Hi all. I'm building my own quadcopter, and using an Arduino Uno as the flight controller. I'm writing the flight controller software entirely from scratch. I'm using the 3DR Radio Telemetry kit (and I tell you what, finding information on this awesome piece of hardware is really hard!).
It was all working fine when I didn't have a common ground (radio powered by lipo battery via 5V power regulator, only connected to Arduino by the two serial wires - Arduino is powered by USB) until I tried to transmit any data. It just refused to do anything, and I had no idea why until I accidentally bumped the heatsink on the power regulator with my finger and the radio's transmit light flashed. I then held my finger on the heatsink and the radio transmitted perfectly. After a couple of minutes of utter confusion, I realised I didn't have a common ground and facepalmed.
I connected the ground of the battery to one of the ground pins on the Arduino, and the radio started working perfectly again. However, when I unplugged the Arduino from the USB, I noticed that the "On" led and the pin-13 led were dimly lit. I panicked slightly and immediately unplugged the ground lead of the lipo from the Arduino.
My question is this: Is what I did safe, and why did the leds light up when I did that? I was under the impression that connecting grounds together could never break anything.
Here's some wiring diagrams in case you couldn't understand my descriptions (I forgot to say in the pictures, the blue wire is RX on the Arduino end and the green wire is TX on the Arduino end):
EDIT: Throughout this message, you can assume the radio remained powered and connected via serial to the Arduino.