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STM32L4 Sensor Tile

Small, connected device for smelling and hearing in any environment.

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This is a 20 mm x 20 mm four-layer pcb tile full of interesting sensors (ICS43434 I2S Digital Microphone, MPU6500 acclerometer/gyro, BME280 pressure/temperature/humidity, and CCS811 air quality) with a Rigado BMD-350 UART BLE bridge for sending data to a smart phone all managed by a STM32L432 host MCU.

The STM32L432 is programmed using the Arduino IDE via the USB connector and serial data can be displayed on the serial monitor to verify performance and proper function, etc. But it is intended to be powered by a small 150 mAH LiPo battery for wireless sensing applications. The STM32L4 is a very low power MCU and with proper sensor and radio management it is possible to get the average power usage down to the ~100uA level, meaning a 150 mAH LiPo battery can run the device for two months on a charge.

There is a 1 MByte SPI flash memory on board which allows either data storage for very low duty cycle BLE radio transmission and/or to store a firmware upgrade for OTA reprogramming of the STM32L4.

There is a MAX1555 LiPo battery charger on board, a power switch that controls the enable on the NCV8170 LDO, and 3V3, GND, SDA, and SCL are exposed as through holes on the board for ease of testing and in case another I2C sensor needs to be added.

The pcbs are available at OSH Park in the shared space.

Arduino-programmable STM32L432 development boards can be purchased here.

Update February 12, 2017

I assembled the first three pcbs I received from OSH Park. and on at least one of them I got almost everything working. Assembly was difficult since the board is crowded on both sides with not a lot of edge to grab onto. I guess this is the price of a small, compact board with lots of sensors!

I verified that the STM32L4 can be programmed via the USB using the Arduino IDE, of course, and that I can blink the on-board led on pin 13, read the internal temperature sensor and VDD voltage, as well as get time and date from the embedded RTC.

I also verified that the BME280 pressure/humidity/temperature sensor, ICS43434 microphone, 1 MByte SPI flash, and MPU6500 accel/gyro work as expected.

I had a bit of trouble with the CCS811 air quality sensor, partly because it is new to me and mostly because the land pattern is pretty tight for a LGA package. I might have to redo the footprint to make it more likely to make a good connection to the board. Of the three boards I (partially) assembled, one has a short between 3V3 and GND due to the poor soldering of the CCS811, one functions well enough but the I2C address of the CCS811 is 0x5B when it should be 0x5A since I set the ADO pin to GND in the design. So there is obviously some kind of connection problem on that one too. The third one seems fine, the I2C address of the CCS811 is 0x5A as it should be. I can't get proper data out of either of these last two and the heater current shows zero Amps and the heater voltage shows 0.83 V so the heater circuit doesn't seem to be working properly. I have asked AMS for access to the application notes so I can figure out how to properly configure this sensor.

The battery charger works as does the battery voltage monitor connected to an ADC of the STM32L432. The 12-bit ADC makes it possible to detect changes in battery voltage of 0.01 V, so one can watch the battery voltage slowly rise in just a few minutes as it charges. The temperature measured by the BME280 and internal STM32L432 temperature both rise several degree when the battery is being charged. I guess this draws a lot of current through the MAX1555 (~100 mA IIRC).

The Rigado BLE module is missing since Digikey won't have them in stock for another week or two. I have two samples from Rigado I assembled into breakouts for testing; I suppose I could harvest one of these to add to the Sensor Tile but I think I will just wait to get a real supply of them.

So what is the rationale for these sensors, are they chosen at random? Not really. The device will find first applications in sensing emplaced capital machinery. The idea is that there are a lot of expensive industrial machines operating in production lines that do not have adequate monitoring of their function and performance. Or such data needs to be manually accessed and printed out of a controller designed twenty years ago, etc. So this sensor tile is designed to be mounted to these machines, and return data wirelessly, automatically, and remotely. So it has to last a long time on a LiPo battery, has to send data that can be funneled up to the cloud or to a server, and has to measure things worth knowing.

One thing worth knowing is whether the machine is operating or not. This is easy to tell just be monitoring the vibrations detected by the accelerometer, which tend to have normal modes at the 60, 120, and 240 Hz supply voltages of AC motors that run them. In...

