Interfacing the MH-Z14A carbon dioxide sensor with OLED display and ToF Laser distance sensor on STM32

When it comes to indoor air quality, especially during the cold days, we usually think of trying to keep the humidity at an acceptable level to prevent irritation of the respiratory tract occurring from dry air caused by ventilation, which reduces the relative humidity of outside air (that was well-saturated with moisture while it was still cold) by heating it up indoors.

However generally avoiding ventilation will sooner or later result in poor air quality, which is way more difficult to measure than humidity itself: indoor pollutants, mostly from human metabolism (but also furniture and everything else in your house) can vary among countless groups of complex substances that would be impossible to quantify analyitically by reasonable means.

The carbon dioxide level is thus often used as an indicator of how much the air is “used up” by human respiration, so you can get a good estimate about air quality in your room or office by just monitoring that single component:

While fresh air is close to the average outdoor level of around 400ppm CO2, you should try to keep your indoor level below 1000ppm to prevent fatigue and reduced cognitive performance.
Exceeding 4000ppm can show significant negative effects on concentration and feeling comfortable.

Devices that measure and display the CO2 concentration (just like we are used to from countless thermometers displaying humidity) are still rare and very expensive, also they would require a little more power than a humidity meter running for years on two AA batteries.

Among the first devices bringing a low-cost solution to the mass market was the weather station from netatmo: The pack contains several sensors connected to a base station that will collect all CO2 measurements and transmit them over WiFi – directly to the netatmo cloud servers. The only way to see your local air quality in the same room is by touching the top of the base, which will then illuminate in one of three colours to give a vague estimate of the situation. Everything else works exclusively through the cloud service: They will monitor human activity (and even sound levels!) in your house and show you fancy-looking graphs on the app or website in return.

If that does not sound like the solution you would want to be running and connected to the internet 24/7, you will quickly stumble upon those MH-Z14 / MH-Z14A sensors that sell on AliExpress for less than $20 USD, shipping included.

They come with their own microcontroller and several interface options like Analog, PWM and UART.

The UART interface is somewhat straightforward, especially if you keep sending the same command over and over to query the current value (so there’s no need to calculate the checksum but rather use the static one that matches the command, as given in the datasheet): Send 9 bytes at 9600 baud and receive 9 bytes with the measurement. There’s not even the need to verify the checksum of the received reply – if you’re lazy, you can just read the value from the third and fourth byte of the reply.

For the beginning, you can just wire it up to some USB CP2102 UART module – the sensor module is perfectly fine with using the 5V from your USB port.

Which results in this tiny little proof-of-concpet Python script that will write the current value to the console each five seconds (make sure to pip install pyserial first and adjust your /dev/ttyUSB or COM port number):

import serial, time

z14 = serial.Serial(port="COM13",baudrate=9600)

while True:
	z14.write(bytearray.fromhex("FF 01 86 00 00 00 00 00 79"))
	result =
	print(result[2] * 256 + result[3])

You will notice it takes a few minutes for the sensor to “heat up” and deliver plausible results.

Also, there seems to be a difference between ZH14 and ZH-14A modules regarding calibration: The 14A variant performs automatic calibration within each interval of 24 hours – in other words, you have to make sure to let in some fresh air from outside side at least once per day, so it can calibrate the lowest reading within the previous 24 hours to be around 400ppm! This seems quite annoying, but makes using the sensor a lot way easier to use on the other hand.

Now a console window scrolling the latest sensor reading may seem quite appealing to the average nerd, but if you want to go for a more netatmo-ish style of standalone device, how about adding a microcontroller with an oled display and a tocuh sensor? The display would show a tiny version of the CO2 graph, similar to the netatmo apps, on the device itself after you activate it with a touch.

And to make it even cooler, I used one of those new-fangled VL53L0X Time-of-flight laser distance sensors from ST – Those are basically the successor of the tiny sensors you have in smartphones to detect user presence (to disable the touch screen while holding the phone close to the head) and allow for range measurements of up to two meters. So you don’t even need to touch the device itself, but could rather hover your hand over the device to switch on the measurements display, or use gesture recognition (upwards or downwards motion) for additional functions.

Here’s some example using an STM32F103C8T6 board (mostly since those are less than $2 on AliExpress for an ARM Cortex MCU and also code for VL53L0x is readily available from ST; also there are Chinese clones of the ST-Link / v2 debugger availably for very little money, compared to e.g. JTAGICE debuggers for AVR controllers, or platforms like ESP8266 or Arduino where you have no live debugging by default at all). Also it has built-in USB that you can use for USB to serial or even mass storage (how about donwloading your measurements from a dynamically created .csv-file for example?).

