Mohammed BENHADINE

I don't really know , i found it in the MQ2 recommendations . I know that in digital inputs it's named a pull down resistor to be sure about the state of the pin if there's fluctuations.

i guess yes,,in the datasheet of the MQ2 they mention that but i didn't tested for the others gas it's tested ;)

Hi,,yes you are right but this circuit was designed to capture sudden movements then format an packet (with some addresses and Data field) and transmit it through an RF infrastructure .I didn't add the transmission part to the code,but i used Arduino to do other works in parallel this why bro.thanks ;)

The maximum current that all outputs combined can receive must not exceed 400mA,,the function is protecting the arduino because we can put any voltage source (more than 5V ),,do u see another function?