It is designed to work properly with 3.3v, so I only supply 3.3v.
Might work with 5v "no problems" from a human perspective, but whats going on internally is a different story. Definitively stressing the components until one day poof the magic smoke wafts through the air.
OK I am a beginner and I'm trying to lean. Mostly from the Internet.
But there are some strange sketches out there ...
Why would one connect 3V3 and 5V from both Arduinos together?
Why put a 220Ohm resistor between TX(ESP8266) and RX(Arduino). Should this be some kind of (failed) voltage devider? If so, shouldn't it between TX(Arduino) and RX(ESP8266)?
@zoomkat@Zealot You probably found it out already, but the chip is 3.3V only, but RX and TX are 5V tolerant, so you can safely use it with a 5V Arduino as long as you only supply 3.3V to the power pin on the ESP8266 module.