Let's imagine we need three servos. As explained before, servos are motors, and motors convert current into movement, driving more current than other kinds of devices such as LEDs or sensors.
If your Arduino project requires a computer, you can supply power to it with the USB as long as you don't go beyond the 500 mA limit. Beyond this, you'd need to use an external power supply for some or all parts of your circuit.
Let's see how it goes with three servos.
An external power supply can be batteries or a wall adapter power supply.
We are going to use basic AA batteries here. This is also a way to supply Arduino if you don't need a computer and want Arduino to be autonomous. We will consider this option in the third part of this book about more advanced concepts.
Let's check the wiring for now:
Three servos wired to an Arduino, and power supplied by two AA batteries
In cases like this, we have to wire the grounds...