I am currently powering my boards with an external power source using the 2.1mm plug with an 8xAA battery holder.
My current problem is that when I'm connecting my UNO with one shield (either the USB host or motor) it works fine, but when I connect them altogether with UNO, USB host then motor shield, it stops working.
I should add that in my motor shield I've connected 2 DC motors (from the 2WD smart car chassis kit) and 2 servos (FEETECH FS90R Micro Continuous Rotation Servo).
Is my current power source too low? I read that the suggested power to supply the Arduino UNO is between 9v-12v, which is why I chose the 8xAA battery holder.
I think the current is too low.
A servo motor could require 500mA (or even 1A) when it starts turning. If only one of those 8 batteries is weak, then you don't have enough power.
The suggested power for an Arduino Uno is 7.5 ... 9V.
Do you power the servo motors with the Arduino 5V pin ? That 5V pin can't supply enough current for a servo motor, not even one servo motor. If not, how do you make 5V or 6V for the servo motors ?