A project log for STM32L4 Sensor Tile

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

Kris WinerKris Winer 09/04/2017 at 22:330 Comments


I finally got the pcbs back from OSHPark with the latest useful changes and a corrected error and put one together this weekend and was pleasantly surprised that everything worked the first time. Always nice.

I moved the BME280 to the back of the board along with the CCS811 so these two "air breathers" could be shielded from sunlight hitting the VEML6040 on the board front. I also got rid of the resistor divider for monitoring LiPo battery voltage relying on the battery voltage, state-of-charge, and charge rate measured by the MAX17048 fuel gauge. I used the analog pin freed up for the orange indicator led that used to be on pin 13 (SPI flash CLK) to allow indication (in this case on for 10 ms whenever the SPI flash page is written --- once every sixty seconds). Lastly, I replaced the 1 MByte SPI NOR flash with an 8 MByte SPI NOR flash for what should be ~24 days of complete, continuous data recording at 0.1 Hz.

I also redesigned the BQ25504 solar cell LiPo battery charger by resizing the resistor network to tighten the under- ( 3.27 --> 3.58 V) and over- (4.27 --> 4.15 V) voltage thresholds as well as changed the layout to conform to the reference design in the datasheet. I also got rid of the battery OK led on the board. This is useful for indication of proper charging but is really not appropriate for a ultra-low-power system.

Last improvement was to take advantage of inexpensive plastic boxes for packaging the whole system in a more convenient and more permanent mounting. The plastic boxes from Bud Industries are perfectly sized for the solar cells I am using and I even made a small hole to expose the VEML6040 to more direct sun to get a more accurate correlation between charging and incident light. For mounting the Bud box on the pole, the solar cell on the outside of the Bud box, and the SensorTile3 on the inside of the Bud box I used pieces of VHB foam tape; a very handy addition to any maker's tool kit!

Overall, the system is a lot closer to a deployable environmental sensor than it has ever been. I want to make sure the new BQ25504 design performs properly and that I can make use of the data on the SPI flash. No reason to think these won't work but it is always prudent to test a new (or newly revised) design just to make sure. So after the test results are analyzed it will be time to put one or more of these devices out into the wild (we live in the middle of an oak forest) and see what can be seen... and smelled... and heard.

Even this last shake-out test is happening at an opportune time since there is a wildfire somewhere nearby producing waves of smoky, stinky air that ebb and flow as the wind shifts; a good test for the CCS811.