http://forum.arduino.cc/index.php?topic=11295.0
You absolutely must have a common ground.
Voltage is a potential difference, and ground is what that difference is compared to. Without a common connection, there is no potential difference.
你绝对必须有一个共同点。
电压是一个潜在的差异,地面是差异相比。没有共同的连接,就没有潜在的差异。