DIY Smart Push-Up Counter

I want to build a pocket-sized fitness sensor that I can stick onto any wall—wood or metal—and have it automatically count push-ups or pull-ups, then send those numbers to my phone so they’re stored in an app... (Budget: $250 - $750 USD, Jobs: 3D Printing, Electronics, Embedded Systems, Microcontroller, PCB Layout, Product Design, Prototyping, Soldering)