In addition, there’s a DPDT relay that is activated by a simple on/off switch at the righthand side. On the cart, the 555 timer takes the place of the on/off switch, by feeding power to the relay.
Suppose that the motor begins with the arm pointing downward, as shown in the top view in Figure 5-99, and the motor is wired so that when it receives negative voltage at its lower terminal and positive at its upper terminal, it rotates counter-clockwise. This is what happens when the on/off switch closes and sends power to the DPDT relay. Positive voltage from the relay contacts cannot pass through the upper diode, but can pass through the upper limit switch, which is closed. Negative voltage cannot pass through the lower limit switch, because it’s open, but can pass through the lower diode. So, the motor starts to turn counterclockwise. During the midpoint of its arc, it receives power through both of the limit switches.
Finally, the motor arm reaches the upper switch, and opens it. This prevents positive voltage from reaching the motor through that switch, and the positive voltage is also blocked by the upper diode. So, at this time, the motor stops.
Now suppose that the on/off switch is opened, as in the top view in Figure 5-100. The relay loses its power, so its contacts relax. The voltage to the motor is now reversed. Negative voltage passes through the upper diode, while positive voltage reaches the motor through the lower limit switch. The motor starts running clockwise, until its arm hits the lower switch, opening it and cutting off power to the motor.
Limit switches are necessary, because if you continue to apply voltage to a simple DC motor that is unable to turn, the motor sucks more current, gets hot, and may burn out.
You can easily see how this kind of system could be used to control the cart’s steering. Even though the motor has only two positions, these are sufficient to make the cart turn when going backward, and proceed straight ahead when going forward.
To reduce power consumption, the DPDT relay could be replaced with a two-coil latching relay. The circuit would then have to be revised so that the relay is flipped to and fro by a pulse to each of its coils.
Fundamentals
All about limit switches (continued)
Figure 5-99. The three diagrams, from top to bottom, show three snapshots of a motor controlled by a DPDT relay and two limit switches. When the on/off switch at bottom-right sends power to the relay, the lower relay contacts cause the motor to run counterclockwise until it stops itself as its arm opens the upper limit switch.
Figure 5-100. When the on/off switch at bottom-right opens, the relay connects its upper contacts. This causes the motor to run clockwise until its arm opens the lower limit switch. Limit switches avoid the overheating and possible damage that are likely when power is delivered to a motor that is prevented from turning.
Fundamentals
All about motors
Brushed DC motor
This is the oldest, simplest design for an electric motor, shown in very simplified form in Figure 5-101. Coils are attached to a shaft where they can interact with stationary magnets around them. The magnetic attraction turns the shaft a little, at which point the next coil on the shaft is energized to turn the shaft a little more, and then the next coil—and so on. To make this happen, electricity has to be fed into the coils by “brushes,” often consisting of soft carbon pads that conduct power to a hub, known as a commutator, divided into sections, each of which is connected to a separate coil.
This basic design has several advantages if we want to build a small motorized gadget, such as a miniature robot or even a model airplane:
Widely available
Low cost
Simple
Reliable
Will run in reverse when voltage reverses
In addition, brushed motors are often sold with reduction gearing built in. Such units are known as gearhead motors or gear motors. They free you from the need to use your own gears or belts to adjust the output speed yourself. You simply choose the motor that fits your specification.
DC stepper motor
This requires a controller, consisting of some electronics to tell the motor to rotate its shaft in small, discrete steps. The advantages of a stepper motor are:
Precise positioning of the shaft
Precise speed adjustment
Stepper motors are ideal for devices such as computer printers, where the paper has to roll up by a precise distance and the print head has to move laterally by an equally precise distance, but they are also useful in robots. If the motor is small enough to draw less than 200mA and will run on 12 volts or less, you can control it with pulses from a 555 timer. I’ll describe stepper motors in more detail in Experiment 33.
Servo motor
This is generally used in conjunction with a programmable microcontroller, which sends instructors to rotate the motor shaft to a specific position and then hold it there. I’ll mention servo motors when I introduce you to microcontrollers, but we won’t be dealing with them in detail.
Other types of motors exist, including brushless DC motors (which require a different type of controller and are found in computer disk drives and CD players), and AC motors (including synchronous motors, which synchronize their rotation with the frequency of AC voltage, and were used extensively in clocks, before clocks mostly became digital).
In this book, I’ll be talking mostly about brushed DC motors and DC stepper motors.
Figure 5-101. The basic principle of a simple DC motor. The commutator passes electricity through a coil, creating a magnetic field that interacts with a magnet around the motor. The coil turns, and the commutator turns with
