Issue with Arduino TX/RX when using different power sources

Question

I have had this issue multiple times and I really want to know what is going on. So I have a bench power supply, and I was powering my ESP8266 off of the bench supply. I then wanted to send it some AT commands, so I set up the Software Serial on the arduino and hooked everything up. All i got on the serial was complete garbage. I decide to disconnect the ESP8266 from the bench supply, and use the 3.3 output of the Arduino. I reset everything, and BOOM, serial monitor is working perfectly and I am no longer getting garbage from the ESP.

This has happened with multiple modules and I am at a total loss. Why would everything work fine when the module is running off of the 3.3v output of the arduino, and NOT when powered from 3.3v off of the bench power supply? Thank you for any clarification!

Answer 1

Your circuit is going to need a common ground between the Arduino and any sensor you're connecting to it.

Voltage is a reference of the potential between 2 points in a circuit. If the sensor and your microcontroller are not sharing a reference for where 0V is, the 3.3V on your Arduino and the 3.3V on your bench supply are "floating" and completely unrelated to each other. The signals from your sensor are as good as noise to your Arduino.

You can solve this by running a line from the GND on the Arduino to the V- on your bench supply, however you're going to want to be darn sure your bench supply is putting out an accurate and stable 3.3V or you risk damaging the Arduino I/O pins.