Perpetual Battery-Free Weather Station

The BFree shield isn’t for sale yet, as it originates from a recent academic project. But you can build your own, using the open-source Eagle design files and component list. Fabrication houses like OSHPark and PCBWay will make the PCB for you for a few bucks.

Next you’ll flash the BFree shield and the Metro M0 Express board with new firmware, so they can work together to restore a CircuitPython program after a power failure.

Follow the instructions at the Github repository to program your BFree shield with its pre-compiled .elf firmware file. (Optionally, the complete source code for the BFree shield, and how to build it from source.)

The BFree-compatible version of CircuitPython is called BFree-core. Download the pre-compiled .elf version of BFree-core, from the folder Metro M0 Express BFree Firmware at the Weather Station repo. Then use your serial wire debugger/programmer to flash BFree-core to the Metro M0 board, following the instructions here. We recommend using the debugger, not the Metro M0 bootloader, because BFree-core is still in development, and you’ll want the debugger to recover from a broken CircuitPython state (if it ever happens).

Attach the BFree shield on top of the Metro M0 (Figure A). The pins of the shield correspond one-to-one with the headers of the Metro.

Next, connect the Metro’s USB port to your PC. The Metro should show up as a mountable device in your operating system as usual.

Now we’ll verify that BFree works as intended, using the Test Code program from the Weather Station repo:

import time

# Setting the counter to 0 will only

# happen the very first boot

counter = 0

# The battery-free program should

# continue counting somewhere in the

# while loop

while True:

counter += ١

print(‘Counter =’, counter)

time.sleep(1)

This simple CircuitPython program increments a variable counter indefinitely. In a typical solar-powered system, the battery will deplete when there’s no sun. At this point, the counter will be reset to the initial value of zero. However, in the BFree-based solar-powered system, the counter will continue increasing from where it left off, once the system recharges after a power outage. To see this effect in action, upload the Test Code to the Metro M0 board, replacing the original main.py program pre-installed on the Metro with this new one. Do not disconnect the Metro from the USB port, and don’t forget to keep the BFree shield connected on top of the Metro M0.

Now monitor the serial port, letting the code run for several seconds. (If you’re not sure how, check this.)

Unplug the Metro M0 from the USB port (cutting off power) and plug it back in again. When the Metro is plugged back in, the counter should continue where it left off instead of resetting back to zero. It’s sustaining the computation state despite power failures!

Figure B shows the serial connection monitored by Minicom (type sudo apt install minicom to install it) with 1-second timestamps. You’ll see that the counter does not reset to zero when the system is unplugged, and that there’s a gap between Counter = 43 and Counter = 55. This is because the Metro M0 with its BFree shield is already continuing the CircuitPython program execution (incrementing the counter) before the USB serial port is even completely initialized.

Now let’s set up the weather station. Follow the wiring diagram (Figure C) to connect the Si7021 temperature and humidity sensor module (black) and the RFM9x LoRa transceiver (blue) to your BFree shield on top of the Metro M0.

Before running your weather station from the solar panel, let’s test that everything works on constant power. Connect the Metro to your computer, and upload the Battery Free Transmitter code from the Weather Station repo. This new main.py program accumulates temperature and humidity samples, averages them, and sends them as a LoRa packet in a continuous, non-breakable loop.

Now monitor the serial port as you connect and disconnect the Metro from your computer. Whenever the station broadcasts a LoRa message, it will also print a message to the serial port containing all the information sent in the LoRa frame and the number of samples collected since the start of the program (Figure D).

Now that your LoRa transmitter is working, you need a receiver. Connect the second RFM9x LoRa transceiver directly to the second Metro M0 Express board, using the same pins as before (Figures E and F). This receiver will be permanently connected to a PC via a USB port and continuously listen for LoRa frames sent by your weather station.

Upload the Receiver code to the Metro M0, from the Weather Station repo. Now whenever your weather station has power and decides to send a message, your receiver should be ready to receive the message, parse it, and print it to the serial terminal.

Before you go battery-free, you should test whether your transmitter and receiver are working as expected on constant power. Connect them both to your PC through individual USB ports, and monitor the serial port that belongs to the receiver. You should see the received messages as shown in Figure G.

Now that everything works on continuous power, you can make your weather station battery-free! You’ll use a solar panel to harvest the required energy and buffer this in a supercapacitor until it gathers enough energy to run a portion of the code.

Multiple factors come into play when choosing the size of the solar panel and supercapacitor. How much energy a solar panel generates dramatically depends on the available light. The same panel can generate 200mA of current when placed in direct sun, but only 0.5mA under office lighting. So if you plan to use your weather station outside, pick a reasonably small solar cell; use a larger one if you use it indoors. Any surplus energy, once the supercapacitor is full, is dumped into a large power resistor on the back of BFree, so make sure your solar cell doesn’t exceed 1.5 watts or this resistor might get too warm.

The capacitor determines how much energy your weather station can store. A larger capacitor can keep the station operating for longer, but also takes longer to charge, so the time between two successive intervals where BFree executes code is longer. If you choose to use a supercapacitor instead of a normal capacitor, check the equivalent series resistance (ESR) value. You ideally want to use a capacitor with an ESR below 5 ohms (5Ω). We chose a 0.22F supercapacitor with an ESR of 2.5Ω for our setup.

Connect your supercapacitor to the terminal block on the BFree shield, shown in dark green in Figure H. Make sure to pay attention to the capacitor polarity. Then connect your solar panel to the header labeled Harvester.

Your weather station is ready to go (Figure I)!