Page 381 - The Mechatronics Handbook

SPDT DPST

FIGURE 19.3 SPDT and DPST switch configurations.

LED photoresistor photodiode phototransistor

+V +V

R 1 R 2
V signal

typical configuration

FIGURE 19.4 Optoelectronic circuit symbols and a typical emitter/detector configuration.

Infrared
Infrared light can be used in a variety of ways to measure linear and rotational displacement. Typically,
an infrared light-emitting diode (LED), or photoemitter, is used as a source, and an infrared sensitive
device is used to detect the emitted light. The detector could be a photoresistor or photocell, a variable
resistor which changes resistance depending on the strength of the incident light (possibly infrared or
visible); a photodiode, which allows the flow of electrical current in one direction in the presence of
infrared light, and otherwise acts as an open circuit; or a phototransistor. In a phototransistor, the incident
infrared light acts as the base current for the transistor, allowing the flow of collector current proportional
to the strength of the received infrared light (up to saturation of the transistor). Circuit symbols for the
various elements are shown in Fig. 19.4.
If the emitter and detector are facing each other, they can be used as a beam-breaker, to detect if
something passes between. This is called a photointerrupter (Fig. 19.5). If the emitter and detector are
free to move along the line connecting them, the strength of the received signal can be used to measure
the distance separating them. Infrared photodetectors may be sensitive to ambient light, however. To
distinguish the photoemitter light from background light, the source can be modulated (i.e., switched
on and off at a high frequency), and the detector circuitry designed to respond only to the modulated
infrared.
An emitter and detector facing the same direction can be used to roughly measure the distance to a
nearby surface by the strength of the returned light reflecting off the surface. This is called a photoreflector
(Fig. 19.6). Alternatively, such a sensor could be used to detect light absorbing or light reflecting surfaces
at a constant distance, as in mobile robot line following. Light polarizing filters can also be used on the
emitter and detector so that the detector only recognizes light reflected by a special “optically active”
retroreflecting surface.

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