Better pcb with sim800c

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Couple of observations,

  • Not a fan on diode approach. It will drop current too. And voltage is not guaranteed at <4.2V. If 4.3V appears there, you are in trouble. You can just use one more regulator to step down the 5V to 4.1V. May be L6932 or another LM2596 or LM2576.
  • Havent used the transistor level translators. I would recommend the TXS0102 or TXS0104 chips. If the transistors work well for you, you can use it.
  • For SIM card, directly connecting them also works, But put decoupling capacitors on all pins and TVS diodes if possible.

Rest all looks ok.