Using STM32 CubeMX from ST you can graphically set your desired pin functions and generate all the hardware-related code, similar to the web-based tool availbale from Microchip / Atmel at for the AVR and ATSAM MCUs).

Here’s my CubeMX file (I actually used both I2C cores for separating the OLED and the ToF sensor for mere simplicity in wiring)

But after all, you should of course use the platform you feel most experienced with, considering that SSD1306 OLED drivers are available almost everywhere, and you can check the VL53L0x library from Pololu for a more lightweight alternative to ST’s bloated C library to control the laser distance sensor.

Next thing would be to make decent case for it: currently my prototype resides in a 3D-printed cylinder without any bottom plate, and I still need to figure out how to fit a battery in there and how to charge it (or probably just run a cable from it to the next usb port).

The final goal would be connecting it to the USB port of some OpenWRT router for data logging and network access, but then again you might as well go for ESP8266 in the first place… And after all, summer is coming closer, making this the ideal project for some spare time in next winter…

pictures will follow 🙂

Yealink IP phones and Sennheiser DW Pro wireless headsets – what’s the magic within the EHS36 box?

If you’ve ever come across wireless headsets for office phones, you might have noticed that “wireless” in this case actually results in a lot more wires on your desk than using a regular wired headset: At least the charging / base station will need an additional power supply, and maybe also an adapter cable to fit the particular phone’s headset jack, since those base stations are rarely sold in multiple variants, tailored to a specific phone model, unlike the most simple wired headsets.

Now there’s one more advantage to wireless than just reducing the chance of accidentally pulling on the cable with an arm (or office chair): You can even leave the desk during calls. Thus it would be great to pickup and hangup calls remotely with a button on the headset – but how should the base station then signal this to the phone?

In mobile phones, this kind of button was considered for wired headsets right from the beginning: The de-facto standard means simply shorting the MIC+ line to GND (directly or with a certain resistor value to distinguish between additional functions like track skipping or volume control).

However, this was never a requirement for desk phones, so the information about ringing, accepting a call or hanging up usually needs to be transmitted out-of-band: Back in the days of analog phones, the most common approach involved mechanical handset lifters that would actually physically move the handset upwards from the phone, so the hook switch would be released or closed when the user presses the button on the headset, thus accepting the call or hanging up.

While this “standard” used to be widely compatible (it just needed to fit the gap underneath the handset), it seemed reasonable for manufacturers of modern ISDN and IP phones to come up with some less hardware-intensive solution – the “Electronic Hook Switch” (EHS).

Obviously, there could not be a common global standard for EHS functionality, but you will probably deal with one of the following (mostly country- or vendor-specific) flavours:

  • DHSG (“Drahtlose Hör-/Sprech-Garnitur”, formal German term for “wireless headset”) – probably the most widely deployed standard worldwide by now
  • AEI (“Additional Equipment Interface”) – by Avaya
  • HHC (“Headset Hookswitch Control”) – for Cisco IP phones
  • MSH (“Microphone Short Hook”, as mentioned above for mobile phones, will short the MIC line to GND to signal pickup / hangup) – available in some Alcatel phones
  • RHL (“Remote Handset Lifter”, apparently standardized interface for mechanical lifters as mentioned above)

I recently had the chance to play with some Sennheiser DW Pro DECT Headsets at the office – they offer DHSG, MSH and a (proprietary?) Handset Lifter. Our phones are Yealink T46G that do not come with EHS functionality out of the box, but you are supposed to purchase the $40 “EHS36” adapter box by Yealink, which is meant to be compatible with several wireless headset manufacturers (it even comes with a bunch of cables), including Sennheiser (in DHSG mode).

While there seems to be nothing wrong with purchasing one of those adapters for each desk, users report they would crash every once in a while and need to be re-plugged. Also it just made me curious that there is absolutely no information about pinouts or even protocols out there yet, not even a picture of the PCB… So here it is:

Yealink EHS36 PCB
Yealink EHS36 PCB

The 6P6C (RJ-12) connector on the right goes into the EXT port of the phone through a 1:1 modular cable.
For Sennheiser, a Y-Cable is included that connects the 8P8C port on the base with the 8P8C (RJ-45) port on the EHS36 (left) and the 4P4C (RJ-9) analog headset connector on the phone. Basically, Pins 3-6 for analog audio go into the Yealink phone in reverse order, while Pins 1, 2, 7 and 8 (DHSG Control Pins) go into Pins 3-6 (in this order) of the EHS36 8P8C connector.

For the detailed DHSG pinout, please see the innovaphone wiki:
The specifications are also available for download from there:

There’s also some info (in German) about DHSG Y-Cables here, the one for Sennheiser should be similar to this one:

I soldered some 0.1″ pin headers to the modular jack pins on the back of the PCB to capture the signalling between phone and headset base while making some test calls.

