Adding a barometric pressure sensor to a LoRa mesh solar node

Recently I added an environmental sensor to a rooftop solar LoRa mesh node.  (There are two popular communities for LoRa mesh, namely Meshtastic and Meshcore.)  This provides telemetry of temperature, humidity, and barometric pressure.  In this article I describe what was required to accomplish this. 

The starting point was that recently I deployed a solar LoRa mesh node.

The device requires no cell towers, no internet, and no electrical power.  It provides LoRa connectivity to some twenty square miles of a rural area.  It is a SenseCAP Solar Node P1-Pro (manufacturer’s web page, Amazon page).

After some time I realized that this device is made so that one can add a sensor for temperature, humidity and barometric pressure.  The sensor is a BME280 (manufacturer’s web page).  The sensor is astonishingly small (2.5 mm by 2.5 mm) and draws astonishingly little electrical power (3.6 μA) and is strikingly inexpensive at less than $8 for a complete development board.

You can see the front and back of the development board (called “GYBMEP”) here.  At left is the sensor side of the board.  The sensor (a small silver-colored square) is at top right.  Across the bottom of the board, four terminals may be seen — VIN (5 volts), GND, SCL (clock), and SDA (data).

You can see a schematic of the GYBMEP board at right.

It turns out that there is a sort of informal industry standard called “Grove” (sponsor web page) for a four-wire bus permitting one or more sensors to be connected to a device.  What I needed to do is find a Grove cable and solder it to this circuit board.  This required that I learn which Grove pin performs which function.  The answer turns out to be:

• Grove pin 1 – ground (customarily black)
• Grove pin 2 – 5 volt power (customarily red)
• Grove pin 3 – SDA (I²C data, customarily white)
• Grove pin 4 – SDC (I²C clock, customarily yellow)

So I soldered the four wires of a Grove cable to the sensor board, as shown at right.  The result was a sensor cable assembly as seen here.

My next step was to open up the solar meshtastic node.  You can see the battery B which is four 18650 cells, each 3350 mAh at 3.6 V. This provides around 48 watt-hours of energy.  (They get charged up by a five-watt solar panel.)  You can see the GNSS chip G which is an L76K GPS module, and its GNSS antenna E.  You can see the Wio-SX1262 LoRa transceiver C.  You can see the XIAO nRF52840 processor F.  And, importantly for this discussion you can see the Grove port D.  And you can see that I added the sensor A.

Having added the sensor A, I closed up the housing and powered up the node.  I logged in at the administrative interface of the node and checked a couple of boxes to turn on “telemetry” mode for the data from this sensor.  And I was delighted to see the temperature, humidity, and air pressure information as shown at the start of the article.

Any user in the service area of this node can click on their device to see the temperature, humidity, and barometric pressure inside the node.

Have you worked with LoRa mesh?  Please post a comment below.