Low Power Environmental Sensor Board

Okay, so, to be blunt, I suck at building cases/enclosures/boxes/whatever... this will be a short final entry.

Simple, but kind of ugly.  :D

I cut a sheet of 4"x6" acrylic (poorly at that), drilled a couple of holes, threw in a few screws, nuts and standoffs and sandwiched it all together.  Attached at the back is a 2 cell AA battery holder with a bit of VHB double-sided tape.

Functional, but it does leave a lot to be desired in the aesthetics department...

Finally...  the PCB's arrived and they look awesome.

Everything worked straight away, much to my surprise.  I've never used ePaper prior and although the driving circuit for it was rather straight-forward, I couldn't really verify it would work until the PCB's came and were built.  Thankfully, nothing blew up or otherwise.

Power consumption at this point (still need to do a lot with the firmware), is about 57 micro-amps during sleep.  As I anticipate only waking things up every 10 minutes or so to take measurements and update the display, two AA batteries will keep this thing running for quite some time.

I did mess up a few things, however.  The FPC connector is too close to the edge of the board and needs to move a millimeter or two deeper; when the ePaper is folded over the board, the contacts on the cable are bending, which probably isn't great.

Also, the gas sensor, the SGP41, may need to be reconsidered...  which is unfortunate.  Sensirion has a nice library that you can send the returned values from the sensor to and it spits out a VOC and NOx index values.  Awesome... except that the datasheet then states, "SGP41 raw signals need to be provided to the Gas Index Algorithm at a sampling rate of 1 second."  Not great...

Seems like they really want this sensor to be operating continuously; not intermittently.  I missed this point prior to picking the SGP41 when skimming (yep, guilty) the datasheet...  and it was a rather important point.  Ah well.  I still get what appears to be valid results, but as I'm not doing it by-the-book, I don't know how much accuracy I'm throwing out the window.

After messing around with this and nailed down the firmware a bit more, it'll be on the revision 2.

I've spent entirely too much time finishing up the schematic and layout; real life tends to intrude entirely too often and for too long.  Wouldn't give it up for the world tho - I've a wonderful and quite understanding family. ;)

At any rate... updates:

• I've swapped out the TPS610994YFFR (which was a tiny BGA part) for a more sane SOT-25 part, the TPS613221A.
• Schematic completed.
• Layout completed.
• Manufacturing files have been sent off as of this afternoon.

To do:

• Check the BOM and see what I have and what I need.
• Order what I need from Digikey.
• Hope and pray that I've not screwed up the schematic and layout...

Schematic:

Layout:

As the ePaper display will be mounted to the "back" of the PCB and I needed mounting holes, I had quite a lot of space to work with.  In regard to the ePaper display, I still need to figure out the best way to mount it; double-sided tape perhaps?

I attempted to keep all of the high frequency switching stuff near the top of the board and away from SPI and I2C traces.  Not entirely certain this is the best layout, but hopefully it'll be good enough for a first revision.

Additionally, for the SHT45, I introduced cutout areas around the sensor and removed all copper below the sensor.  Hopefully, this will prevent inaccuracies (as best as I can do, at least) in temperature due to any heat generated by the SGP41's built in heater.  Then again, as I anticipate only performing sensor readings every 15-30 minutes, this may be overkill as the board will generally be cool - or at near ambient temperature - when the uC wakes up and grabs sensor values.

- Ordered 5 ePaper displays from buy-lcd.com

- Gathered KiCad symbols/footprints from Digikey and SnapEDA

Datasheets

https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bmp390-ds002.pdf

https://sensirion.com/media/documents/5FE8673C/61E96F50/Sensirion_Gas_Sensors_Datasheet_SGP41.pdf

https://sensirion.com/media/documents/33FD6951/662A593A/HT_DS_Datasheet_SHT4x.pdf

- BMP390 - Pressure Sensor (I2C)

- SGP41 - Gas Sensor


- SHT45 - Temperature Sensor

This is a very early thought-dump for a dual-AA battery powered indoor sensor with E-Paper display.

Specifications:

- Preferably AA battery powered - Why no lithium?  Kids... and their propensity to play with things that they probably shouldn't.

- Low power - Keep the uC in sleep for as long as possible.  Thinking about grabbing sensor values and updating the display every 10-15 minutes or so.

- E-Paper Display - Good power efficiency when compared to LCD's and OLED's.  Just not sure what the best way is to attach the panel to the PCB?  Double sided tape?  Nails through the display?

Components:

- STM32L431CBT6

- TPS610994YFFR (Low Iq 3.3v Boost)

- SGP41-D-R4 (Gas)

- SHT45-AD1B-R3 (Temp/Humidity)

- BME280 (Pressure)

- Good Display E-Paper Panel GDEY0213B74

Briefly checking around for STM32, CubeMX code examples, found some Arduino code fromSensirion AG for their sensors.  Should be easy enough to port over with the official datasheets:

Sites - Code Examples

• Sensirion Gas Index Algorithm
• Sensirion Core
• Sensirion I2C SGP41
• Sensirion I2C SHT
  

Good Display / Buy-LCD E-Paper Display GDEY0213B74 -- from their datasheet, a reference circuit for driving the display.  Seems simple enough:

Another example schematic that may prove useful:
https://www.crystalfontz.com/products/document/3991/CFA-10084AdapterBoardDatasheet.pdf


Panel dimensions are approximately 60 mm x 30 mm (2.4" x 1.2"), so a PCB a bit larger to accommodate mounting holes on each corner should fit all of the electronics without any issue... and slapping the E-Paper panel on the back of the PCB seems like it would work nicely.


Headers
- SWD Header (5 pin - 3.3v, SWCLK, SWDIO, NRST, GND)
- UART Header(3 pin - TX, RX, GND)