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  • Upgrading BMD-350 Firmware, Hot Swapping AT Mode

    Kris Winer07/19/2017 at 03:03 0 comments

    07/18/2017

    The BMD-350 comes loaded with firmware that creates a BLE UART bridge such that simply writing Serial2.print (the BMD-350 is on the STM32L432 host's Serial 2 port) in the host's Arduino sketch sends anything one would normally write to an IDE serial monitor to the UART console in the Rigado Toolbox app on the smart device (iPhone in my case). This is convenient for checking on current conditions or proper sensor operation during a logging session, for example. And the BMD-350 UART configuration (baud rate, TX power, etc) can be changed on the fly using the Rigado Toolbox app.

    The way the BMD-350 is configured in the host sketch is by entering AT Command mode, passing some configuration statements via Serial2.write, and then entering UART bridge mode for normal functioning. In order to enter AT Command mode one has to 1) hold the AT Mode pin on the BMD-350 module LOW, 2) then hold and release BMD-350 Reset LOW to reset the module. Of course, these pins are connected to STM32L432 GPIOs for this purpose. When the module powers on it senses the AT Mode pin and if it is LOW, the module enters AT Command mode. After the commands have been entered, the reverse process puts the BMD-350 back into UART bridge mode for normal operation. All of this takes a bit of time (~seconds), but for a one-time initial configuration (i.e., in setup) this is fine.

    Rigado has impemented a hot swap AT mode such that if the hot swap is enabled during the BMD-350 initialization, then simply holding the AT Mode pin LOW during the running of the sketch will put the BMD-350 in AT mode and the configuration can be changed "on the fly" in the same way the configuration can be changed from the Rigado Toolbox app on the smart phone. Resetting the AT Mode pin HIGH returns the BMD-350 to BLE UART bridge mode. This is very convenient. More on this in a moment...

    I upgraded the firmware on the BMD-350 nRF52 BLE module on the Sensor Tile since I wanted to be able to use the hot swap capability just added to the new protocol 3 firmware; the BMD-350 came with protocol 2 from the factory. After some initial fussing and after I finally decided to actually follow the directions on the Rigado website I succeeded in getting the firmware update to work. The update process was accomplished via OTA from the iPhone and required e-mailing myself three separate .bin files, a bootloader, the S132 soft device and the new BMDware (firmware). For each file I used the Rigado toolbox firmware upgrade tool to load the corresponding .bin file and after a few false starts got everything properly loaded. In order to make my Sensor Tile work again with the new firmware I had to "restart" bluetooth on my iPhone, meaning I had to disable and re-enable it in Settings for the changes to "take". Don't ask me why, it all finally worked so---no harm no foul!

    Now the initial idea for my interest in hot swapping was so I could set the Tx power to the lowest setting, as I have been doing, but just before a Serial2.write I would bump up the Tx power to 11 or at least a higher-than-minimum value to temporarily increase the range. Then after the message was sent, bump it back down to the minimum to save power. Nice idea, but it takes time to effect the mode changes even without having to reset the module twice. For data update rates of ~1 Hz like I am using on the Sensor Tile this is simply not practical. The reason is that it takes ~100 ms or so each direction. So my idea of using hot swapping as a power saving strategy isn't going to work.

    But there is another potential application of hot swapping that might.

    Turns out BLE, despite the hype about BLE 5.0, is often best as a near-field protocol. Meaning for a lot of applications, people choose BLE because it requires a lot less power than wifi. The lowest power is achieved with the lowest Tx power settings, meaning using the BLE as a near-field communication protocol for when someone brings a smartphone close, say within a few feet, is where...

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  • Sensor Tile Ears to Add to the Nose

    Kris Winer07/16/2017 at 22:52 0 comments

    07/16/2017

    I finally got around to playing with the ICS43434 I2S digital microphone on the Sensor Tile. I used the I2S library that is part of the STM32L4 Arduino core and Sandeep Mistry's Arduino Sound library for the FFT analysis. Both are very easy to use. But for some reason the I2S mic did not play well with the SPIFlash.h class I created. Both work well without the other and the SPIFlash class works well all of the time but there is some conflict I haven't yet discovered so in order to make some progress, I just regressed to having the SPIFlash functions in the main code. Now all is well.

