But why do I get 7V when it's 3.3V from arduino (no PWM), and 9V batteries to the op amp circuit?
First thing to fix is adding a wire from the 9 volt battery's negitive terminal to a Arduino ground pin. Your external circuitry and the arduino must share the same common reference ground.
Also about that meter? does it really measure something using just one lead?
I'll have to look up why !
Hmm, I didn't need the resistor or the capacitor with the external 9V supply.Pin 9 PWM = 2.49V from the op amp output