FOR FULL DETAILS ON HACKING THE M3 ACTIVITY TRACKER AS WELL AS PURCHASE LINKS SEE MY Hacking a $25 nRF51 ARM Cortex Activity Tracker PROJECT

M3 Specs:

• Display: 0.69" 16*64 OLED
• MCU/SoC: nRF51822 258kB Flash Memory 32kB RAM
• Accelerometer: Kionix KX022-1020 using SPI interface
• Heart Monitor: PixArt PAH8001 green LED PPG
• Battery: 40 mAh lithium polymer
• Waterproof: IP67
• Device size: 18.0*11.2mm

This is a high resolution microscope image of both sides of the main board aligned so that pins and traces can easily be mapped. I currently use this for reference purposes when using the hacked M3 as a development platform.

Purchase Links

• https://www.aliexpress.com/item/HL-2017-M3-Sports-Smart-Watch-Heart-Rate-Calories-Wrist-Watch-Smart-Bracelet-Watch-ma29-Levert/32804495965.html
• https://www.aliexpress.com/store/product/Smart-Watch-Wrist-Band-Blood-Pressure-Heart-Rate-Monitor-M3-Bracelet-Sport-Activity-Clock-Wristband-Bluetooth/1961463_32803053240.html
• http://techshopstore.com/products/m3-ip67-life-waterproof-sport-smart-wristbands-heart-rate-calories-wrist-watch-smart-bracelet-watch-0-69-inch-hd-for-ios-android/ 
• https://www.amazon.com/Aurorax-Fitness-Waterproof-Tracker-Monitor/dp/B0765RG26M/ref=sr_1_5