    I am reading the microphone data at 8000 samples per second, rather low for CD-quality music but perfectly fine for voices and machine chatter, etc. I capture 128 sound samples, perform an FFT and then normalize the spectrum and output only those frequencies that are 50% or higher of the maximum intensity of 64 frequencies in the spectrum. So if there is nothing but low ambient sound, nothing is output onto the serial monitor. If I speak, then a handful of frequencies appear. I am only plotting the few major modes once a second just to get a feel for what the data might look like.

    Here is an example of the output on the serial monitor:

    This from me holding some more or less constant tone voice sound. Apparently the sound has a dominant frequency of ~250 Hz. It is interesting to see the normal modes, and it would be easy to use this capability to detect clapping, for example, or detect when someone was speaking. But I can see already that I will want to capture complete time series of the data for speech recognition. How cool would it be if the Sensor Tile could recognize and respond to the key words "Open the pod bay door HAL" or "Soylent Green is people"!

    I managed to send these dominant normal modes to the iPhone via BLE so when I speak I can see the response on the UART console. I had been converting the floats to strings using dtosrtf and assembling into a packet for transmission via serial and the BLE UART bridge using sprintf but I finally realized all I need to do is a Serial2.print to send the data to the iPhone serial console! Not sure why I started with the string packaging. Not necessary in Arduino.

    If I hum a constant note (as well as I can anyway) I see one or two dominant frequencies at 250 Hz or 300 Hz or whatever note I can hold (see above). I checked with a metronome tone generator set for 440 Hz and I see 437 Hz with a small side band at 500 hz. I think the way I have the FFT set up the frequency resolution is only 62.5 Hz (8000 Hz sample rate divided by 128 FFT size) so I am not discriminating very finely yet. For normal voice recognition I should probably set the sample rate to 2000 or 4000 Hz or so. Or I can increase the FFT size to 256 or 512 to get better frequency resolution, but at the cost in compute power. The STM32L432 has a Cortex M4F processor and the floating point engine is not really taxed at these kinds of rates. Not sure where the limit is though.

    The bigger problem is the flash memory required to hold the FFT code. The full sketch is 214 kB of which ~175 kB is the FFT package alone; this is getting close to the ~245 kB flash memory limit on the STM32L432. I think there are a lot of lookup tables and what not driving this load and I would like to find some lighter implementation of the FFT analyzer, but as far as I can tell everyone uses this CMSIS FFT package.

    The point of all this is that all of the sensors including the I2S microphone now work and work together with a lot of the relevant data being sent to the smartphone via BLE.

    The smartphone is convenient as a check but I want to start using the nRF52 in central role as a point-to-point gateway so I can get the data onto the Arduino IDE serial monitor running on the laptop. I am having the nRF52 add-on I designed for the Butterfly produced in China and I expect to start using this to monitor several of these Sensor Tiles at once for real-time data analysis....

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  • Sensor Tile as an air quality sensor

    Kris Winer07/09/2017 at 01:49 0 comments

    07/08/17

    The latest experiment was a partial success. I essentially repeated the last experiment but placed the Sensor Tile outside the house, near the threshold on a portico support pillar out of the sun. Disappointingly, the first several hours of data recorded on the SPI flash were all zeroes, so something is going wrong in the packet construction. I will have to sort this out. I got 53 hours of actual data when I should have gotten ~80.

    This time I recorded the equivalent CO2 (eCO2) and total volatile organics (TVOC) from the CCS811 measured at 60 second intervals, as well as the usual pressure, humidity, temperature, etc. The temperature data look familiar:

    showing peaks at ~2 pm and troughs at ~4 am every day (again, this should be July not January! I have to find and fix this problem too).

    The eCO2 and TVOC data are interesting:

    The eCO2 and TVOC levels shows a lot more sensitivity than I would have expected. Since I am compensating the CCS811 resistance measurement with current temperature and humidity data I am wondering if the large variation in eCO2 and TVOC levels is an artifact of the underlying variations of these instead. Although it makes intuitive sense that pollutants in the air would be maximum 1) in the middle of the day when people and machines are maximally active, and 2) that the volatiles and pollutants would be maximum at the hottest part of the day since this would be the condition for maximum outgassing of rotting vegetation, etc. I think the shorter duration excursions at ~10 pm and ~10 am are due to our old car arriving for the night or leaving in the morning, respectively. The Sensor Tile is close to where we park our cars.

