Low Power LoRa

Low Power LoRa Weather Station

A solar powered, batteryless LoRa (expandable) sensor node powered from a supercapacitor charged by a small solar cell using an energy harvester. This was built around the Semtech SX1262 LoRa transceiver (US band; 915 MHz) and the low power MSPM0L110x MCU (TI; M0+), this board is a monitoring platform for anything you might want! The board is made around the solar cell and is roughly 23x24mm.

Overview

When researching this concept, I had found a few previous examples of this being done (e.g. nootropicdesign). I had not found an example, however, of a board that was fully self contained. When I was researching the component selection I really wanted it to be fully on board, which drove my decision to use a supercapacitor over a traditional Li-ION cell (almost none are SMD) and made me want to use a chip or PCB antenna. I found a supercapacitor that should work (this is my first time working with them), but I did not have enough room for an onboard antenna. Because of this I decided to add an external antenna through a connector. Another reason for this is the solar cell can block the RF waves from propagating properly. My project is still fairly unique from any existing solution, because I have found that it is rare to use a SMD solar cell. This is one of the biggest ones available on the market, as most are designed for handheld calculators. Figuring out the power budget was very difficult for this project, and that was probably the thing I spent the most time on. Thankfully, the SX1262 datasheet had detailed power budget options, and I was able to base it off of that (almost all of my battery consumption should be from that). This project taught me a lot about the importance of component selection! (I probably spent 2 times longer on this than any other project I had done vs the routing). When I get the board, I hope I can get it so that it is watertight. I have already done some research into gaskets for the humidity sensor (as this needs access to the outside world to get a good reading), but this seems pretty hard to do. There is a version of the sensor that has this built in, but it is far more expensive and is hard to get from most suppliers. I plan to use silicone around the PCB to protect the border of the solar cell from accidental damage, and put a piece of fiberglass over it. I made sure to keep all the parts that would petrude together, so I should have a slightly easier time designing the case. I hope I can get this board and test it out soon!

Key Features

• Ultra Low Power This PCB should run at ultra low power, and may be able to run continuously in good sunlight (although it is designed for far less).
• Small Board Size The board for this is quite compact, and is mainly constrained by the size of the solar cell.
• LoRa Transmit This should be able to transfer over very long distances, and should be able to also extend range of other forming a "mesh"
• Future expansion Plentiful GPIO pins are broken out, so if one wanted to add more sensors, it would be as easy as soldering additional connectors to the PCB test pads.

How It Works

1. The solar cell charges a supercapacitor through the BQ25504 energy harvesting IC, which is designed to extract power efficiently over a long period of time from very low amperage power sources. The datasheet mentioned being able to charge off of a piezoelectric element which would be way under the amount of current generated by a solar cell. Hopefully, this will overcome the natural discharge of the supercap.
2. A PMOS, switched by the MCU, gates power (VSYS) to the SX1262 and sensors, keeping the idle current as low as possible between transmissions because far less of the passive components are powered, and everything remains fully off.
3. The MSPM0L110x wakes periodically (very low sleep current ~<1uA; nothing compared to capacitor discharge), turns on the MOSFET to wake the sensors, reads the temperature/humidity and pressure sensors over I2C, and transmits the data over LoRa using the SX1262. Monitors the BAT_GOOD pin and turns everything off when it needs to!
4. The system was sized so that the supercapacitor can supply more than enough for the estimated ~15–30 mJ needed for a transmit burst. This part selection could be more conservative, but it was outside the range of ceramic capacitors, and there were not a lot of good low profile supercapacitor options.

Hardware

3D Renders

PCB Layers

PCB Layers

Schematic: Low Power LoRa Schematic.pdf

Repository Structure

.
├── KiCAD/
│  ├── Low Power LoRa.kicad_pcb     # PCB layout
│  ├── Low Power LoRa.kicad_prl     # Project local settings
│  ├── Low Power LoRa.kicad_pro     # Project file
│  └── Low Power LoRa.kicad_sch     # Schematic
├── fabrication/
│  ├── bom.csv                      # Bill of materials
│  ├── gerbers.zip                  # Gerber files for PCB fabrication
│  └── pick_and_place.csv           # Pick-and-place file for assembly
├── images/
│  ├── Low Power LoRa Bottom View 3D.png
│  ├── Low Power LoRa Front View.png
│  ├── Low Power LoRa Layer 1.png
│  ├── Low Power LoRa Layer 2.png
│  ├── Low Power LoRa Layer 3.png
│  ├── Low Power LoRa Layer 4.png
│  └── Low Power LoRa Schematic.pdf
├── LICENSE
└── README.md

Ordering the Board

Fabrication files can be found here: fabrication/ folder: The BOM was generated using LCSC, so ignore the minimum part quantities (these get messed up due to MOQ). You should be able to use the KiCAD files to generate your own.

Programming

The MSPM0L110x is programmed with the SWD header. I do not have the board yet, so I will update this section more later when I have more information about real-world testing.

Acknowledgments

• SX1262 RF matching network and BQ25504 based on respective companies reference design
• Review from r/PrintedCircuitBoard! (Thank you!)

License

See LICENSE for details.