>ADCSRA |= _BV(ADPS2)|_BV(ADPS1); // set ADC division factor 64;

>F_CPU=9600000

Are you sampling ADC with 150kHz ? and if so why ?

A fun twist of your code would be to put it inside a flash-light driver

http://flashlightwiki.com/AVR_Drivers

Hi, after I read your comment I realized that I made a copy-paste
mistake. Of course, this example will work with higher sampling rate,
but it result in poor quality. Project has been tested using @1.2MHz
internal clock source. ADC division factor is set to 64 so the ADC
sampling frequency is 18750Hz (F_CPU/64). Thank you!

NOTE: “copy-paste mistake” has been fixed on both blog and github.

NICE ! ( less is more ;-)

You should put it into 1k challenge ...

/JB