    I recorded all data at 10 second intervals but the CCS811 has a new measurement only once every sixty seconds. Yet, this was enough to capture the relatively rapid change in the eCO2 and TVOC levels. I probably do not need to measure the temperature, humidity, and pressure at 10 second intervals so I might reduce the data logging interval to sixty seconds generally since SPI flash writes use up some power. Also, with eight 32-byte samples per page and 4096 pages, there is a lot of data (even when half of it is lost) and Open Office is having trouble crunching all of it.

    Astute observers will note that I have recorded the components of acceleration as well. These were pretty boring in general. I need to configure the accel with a high-pass filter and then interrupt on significant motion to see if I can capture vibrations above ambient. I have the second interrupt already configured as a tap detector. With the high-pass filter I should be able to analyze any high-frequency acceleration data with a FFT to extract normal modes. This is an outline for an earthquake detector when logging environmental data as in this experiment, or as a diagnostic for equipment faults in an industrial application.

    Lot's to do yet...

  • Sensor Tile Update-Power measurements and data logging

    Kris Winer07/04/2017 at 20:55 0 comments

    07/04/2017

    I ran my first really successful test of the logging features of the Sensor Tile in the last few days and collected some interesting power efficiency information.

    Firstly, I just ran the Sensor Tile with no attempt to manage the power usage. I was collecting data from the CCS811 air quality sensor (one of two big power users on the board) at 10 second intervals, with the BMD-350 nRF52 module (the other big power user on the board) operating with its defaults and sending data via BLE also every 10 seconds. In all of these experiments, I was simultaneously logging data to the 1 Mbyte SPI flash, although it took me a while to get this right. The result of the first experiment was a total run time of 31.5 hours on a 150 mAH LiPo battery; just under 4.8 mA average power usage. Not really great. Furthermore, I made several errors in setting up the SPI flash logging and ended up in these first few experiments with all zeros or a completely erased flash. But I finally figured this out too.

    I was able to configure the TX power of the BMD-350 to its lowest power level (-30 dB from the -4 dB default) and maxed out the advertising interval to 2.5 s (from the 1.285 s default). These are the lowest power setting of the BMD-350 module when advertising BLE data to a central (my iPhone).

    There is a way to enable hot swapping of the AT mode where these parameters can be set without resetting the BMD-350 module. This is useful for keeping the power minimized except when data has been packaged for transmission, when the TX power would be set to maximum (or at least a higher value than -30 dB), the data sent, the serial buffer cleared (TX complete) and then the power could again be minimized until the next data package was ready for transmission. This is the righter way to do things, but the firmware currently on my BMD-350 (Protocol 2) doesn't support this feature. Fortunately, it is straightforward to upgrade the firmware via OTA, I just haven't gotten around to doing so yet. So in these experiments, I simply kept the TX power (which affects the reception range) and advertising interval (how long one has to wait when pairing with the iPhone) at their low power limits. Even so I had no trouble reading the BLE data whenever I wanted within 10 feet of the Sensor Tile, which is good enough for these experiments.

    The next thing I tried was some basic power management. I decreased the duty cycle of the CCS811 air quality sensor to once every 60 seconds, put the BMA280 in low-power mode (not really a big player in power consumption anyway) with a 50 ms sleep interval (effectively a sample rate of 20 Hz) and with the BMD-350 in its lowest power mode I asked for BLE data at 1 Hz and again attempted to capture all of the data on the SPI flash. In the end, I got all zeroes again on the flash but managed to get streaming BLE at least for 90 hours on a 150 mAH LiPo battery for an average power usage of just under 1.8 mA. This is a significant improvement in power usage!

    The CCS811 is really quite a power hog, supposedly requiring 7 mW (~2.1 mA @ 3.3 V) at the 10 second per sample setting and 1.2 mW (360 uA @ 3.3 V) of power at the 60 second per sample setting per the data sheet. I wanted to figure out how much the CCS811 is really costing me. So I put the CCS811 in idle mode (where it presumably takes ~10 uA) and essentially just repeated the last experiment. To my pleasant surprise the Sensor Tile lasted at least 120 hours (having died sometime between midnight and 9 AM) for an average power usage without the CCS811 of < 1.25 mA. So running the CCS811 once per 60 seconds is actually costing >550 uA, a bit more that the data sheet (which in fairness is spec'd at 1.8 V and I am at 3.3V).
    Even better, I got almost exactly 96 hours of sensor data recorded every ten seconds on the SPI flash for later analysis. I discovered yet one more error in that I had set the maximum number of pages to 0x0EFF instead of the 0X0FFF it should have been for the 4096, 256-byte...

