#define CHECK_FREQUENCY 1000;    // check sensors every second
#define HUMIDITY_DESIRED 75;
#define HUMIDITY_VARIATION 3; // ultrasonic turns on at (75 - 3 = 72) and off at (75 + 3 = 78)

int humidity () { ; }
int tempature () { ; }
int co2 () { ; }
bool ultrasonic_start () { ; }
bool ultrasonic_stop () { ; }
bool fan_start () { ; }
bool fan_stop () { ; }
bool log ( int humidity, int temp, int co2 ) { ; }

void loop() {
     delay(CHECK_FREQUENCY);
     int humidity = humidity();
     int temp = temp();
     int co2 = co2();

     if ( humidity < (HUMIDITY_DESIRED - HUMIDITY_VARIATION) ) {
         ultrasonic_start();
         fan_stop();
     } elseif ( humidity > (HUMIDITY_DESIRED + HUMIDITY_VARIATION) ) {
         fan_start();
         ultrasonic_stop();
     } else {
        ; // keep going
     }

     log(humidity, temp, co2);
}