The DHSG part looks quite straightforward and can also be found in the specifications above:

DHSG / EHS wireless headset serial bus protocol signalling / commands specification
DHSG / EHS wireless headset serial bus protocol signalling / commands specification

8P8C pinout at EHS36 (only 3-6 seem to be connected):

Pin 3: GND
Pin 4: DHSG RX (“Bus Send”: headset -> EHS36)
Pin 5: +3.3V
Pin 6: DHSG TX (“Bus Receive”: EHS36 -> headset)

Due to the pause after each bit, it will not work with standard 8 bit hardware UART though.

However I will focus on the interface between phone and EHS36 here:

Here we have SPI with CPOL = 1, CPHA = 0 (“Mode 2”) and a variable bit count, so you probably cannot use most of the hardware implementations available in microcontrollers for this part either, since most of them can only handle fixed 8 bit data chunks.
There’s also an LDO voltage regulator on board that generates 3.3V for the logic levels from the phone’s 5V supply.

6P6C Pinout at EHS36 (conntected to phone’s EXT port):

Pin 1: MISO
Pin 2: MOSI
Pin 3: SS
Pin 4: GND
Pin 5: Vin (+5V)
Pin 6: SCLK

Let’s start with the most frequent signal:
Exactly once per second, the phone will send a 15bit poll message to the EHS box:

15 bit poll:
MOSI: 101100001100000
MISO: 000000000000101

(it seems that somewhere between bit 9 and 11 the EHS36 eventually decides to reply with …101)

Periodic “poll” signal (1 Hz) between Yealink T46G phone and EHS36

The same “poll” also occurs in between the 1Hz interval, directly before any other command will be sent.

So let’s ask the EHS36 to signal an incoming call to the headset: On SPI we will have the above “poll” followed by another message (/SS released shortly in between!) that is 21bits in length:

21 bits ring:
MOSI: 101000001000000000010
MISO: 000000000000000000000

The EHS36 then signals DHSG Code 1 to the headset base, and you can hear the ringing indicator in the earpiece.

Now what if you press pickup on the headset?
Upon receiving DHSG Code 2, EHS36 will set MISO high, which makes the phone immediately pull /SS low and start generating the clock for a transfer:

At first we see the same “poll” command as above, however MISO is still high during the first 12 bits:

MOSI: 101100001100000
MISO: 111111111111101 (apparently, only the last 3 bits are considered as acknowledge from EHS36 to the polling?)

After that, /SS will be released shortly and a new 19bits transfer is started – the phone will read the command from EHS:

19 bits pickup:
MOSI: 0111111111111111111
MISO: 1110100000100011001

EHS36 now confirms with DHSG Code 2 and the call is being accepted by the phone; curiously this is followed by two more “poll” commands with sometimes very delayed SPI clock (phone seems to be busy handling the call). After several hundred milliseconds to establish the call, the phone notifies the headset about the successfull connection again with DHSG Code 2 (this will also happen when you press accept on the phone rather than headset, or after you have dialled a number and the remote party accepts):

Again, we see a “poll” command followed by a signal that is 21 bits in length:

21 bits call established:
MOSI: 101000001000000000000
MISO: 000000000000000000000
EHS36 logic signals when picking up a call by pressing the headset button
EHS36 logic signals when picking up a call by pressing the headset button

What’s missing? Hanging up by pressing the headset button!

If the headset base sends DHSG Code 1 during a call, exactly the same will happen on SPI side as for pickup:
A “poll” with first 12 bits MISO high, followed by 19 bits pickup command. This is followed by two “regular poll” commands.
The base then receives DHSG Code 3 as an indicator that the call has been terminated.

Where to go from here?

Using the information above, it should be theoretically possible to build your own EHS-adapter for Yealink <-> DHSG, though this would most probably not be cheaper than buying the original one in the small scale.

However, you could add some nice features like Busy Light Indication – think of a little LED sign on your desk that shows clearly whether you are available or not?

There seems to be no busy light on the market yet that natively uses the EXT port on Yealink phones – all that money can buy at the moment will use workarounds like monitoring the state of the microphone audio line between phone and headset – through another little black plastic box on your desk… *sigh*

For the kuando Busylight Combi, for example, there’s even a dual version that allows you to use *either* handset or external headset, by looping both microphone lines through the box…

So stay tuned for my future attempts of putting EHS, Sennheiser Y-Cable and Busy Light Ddetection all into one little box 🙂

DVB-T2 HD auf billiger China-Hardware?

Gerüchte um die Abschaltung von DVB-T zugunsten eines Nachfolgestandards gibt es schon seit Jahren, vor einiger Zeit wurde es dann konkreter: Multinationale Ballsportereignisse zeigten sich ja schon in der Vergangenheit als äußerst geeignet, um neue Generationen von Entertainment-Hardware unter das konsumfreudige Volk zu bringen.

