A DIY Heartbeat Display for Polar Chest Strap Monitors

Your heart rate can tell you a lot about the efficacy of your workout. If your goal is moderate to vigorous exercise, you’ll probably want to aim for somewhere around 70% of your maximum heart rate — though medical conditions, like heart defects, can certainly change that target. It is, therefore, a good idea to monitor your heart rate throughout your workout. To make that as easy as possible, Jürgen Schmidt designed this DIY heartbeat display that works with Polar brand chest strap monitors.

There are many, many different devices on the market that can check your heart rate and tell you the results, but Schmidt wasn’t happy with the options. Smartwatch and fitness tracker-style devices tend to be less accurate and they force you to look down at your wrist whenever you want to see your heart rate. The wearable devices that display results on your smartphone are even more inconvenient. Schmidt found that heart rate monitors that strap onto the chest work best and so he built this display that shows the data coming from a Polar brand model.

Specifically, Schmidt’s display works with Polar H7 chest strap heart rate monitors that feature Bluetooth connectivity. Those can send their data to smartphone apps, but Polar specifically designed them to be compatible with a wide range of other devices. For that reason, they are mostly “open” and simply send unencrypted heart rate data to any devices they connect to via Bluetooth.

To take advantage of that, Schmidt’s display contains an Adafruit ESP32 Feather V2 development board. That contains an ESP32 microcontroller that has a built-in Bluetooth adapter. Schmidt originally used an Arduino Uno R4 WiFi development board for the initial prototyping, but switched to the Feather because it is smaller and more affordable.

The Adafruit Feather fits inside a 3D-printed enclosure alongside a 2.4” OLED screen that shows the user’s heart rate. Schmidt programmed this to show the heart rate in huge characters that are easily readable from across the room, so it is convenient to use while exercising. Power comes from a 1200mAh lithium battery from Adafruit, which works with the Feather’s onboard charging circuitry. A switch on top of the enclosure lets the user turn off power whenever the system isn’t in use.

For people who like chest strap heart rate monitors, this could be a very useful project.