I am building a robot that runs only when a matching rfid tag is identified..for that i need motor driver shield for running the motors and rfid module. The problem is if i attach the motor driver shield directly to arduino (MEGA) there won't be any 3.3v left which is necessary to power rfid so what i am thinking is connecting all the pins of arduino and shield seperately using jumper wires and leaving the 6 power sockets unconnected. If i connect 9v battery to arduino mega to power the rfid and micro-controller itself, what battery should be connected to the shield? Is it necessary to connect the GND of arduino awa shield even if power sources are seperate? Is it necessary to attach 2 switches for each source?