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MEP EP#338: Feature Complete or Minimal Viable Products

Podcast Notes

Project DiffProbe

  • Landed on the topology Stephen wants
    • High voltage dividers
      • Compensation capacitors
      • CMRR calibration (120dB rejection)
      • Offset correction
    • Buffers and Instrumentation Amplifiers
    • Differential Stage
    • 10x and 100x attenuation means offsets have to be bang on
  • Opamps (3 of them)
    • Buffers
      • Low input bias current
      • Low offset
      • Bandwidth?
    • Differential amp
      • Low offset. Capable of driving 50 ohm load to the scope
    • Gain setting resistors
      • Vishay ACAS 0612 – Precision resistor network
      • 4×0603 resistors to make a diff amplifier with a gain of 2 
      • Relative tolerance of 0.05%
      • Worst case relative temp-coefficient 15ppm

Project Snackey

  • Firmware is “feature complete”
  • Display Commands
  • How the vending motors actually work
    • INA219 Current Sensor
    • Interrupts
    • Improved method for next revision?
  • The one byte Serial Buffer problem
    • NEOPIXEL (WS2812) vs APA102 (SK9822)
    • Blocking functions
    • DMA – Direct memory access
    • Importance of full system integration testing 
  • Timer Lockouts for motors
    • Is there a better way?

Special thanks to whixr over at Tymkrs for the intro and outro!