Optimised Rx 5v UART Protection

[glynhudson]

The ESP is a 3.3V max device, using a 5V FTDI cable (even just for Rx + Tx) can damage it. The HP monitor currently includes a voltage divider on the RX input (R6 10k, R5 5.6K). This does the job but the same can be achieved with a single 1N4148 diode in reverse bias

Whenever the 5V device transmits, it pulls the bus down, and the diode conducts to 0.5~7V, when it releases or pulls the bus UP to 5V, the diode blocks the current from flowing, and a resistor on the 3.3V device should pull the bus on the 3.3V side back to 3.3V.

http://www.ceus-now.com/how-does-this-logic-level-converter-on-the-adafruit-huzzah-work/

No protection is needed on the Tx since this is transmitting (at 3.3V) from the ESP.

Solution:

Put a 1N4148 diode in series on the Rx line with cathode (-) connected to to the 5V FTDI cable and anode (+) connecting to the ESP, see Huzza shematic: