Digital RGB LED Strip Lighting Up On Its Own

[tjsavage]

I've run into an interesting issue: I have a 32 pixel LED Strip wired up to a Raspberry Pi like this: http://learn.adafruit.com/assets/13630 - but with one change: I power the strip and the Raspberry Pi each through their own 5V inputs, and don't connect anything to the 5V Raspberry Pi GPIO pin. They do share a GND.

I've found that when the light strip is off, if I suddenly turn off a significantly-powered device in the same room as the light strip, a number of the light strip's pixels will suddenly turn on to full white. The faster I turn off the other device, the more pixels on the LED strip go on (turning off the space heater seems to be the biggest culprit).

I have a feeling that by quickly turning off a heater I'm introducing noise into my house's circuit, and that a capacitor may help smooth the signal and avoid this issue, but I'm unclear on if that would mess up the digital signal from the RPi.

[adafruit_support_mike]

All voltages are relative.. you always measure voltage between two points, and usually compare a voltage in one place to a voltage somewhere else.

If you connect a smooth, constant voltage to a circuit with noisy supply rails, the circuit will see noise when comparing the input voltage to the supply voltage. The noise actually comes from the supply rails, but the circuit usually won't know that.. all it knows is that there was noise somewhere.

It's also possible that the noise takes different amounts of time to reach the LED strip and the RasPi. That would cause an effect called 'glitching', which a circuit can also interpret as a signal.

Generally speaking, it's not the kind of thing you want to do if you can avoid it. There's always a chance that whatever is happening will happen in a way that damages something in either the RasPi or the LED strips. If you have a UPS for your computer, you might want to plug the electronics gear into that.