digitalWrite(led_red_pin, co2 >= co2_alarm_thr);
digitalWrite(led_yellow_pin, co2 >= co2_warning_thr && co2 < co2_alarm_thr);
digitalWrite(led_green_pin, co2 < co2_warning_thr);
digitalWrite(led_red_pin, co2 >= co2_alarm_thr);
digitalWrite(led_yellow_pin, co2 >= co2_warning_thr && co2 < co2_alarm_thr);
digitalWrite(led_green_pin, co2 < co2_warning_thr);