122 is provided at both sides thereof respectively with an air chamber 1252 and an air chamber 1242 which are opened on their tops the air chamber 1252 and the air chamber 1242 are provided on the bottoms thereof respectively with a vertical lower air pipe 1253 and a vertical lower air pipe 1243 communicating respectively with the interiors of the air pipe 1251 and the air pipe 1241 the air chambers 1252 and 1242 of the lower seat 12 are provided at the outer lateral sides thereof respectively with a receiving seat 126 and a receiving seat 127 for mounting therein two micro switches 31 32 as well as a micro switch 33 and a pad 34 by a plurality of screws 39 the second air valve 2 is comprised of an upper rotation lid 21 and a lower seat 22 the upper rotation lid 21 is mounted on the lower seat 22 the upper rotation lid 21 and the lower seat 22 are provided at the centers thereof respectively with a through hole 210 and a through hole 220 a driving member 218 is provided on the top of the upper rotation lid 21 thereby an external motor not shown can drive the upper rotation lid 21 to rotate while the lower seat 22 maintains unmoved the top surface of the upper rotation lid.