I'm having some very unexpected trouble with my arduino uno (actually a SparkFun Redboard). Basically there seems to be a sine wave superimposed on all of my output signals for all of the output pins. When I just set a pin to HIGH, what comes out is a ~4V pk-pk ~120 Hz sine wave centered around ground. When I set a pin to a frequency using the tone function, I get a square wave of the correct frequency "centered around" the same sine wave (if that makes sense -- can't take pictures atm).
I could not seem to find any information on this issue online. Is my arduino broken? It was working a few days ago, and this issue seems to have come out of nowhere.
It sounds to me like a power supply problem. If you're powering it via the USB port, try using a different USB outlet, such as a phone charging block or another computer. Try powering it from 7 to 12 volts through the Vin pin or DC barrel jack. Try powering it from 3 AA batteries via the 5V pin. See how the sine wave thing goes in those 3 different scenarios. You can also try disconnecting any sensors or wiring external of the Redboard.