Meet the OpenBarbell V2: a smart velocity based trainer that combines with your barbells to track average velocity, range of motion, rep duration, bar position, and time between sets. It comes with an open-source app. The device is based on a draw-string rotary sensor to measure barbell velocity accurately.
Just connect the device to the end of your barbell via a magnetic hook and stainless steel wire. You get instant feedback to maximize your workouts. A wide variety of workouts can be performed with this gadget.