I am still away and responding on my phone but basically:
Take a wire from the arduino pinout to the +leg of a capacitor.
Connect -leg of the capacitor to a diode going to ground and
Connect same -leg of the capacitor to voltage out.
You end up with pwm going from 0 to +5v from the pin, converted to -5v to 0v to Vout.
It's half of the standard circuit to convert positive pwm voltage to a sustained negative voltage.
Now I just have to get back home from my week of travel and go ahead and do it.