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  • First revision of the Sensor Tile

    Kris Winer06/18/2017 at 06:33 0 comments

    06/16/2017

    I redesigned the Sensor Tile slightly replacing the power switch with a smaller ALPS SSAJ110100 switch, replacing the 3 mm x 3 mm MPU6500 with a 2mm x 2 mm BMA280 accelerometer but otherwise it's still the same concept. The accelerometer change is to reduce the cost and improve the performance. The BMA280 has two multiply-configurable interrupts, is much lower power than the MPU6500 (I never really had a use for the gyro anyway) and offers a rich variety of interrupt configurations (single and double tap, pan, tilt, portrait and landscape detection), as well as a high-pass and low pass filter. It is much more versatlie than the MPU6500 and a better choice for a low-power wearable device.

    The goals of the project remain the same. And I expect the BMA280 will make achieving them a little easier since I can run the accelerometer at 2 kHz (unfiltered) which means I can detect vibration and sound up to 1 kHz or so. This nicely complements the ICS43434 I2S digital microphone (50 - 20,000 Hz).

    While the board has been slightly modified, I finally got around to learning how to construct "proper" C++ libraries and rewrote the single, integrated Arduino sketch I started with into a series of individual sensor libraries (with .h and .cpp files) and now I am able to control the I2C sensors with proper constructors resulting in a slimmer main code (still ~400 lines) that is a little easier to manage. Here is the library.

    I am using interrupts wherever possible (BMA280, CCS811, and RTC) but for the devices that have no interrupt (ICS43434 and BME280) I am using the RTC alarm to interrupt in order to set the duty cycle for reading and reporting data, either to the serial monitor or to the smartphone via the UART bridge provided by the BMD-350 (nRF52) BLE module.

    I still need to figure out how to efficiently transfer the data via BLE so I can run the Sensor Tile at the lowest CPU clock speeds (~1 MHz) and use the lowest power mode (STM32.stop between interrupts) to cut the average power usage down as low as possible while still getting useful data via BLE to a laptop.

    To this end I have designed an nRF52 add-on for the Butterfly (STM32L433 development board, sister to the STM32L432 used as host MCU for the Sensor Tile) that will allow the nRF52 to operate in central role and pass data to the Butterfly via its own UART bridge and then on to the laptop via the USB cable. Thus I expect to have a wireless Sensor Tile spewing lots of data wirelessly to the laptop for capture and analysis.

    Still a lot to do but the path is getting clearer and I see no showstoppers along the way to prevent eventual success.

  • Update for March 9, 2017

    Kris Winer03/10/2017 at 05:04 0 comments

    I finally got back to this project and made some progress today on several fronts.

    First on the anomalous I2C address of the CCS811, which occasionally shows 0x5B even though I connected the ADO pad to GND in the design. I discovered that since this pad is connected to the large ground pad in the center of the CCS811 that the solder will wick away from the ADO pad during assembly and result in no contact between the address pad and GND, and a subsequent setting of the I2C address to 0x5B which seems to be the default. That is, the pad is pulled up by default. The solution is to butter this pad with solder paste either before (best) or after assembly (what I did) to ensure good contact. This kind of thing is a common problem with LGA packages, as I have learned the hard way on the LSM303D.

    Secondly, I was getting weird results from the CCS811. I got CO2 levels way above what should have been the maximum range and the VOC always showed 144 ppb. Well, it turns out I had the register indices reversed, and now I am getting ~400 ppm CO2 and ~10 ppb VoC, unless I breathe on the Sensor Tile, and then these values go up in a pleasingly responsive way-- I have a bad breath detector!

    Lastly, I took advantage of the BLE modules I finally received and assembled and started to set up broadcast of the data to the Rigado toolbox. So far I have battery voltage, BME280 pressure and humidity being written to the UART console on my iPhone every second via BLE and the BMD-350 UART bridge. It is not elegant but for the first step in a remote environmental sensor it will do.

