plans/ schematic for a guitar/ instrument string sustaining device??

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:

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