To begin, assemble the two seven segment display FeatherWings as shown here.
Solder male headers to the Feather nRF52840 as shown here.
Then, add the plain female headers to the FeatherWing Tripler as shown here.
I2C Address Jumper
In order to use two displays on one Feather, we need to give the boards unique I2C addresses.
Leave the red BPM display board at its default state, which will use 0x70 as its address.
For the blue heart rate zone percentage display, we'll solder the jumper pad marked A0, which will give the board the address 0x71.
I did this by soldering a small piece of wire across the two pads as shown here.
It's best to install the Feather nRF52840 at the top of the trippler so the battery cable doesn't interfere with the displays. (This won't matter if you choose to power over USB instead.)
Then, place the red seven segment display at the middle position.
Place the blue display at the bottom position.
You can power the Heart Rate Zone Trainer from a LiPoly battery, by plugging it into the Feather's battery plug.
Use a little bit of double stick foam tape to adhere it to the back of the board, and snake the wire under the Feather to keep it out of the way.
To charge the battery, simply plug the Feather into USB power.
One nice way to improve the look of your LED displays is with a small piece of colored gel filter designed for film and theatrical lighting. It hides the white unlit segments so the lit ones really stand out. In this project I'm using a deep purple gel which gives both the red and blue displays a very nice look. You can experiment with different filters if you use different display colors.
Cut a small section of gel from your sheet to fit the displays.
To adhere them to the displays, I used a couple of thin strips of 3M double stick transparent tape at the top and bottom of the display pair.
You can attach a strip across the top as shown here, and then trim away the excess with a hobby knife.
Repeat this for the bottom display's bottom edge, and then press the gel filter into place.
Once you code the Feather in CircuitPython on the next page you'll be able to see the beautiful impact of the filter on the displays!