    Next step is to package all of the relevant data into a data packet and send the packet every 20 - 50 ms to the laptop, where the packet sniffer Wireshark will allow me to grab it for parsing and display. Packaging pressure, humidity, temperature, acceleration (or vibration), CO2 and VOC along with time and date will need a packet of about 60 bytes or so, small enough for BLE to handle. But there is an I2S microphone on the Sensor Tile and apart from analyzing normal modes via FFT and passing these along with the data packet, it will take some more thought to allow sound recording to be sent via BLE. There is a 1 MByte SPI flash on the board so I could simply record the I2S data from the microphone and then send it periodically in a dedicated bit stream via BLE. Preferable would be real time voice/sound transmission, but this might require wifi, as in the ESP8285, to be practical. There are a lot of possibilities in this sector, and the least mature aspect of the Sensor Tile.

    The Sensor Tile is sort of a combination Start Trek Tricorder and Communicator rolled into one, but it will take me a bit longer and a lot more work to get the most utility out of it.

  • Added Rigado BMD-350 BLE module

    Kris Winer02/26/2017 at 04:24 0 comments

    February 25, 2017-Finally got a hold of some of the Rigado BMD-350 nRF52 BLE modules; Digikey finally had some in stock so I bought 100 for prototyping, etc. Soldering them onto the board was a piece of cake. They are very small, and the pcb that supports the module is quite thin, I think 0.5-mm.

    I had written a program last year when I got a couple of samples from Rigado to test the functionality of the UART bridge. The Arduino sketch (run on the STM32L432 on the Sensor Tile) instantiates two serial ports, one between the STM32L432 and the BMD-350 and one between the STM32L432 and the laptop through the USB cable.

    The program starts by putting the module into AT command mode. There I check the module, bootloader, and firmware version numbers set the baud rate, query and/or set various parameters like flow control, parity and whatever else I need to configure the UART bridge parameters. Then the program puts the module back into UART bridge mode and the program waits for valid data to be sent from the laptop console, which is then sent on to a UART console running from Rigado's toolbox on my iPhone as a UART console app. Whatever I type in the UART console on the iPhone app shows up after some delay on the laptop and vice versa.

    The speed of the transmission is surprisingly slow. The top speed I expect per Rigado's data sheet is 1 kByte/s, appallingly slow compared to wifi, for example. But the words that appear on either console take a second or two to spell themselves out so the rate seems even slower than this.

    Next to do is to package the sensor data and have it sent by the program to the console. I think I am going to have to figure out how to write iOS apps if I want to have a custom interface to accept my Sensor Tile data. Well, at least all of the hardware is working as it should!

  • February 14, 2017

    Kris Winer02/15/2017 at 05:14 0 comments

    I received an application note from AMS on how to interface with and program the CCS811 Air Quality sensor and discovered the missing bit of information, which was I needed to write to the application ready register to take the CCS811 out of boot mode (into which it enters on power up) and into application mode. Once I did this I stopped getting I2C read and write errors and started getting data as well as a data ready bit in the status register. I got the interrupt working as well as the humidity and temperature compensation from the BME280 sensor to improve CCS811 accuracy. Over all the sensor seems to be working as designed. I need to let it stay on for a while to "burn in" before I can get reliable readings, but overall the initial phase of the sensor tile is a success. All sensors on board are functioning and returning data.

    I measured the current used by the Sensor tile with no great effort to minimize power usage yet and found that when the BME280 and MPU6500 are running normally as well as having the STM32L4 host run at 80 MHz the current is about 6.25 mA, more or less what I expect. However, when the CCS811 air quality sensor is enabled once every 10 seconds in the present operating mode the current jumps to about 25 mA. If it takes ~1 second to get the air quality data, this is an additional ~25/10 ~ 2.5 mA for a total of almost 9 mA average current. Way too high to be practical as a remote sensing device. So as I work towards lowering the average power used by this device I will have to pay close attention to how long I enable the CCS811. In the application note there is an example of enabling it just long enough to read I2C data, but I think the best method is to take advantage of the rather long duty cycle (once per 60 second measurement) available and interrupt on threshold capability to save power.

  • First build

    Kris Winer02/12/2017 at 19:14 0 comments

    February 12, 2017:

    I added some details about the results of the first assembly of the Sensor Tile. Overall everything works, but I haven't got the CCS811 air quality sensor to work properly yet and I am still waiting for the Rigado nRF52 BLE modules to become available.

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