SOME THOUGHTS ON GETTING THE PITCH YOU WANT 

It is important for me to have the string pitch in a comfortable singing range, which has required some trial and error. This will make the difference between a diddley bow that is useable and one that is simply a curiosity that you build, noodle around with, and then ends up collecting dust in the corner.  The pitch of the open string depends on three things:  the mass of the string (i.e., 0.055" wire will be lower in pitch than 0.047" wire at the same tension and length); the length of the string between the bridge and the nut (scrap of wood) (i.e., for the same mass and tension, a longer vibrating string length will be lower in pitch than a shorter vibrating string length); and the tension on the string (all other things being equal, a string with more tension will be higher in pitch than a string with less tension).  I have not made a completely systematic investigation of these three parameters for this instrument, but I can provide some guidance.  I will state my preference here for the meaty sound of a heavy string—thin strings sound thin, and are not to my taste.  I have measured the string length and pitch of the diddley bows I have made, and have interpolated and extrapolated the relationship of string length (in inches) to pitch for various string materials in the following table:

These pitches may not be the highest achievable for a given wire and string length, but they are as high as I have been able to get.  It is possible to have the pitch a little lower by reducing the tension slightly, and in fact, the wire tends to stretch over time, so the pitch drifts down.  Therefore I suggest you make the diddley bow a little higher pitched than you think you will need.  Note that the string length is the distance between the top of the bridge to the nut, NOT the distance between the nails.  On the diddley bows I have built, the string length is typically 3-4 inches shorter than the distance between the nails.

Once you have made a basic instrument or two, you may wish to build a diddley bow with a heavy guitar string and a cheap guitar tuning machine.  Wound phosphor bronze strings tend to have good sustain (the notes ring for a long time).