pulling straps 31 to extend through blocks 321 a and 331 b are provided at the center portions of the slots 32 a and 33 b every two adjacent hangers including the active hanger 3 a and the driven hangers 3 b are connected by an independent pulling strap 31 which comprises a securing end 311 and a terminal end 312 both the securing end 311 and the terminal end 312 are larger in size than the slots 32 a ad 33 b the securing end 311 is secured to the block 321 a and the terminal end 312 is able to engage with the block 331 b or disengage from the block 331 b the motor 4 is connected to the transmission gear 21 of the transmission unit 2 and outputs a power to link the hanging unit 3 to slide along with the transmission belt 22 through the transmission gear 21 as shown in fig1 the control unit 5 as shown in fig1 4 and 5 comprises a power switch 51 an automatic control key 52 a manual control key 53 a light sensor 54 a water drop sensor 55 and a timer 56 which is electrically connected with a time display panel 561 any of the light sensor 54 the water drop sensor 55 and the timer 56 can trigger the motor 4 to activate the transmission belt 22 through the transmission gear 21 to slide the hanging unit 3 to stay within the tent 13 or to extend outwardly the automatic control key 52 controls the light sensor 54 the water drop sensor 55 and the timer 56 the light sensor 54 the water.