Detecting voltage change

[GM_Soulless]

Ok, might need some help here guys- trying to set the pi up so it can detect a voltage change over a port (high<->low) for integration in some machinery I'm running.
Was wondering whether it would be possible to do this with the USB port, or would that be far, far too complex?
Would using the GPIO be a better option? Or do you guys have any other suggestions
Sorry if this is in the wrong board, wasn't quite sure where it would fit

[PiGraham]

What voltage(s) do you want to detect? Is it AC or DC? How quickly does it change?

[GM_Soulless]

Whoops, was wondering why I wasn't getting replies, obviously my last post didn't actually post.
It would be an on/off check basically. As my tech guy said- "Since the Vcc is 3.3 v, we can safely say less than 0.5 V is off; greater than 2.0 V is on." It wouldn't be rapidly changing

[MarkHaysHarris777]

Whoops, was wondering why I wasn't getting replies, obviously my last post didn't actually post.
It would be an on/off check basically. As my tech guy said- "Since the Vcc is 3.3 v, we can safely say less than 0.5 V is off; greater than 2.0 V is on." It wouldn't be rapidly changing

If its a digital voltage, and you can guarantee that it will not exceed 3v3, then hook the voltage to your GPIO of choice and use a simple python script to read the pin.

If there is even the slightest chance that the pin will go higher than 3v3, you should run it through a buffer (or voltage shifter) similar to the buffers on the Gertboard, or the level shifters from AdaFruit, before you run to the GPIO pin.

If its an analog voltage, use an ADC (like the one on the Gertboard) to read the voltage, and then use SPI to read the ADC to get the correct digital correlation.

[GM_Soulless]

Cheers Marcus, informative reply. I'll check with my guy as to whether it'll be guaranteed under the 3.3v limit because i wouldn't want to cause any malfunctions.