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.

Assembly

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.

Battery Power

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.

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.