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.
6. TEST THE LORA COMMUNICATION
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
.
7. MAKE IT BATTERY FREE
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.
Vito Kortbeek and Przemysław Pawełczak
H
87
make.co
M82_82-89_BatteryFreeComp_F1.indd 87M82_82-89_BatteryFreeComp_F1.indd 87 7/11/22 4:34 PM7/11/22 4:34 PM
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