The PWM output is set as an external voltage using the “Inputs and Sources” components (remember, the output from the Arduino is the input to the circuit). Conversely, the comparator output ...

The basic idea of the circuit is a NE555P based astable oscillator driving the clock of an 74HC595 shift register which has it's serial data input (SER) line pulled high, it's SRCLK connected to RCLK, ...