Also mal spaßeshalber bei AliExpress den nächstbilligsten China-DVB-T2-Stick für $18 bestellt, nur so zum Spielen, ohne Garantie auf Empfangbarkeit der terrestrischen Ausstrahlungen in Deutschland. Seit dem 31.05.2016 gibt es nun auch den ersten Pilotmultiplex in den Ballungsräumen.

Es folgt die Essenz aus stundenlangem Wälzen von Bullshit-Bingo und mehr oder weniger hilfreichem technischen Wissen und Halbwissen, das man zu dem Thema finden kann:

DVB-T2 wurde als ETSI-Standard ursprünglich mit MPEG-4 (H.264) eingeführt
in Deutschland wird als Videocodec HEVC (H.265) genutzt,
außerdem eine Modulation nach aktuellerer Spezifikation mit Multi-PLP (Physical Layer Pipes)
geschützte Wort-/Bildmarke "DVB-T2 HD" der Landesmedienanstalten als Zertifizierung für Geräte
Übertragung in 1080p50, öffentlich-rechtlich meist hochskaliert, EM-Spiele jedoch nativ in 1080p und damit momentan besser als über Satellit / Kabel!
momentan billigster China-Gadget DVB-T2-Stick von Astrometa:
RTL2832P + R828D (diese Kombination klingt doch irgendwoher vertraut)
+ dazu noch ein externer Demodulator für DVB-T2, fertig ist der Stick für DVB-T2
Demodulator-IC von Panasonic: alte Version MN88472, unterstützt kein M-PLP
Stick aufmachen: neue Hardware-Revision mit MN88473 drin?
juhuu, dann fehlt jetzt nur noch die Software!
Klickibunti-Player TVR von Astrometa für Windows:
immerhin halbwegs zum Scannen geeignet, will bei mir aber auch nach Installation von LAV Filters kein H.265 decodieren

E drücken für Einstellungen
Strg + Alt + Shift + D für erweiterte Hardware-Informationen
VLC unterstützt H.265 (und/oder den Demodulator!?) erst ab Version 3, also aktuelle Nightlies runterladen!
Im VLC dann nicht drauf reinfallen und "DVB-T2" wählen, sondern DVB-T!

What’s the frequency, Kenneth?

Schon bei der Einführung von DVB-T hielt man es seitens der Sendeanstalten offenbar nicht für nötig, die Sendefrequenzen rauszurücken. Neben detaillierten Übersichten zu FM-Radiofrequenzen in den Regionen und den Parametern für den SAT-Empfang wurden DVB-T-Nutzer damals lediglich darum gebeten, auf ihrem Klickibunti-Gerät “einen Suchlauf” durchzuführen.

Geht mit VLC leider nicht, muss auch nicht (man könnte aber sicher einfach ein Batch-/Shell-Skript basteln).

Eine Übersicht der Frequenzen für den DVB-T2 HD Pilotmultiplex (sowie viele weitere Informationen – bisher die mit Abstand hilfreichste Seite) gibt es bei

Im VLC wird der Wert in kHz eingegeben, also z.B. 626000 für Hannover, 658000 für Hamburg etc.

Falls der Empfang in ländlicheren Regionen nur auf dem Dachboden wirklich brauchbar ist, kann man so im Gegensatz zur Klickibunti-Software immerhin auch mit wenigen Klicks einen Streaming-Server aufmachen und übers lokale Netz womöglich sogar direkt auf den Fernseher im Wohnzimmer streamen – falls man sowas besitzt.

Mir hat in den letzten Jahren jedenfalls ein DVB-T-Stick für sporadisches Fernsehen völlig ausgereicht.

Wenn die Privaten nun langfristig mit der DVB-T2-Plattform Freenet.TV planen, ihre Reichweite in den Ballungsräumen zu reduzieren und terrestrisch nur noch verschlüsselt zu senden, sollen sie das gerne tun.

Wenn man dann mittelfristig noch mit der analogen Kabel-Abschaltung rechnen kann, bliebe eigentlich nur die SD-Übertragung auf 19,2° Ost, die dem Deutschen Privatfernsehen überhaupt noch einen Rundfunk-Charakter geben würde – alles andere sind meiner Meinung nach längst Telemediendienste.

Möglicherweise ist das auch mittelfristig die Intention, die lästigen Bedingungen für eine Vollprogramm-Lizenz eines Tages endlich loswerden und über sämtliche Inhalte frei entscheiden zu können?

Vielleicht gibt es dann irgendwann wieder nur noch öffentlich-rechtliches Fernsehen in Deutschland, wie vor über 30 Jahren…