How do I hack a body weight scale?

Using Arduino Sensors
2

Generally there are two ways to hack such a device

  1. find some meaningfull signal on the PCB, with a bit of luck such a signal can be found, might need some decode

  2. reverse engineer the signals on the display, you probably need many lines to monitor, doable (with 4 shift registers you have enough lines)

Finally you need to find the signal lines that indicate [wakeup] [ready for measure] [measure done] These status lines indicates when the measurement-data makes sense.