z19B sensor with unlabeled SWD pads on the right:

Front side of the (4-layer) PCB:

Partial schematics:

Firmware

MCU firmware can be easily dumped through SWD pads. Pinout:

Pin	Signal
1 (square)	3V3
2	GND
3	SWDIO
4	SWCLK
5	RESET (active low)

Additionally, bootloader asks for a firmware update during the first 20s of startup.

Operation

Firmware turns on lamp for 400 ms and measures sensor response along with the voltage of the voltage reference. This happens again and again in cycle. Default cycle length is 5s.

Firmware measures only a part of the response signal: 180..980 ms interval in 10ms steps. It integrates 180..680 ms interval and removes constant component (1.026V on the image above) based on values from the 930..980 ms interval. Obtained result after some smoothing is available (divided by 2) via command 0x84. This value is used for subsequent computations.

Another firmware turns on lamp for 500 ms and measures 450..500 ms and 1800..1850 ms intervals, then uses the second one to remove constant component from the first one.

As stated in the datasheet, MH-Z19B should be kept away from heat. Experiment showed that the light sensor inside MH-Z19B stops working when MH-Z19B is exposed to an air stream from a heat gun.

Interface

Sensor provides UART, PWM and analog output interfaces. Both PWM and analog output provide CO2 measurement result limited by some bounds. UART interface implements one of several protocols. Default protocol ("operation mode 0") is the richest one.

Protocol (operation mode 0)

All commands and responses are 9 bytes in size. Most of the commands have a response, but some -- don't. Response contains command code, both command and response contain checksum value.

Command format: 0xFF, 0x01, CMD, (5 bytes of parameters), CKSUM

Response format: 0xFF, CMD, (6 bytes of response), CKSUM

For short, hereinafter this response will be called "ACK": 0xFF, CMD, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, CKSUM

In the following table in Request and Response fields only the meaningful bytes will be shown. For example, b[3] is the first parameter byte and b[3..4] is the first two parameter bytes forming a word. "???" means "isn't investigated yet", an empty field means "no parameters".

Command	Request	Response	Description
0x78	b[3]	no response	Changes operation mode and performs MCU reset
0x79	b[3]	ACK	Turns ABC logic on or off (b[3] == 0xA0 - on, 0x00 - off)
0x7D		r[7]	Returns ABC logic status (1 - enabled, 0 - disabled)
0x7E	b[3], b[4..5]	r[2..3]	Sets timer cycle length (b[4..5]) in seconds and returns current value. b[3] should be 2 to update the length
0x80..0x83	b[3], b[4..7]	no response	Writes one double word to the 1024-byte configuration area (offset = b[3] + (CMD - 0x80) * 256). Command 0x80 with b[3]==0 clears the whole configuration area. On timeout this configuration area will be written to flash.
0x84		r[2..3], r[4..5], r[6..7]	Returns "raw" values. r[2..3] is a half of the raw light sensor value. r[4..5] is constant 32000. r[6..7] is different in different versions of firmware: bit field or maximum light ADC value
0x85		r[2..3], r[4..5], r[6..7]	Returns "raw" values. b[2..3] is smoothed temperature ADC value. b[4..5] is CO2 level before clamping. b[6..7] is minimum light ADC value
0x86		r[2..3], r[4], r[5], r[6], r[7]	r[2..3] - "final" CO2 level. r[6] and r[7] - if ABC turned on - counter in "ticks" within a calibration cycle and the number of performed calibration cycles.
0x87	b[5], b[6], b[7]	r[2], r[3..6]	???
0x88	b[3..4]	r[2..3]=b[3..4]	???
0x8D		no response	MCU reset
0x90..0x93	b[3]	r[2..5]	Reads a double word from configuration area by offset (b[3] + (CMD - 0x90) * 256)
0x94	b[3], b[4..6]	r[2..6]	Changes 3 bytes (b[4..6]) of "id string" with offset (3*(b[3] - 1))
0x95	b[3]	r[2], r[3], r[4..6]	Reads 3 bytes of "id string" (b[4..6]) with offset
0x99	b[4..7]	ACK	Sets sensor range. Note that parameter bytes differ from those in the datasheet.
0x9A	b[4]	ACK	???
0x9B		r[2..5], r[6], r[7]	Returns sensor range (r[2..5])
0x9C		r[2..5], r[6]=1	???
0x9F	b[3]	ACK	???
0xA0		r[2..5]	Firmware version string? "0430" and "0443" observed
0xA1	b[3]	ACK	???
0xA2	b[3]	r[2], r[3..6]	???
0xA3		r[2..3]	???
0xA4	b[3..4], b[5..6]	r[2]	Setting bounds for DAC output, r[2]==1 on success
0xA5		r[2..3], r[4..5]	Reading bounds for DAC output
0xAA	b[3], b[4]	r[2], r[3]	???
0xAB	b[3], b[4]	r[2], r[3]	???
0xAC	b[3]	r[2], r[3..6]	???