Arduino readable non-contact voltage detector (looking to buy, not make)

We're making a robot that needs to have a voltage detector for electrified surfaces. We already tried making a noncontact voltage detector ourselves and weren't satisfied with the quality of what we produced. Now we're looking for a commercial design that can feed data into the arduino with minimal hardware hacking. It'd be really nice if it was a simple analog value that we could read off a single pin.

Any suggestions?

How, exactly, are these surfaces "electrified" and what are your requirements for voltage range, voltage source frequency and accuracy?