Arduino Yún Sketch Programming - The Hands-on Arduino Yun Manual Lab - page 75

Now you can write the program.
Open Arduino Sketch and write this code.
#include <Wire.h>
# define PCF8591 ( 0x90 >> 1 ) // Device
address
# define PCF8591_ADC_CH0 0x40 // thermistor
# define PCF8591_ADC_CH1 0x41 // photo-voltaic cell
# define PCF8591_ADC_CH2 0x42
# define PCF8591_ADC_CH3 0x43 // potentiometer