Using Transistors and Driving Motors - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 82

Figure 4-10: Servo motor timing diagram
Note that in each of the examples in Figure 4-10 the pulse is sent every 20ms.
As the pulse length increases from 1ms to 2ms, the angle of rotation of the
motor (shown to the right of the pulse graph) increases from 0 to 180 degrees.
As mentioned before, servos can draw more current than your Arduino may
be able to provide. However, most servos are designed to run at 5V, not 9V or
12V like a DC motor. Even though the voltage is the same as that of an Arduino,
you want to use a separate power source that can supply more current.
To do this, you learn here how to use a 9V battery and a linear regulator to
generate a 5V supply from your 9V battery. A linear regulator is an extremely
simple device that generally has three pins: input voltage, output voltage, and
ground. The ground pin is connected to both the ground of the input supply
and to the ground of the output. In the case of linear-voltage regulators, the
input voltage always must be higher than the output voltage, and the output
voltage is set at a fixed value depending on the regulator you use.
The voltage drop between the input and the output is burned off as heat,
and the regulator takes care of ensuring that the output always remains the
same, even as the voltage of the input drops (in the case of a battery discharg-
ing over time). For these experiments, you use an L4940V5 5V voltage regulator.
It's capable of supplying up to 1.5 amps at 5V. Figure 4-11 shows a schematic of
how to hook up the regulator.