Pin 9 generates a PWM signal using a time-honored technique. Suppose you want to generate 20/255 (about 8% duty cycle). You take an 8-bit accumulator and add 20 to it repeatedly.

Pulse-width modulation (PWM) Pulse width modulation (PWM) is a powerful technique for controlling analog circuits with a microprocessor’s digital outputs. Pulse-width modulation,as it applies to LED ...

But generating a smooth voltage in this way takes just a few more parts. [Scott Daniels] spent some time discussing the process of smoothing a PWM output by using a low pass filter.