Page 91 - Programming the Photon Getting Started With the Internet of Things

DC motors are excellent drive motors, but they are not really good for precision work
because there is no feedback. Without using some sort of encoder device you will never
know the exact position of a DC motor. Servo motors, or servos, are unique in that you

can command them to rotate to a particular position and stay there until you tell it
otherwise. A good example use of a servo motor is an actuating door lock.

When using servos, there are a couple of different types to consider, such as standard
and continuous rotation. Standard servos operate from 0 to 180 degrees. Servo control is

achieved by sending a pulse of a particular length. The length of the pulse determines the
absolute position that the servo will rotate to. This is due to a small potentiometer in the
servo that measures its position; when you remove the potentiometer, it becomes free to
continuously rotate.

Unlike a standard DC motor, servo motors have three pins: power (red), ground (black

or brown), and signal (white or orange). The wires are color coded and typically in order;
they look like the ones shown in Figure 4.11.

Figure 4.11 Servo motor.

Servos have a dedicated control pin, unlike standard DC motors, that tells the servo
which position to turn to. The power and ground lines of a servo should always be
connected to a power source. Servos are controlled using adjustable pulse widths on the
signal line. For a standard servo sending a 1-ms 5-V pulse turns the motor to 0 degrees
and sending a 2-ms 5-V pulse turns the motor to 180 degrees. Once a pulse has been sent

to a servo, it then turns to that position and will remain there until instructed to move to
another position by the sending of another pulse signal. If you want the servo to hold its
position—that is, resist any movement, then you need to resend the same command every

20 ms or so.

In this experiment we are going to use a standard servo motor that operates from 0 to
180 degrees and we will control it using a simple potentiometer. We will read the value of
the potentiometer, and as we rotate it from one value to another the servo motor will rotate