You can build a Heart Rate Trainer on Feather nRF52840 Bluefruit in CircuitPython that displays heart rate and max heart rate percentage training zones. John Park's Learn Guide coming soon.
Visit the Adafruit shop online - http://www.adafruit.com
LIVE CHAT IS HERE! http://adafru.it/discord
Adafruit on Instagram: https://www.instagram.com/adafruit
Subscribe to Adafruit on YouTube: http://adafru.it/subscribe