How the bus door opening and closing warning works

Atmospheric pressure generates power.

The working principle of the bus door opening and closing warning is that atmospheric pressure generates power, and the compressed air generated by the air pump acts on the cylinder through the electromagnetic reversing valve.

When opening and closing the door of a bus, a slogan should be posted to prevent accidental injury to passengers. You can write: Be careful of the door crack, watch your step; for the safety of you and others, please watch your step, etc.