有一次,我到外地参加培训,入住酒店的房间门“钥匙”是一张写有房间号码的硬卡,打开房门后,把房卡插入房间总开关的插座内,房间就有电了。取下房卡,房间内就没有电了。总开关插座在没有插入房卡时,有一个小指示灯会亮 Once, I went to the field to attend training, stay in the hotel room door “key ” is a hard card with the room number written, open the door, the room card into the main switch socket, the room has electricity It's Remove the card, there is no electricity in the room. The main switch socket in the absence of room card inserted, there is a small light will be bright