plans/ schematic for a guitar/ instrument string sustaining

[Sam Waterston]

I'm trying to locate a schematic for a circuit that allows a sustained
resonance of guitar or instrument strings. There is already such a device
called an Ebow, found here:

http://www.ebow.com/ebow/brochure.htm

However, this unit doesn't allow the oscillator to be varied, and I'd like
to vary the string frequency this way.

If anyone knows where I could locate a circuit schematic or plans for either
the above device or something similar, please share it if you would.

Thanks,
Sam