CHAPTER 27 ■ ENCOREBy observing the time between the appearances of a signal, an appliance can decode commands,
which is the basis for remote control. In fact, the target market for these parts is actually the remote control
industry (blu-ray players, TVs, and stereos). That large audience has caused the standardization of the signal
frequency and dramatically decreased price of the parts.
Some robots surround themselves with photodetectors. By turning on different infrared LEDs at
different times, and then reading each of the photodetector’s outputs, the robot can get a rough idea of the
locations of nearby objects. Other robots use the photodetectors in a classic way, as a remote control, or in a
fancy way, as a method of data communication.
Distance and Object Sensor
Standard and remote control photodetectors can tell a robot whether an object is nearby, but can’t tell the
robot exactly how far away the object resides.
Sharp makes a variety of fantastic distance sensors (see Figure 27-15) with which the robot can
determine not only if an object exists, but also how far away the object is. The sensor emits an infrared beam
and detects the angle of return in a lens. It can detect objects and walls at distances between 10 cm and 80 cm.
Figure 27-15. Sharp GP2D02 infrared ranger
The distance sensor is available with either a digital-numeric output or a variable-voltage output. You
could mount a pair with variable-voltage outputs slightly forward on the sides of the robot, replacing test
point 1 and test point 2 of the line-following robot’s brightness sensor. In that case, Sandwich might be able
to drive down a corridor, centering itself.
You can learn more about Sharp distance sensors by visiting http://www.robotroom.com/
DistanceSensor.html.
Oscillators and Crystals
Oscillators provide accurate timing to microcontrollers, power converters, motor controllers, or any time-
sensitive circuit. The timing signal, often called “the clock,” provides the drumbeat that keeps all the chips
and components working together at the same pace, like clockwork.
Crystals are a portion of an oscillator circuit. The crystal itself has a natural vibration that can be
converted to an electrical signal, and then amplified by other components in an oscillator circuit. A canned
oscillator is a component that consists of the crystal plus the rest of the oscillator circuit inside a single piece.