Page 173 - Sensors and Control Systems in Manufacturing

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LED Photo
Detector
Output
Contact
Source Detector
FIGURE 3.1 Photoelectric sensor.

FIGURE 3.2 Early photoelectric control.

Early industrial photoelectric controls used focused light from
incandescent bulbs to activate a cadmium sulfide photocell (Fig. 3.2).
Since they were not modulated, ambient light such as that from arc
welders, sunlight, or fluorescent light fixtures could easily false-
trigger these devices. Also, the delicate filament in the incandescent
bulbs had a relatively short life span, and did not hold up well under
high vibration and the kind of shock loads normally found in an
industrial environment. Switching speed was also limited by the slow
response of the photocell to light/dark changes (Fig. 3.1).

3.1.1 Light-Emitting Diodes
Photoelectric sensors use an effective light source, light-emitting
diodes (LEDs), which were developed in the early 1960s. LEDs are
solid-state devices that emit light when current is applied (Fig. 3.3).
This is the exact opposite of the photodetector, which emits current
when light is received.
LEDs have several advantages over incandescent bulbs and other
light sources. LEDs can be turned on and off very rapidly, are
extremely small, consume little power, and last as long as 100,000
continuous hours. Also, since LEDs are solid-state devices, they are
much more immune to vibration than incandescent bulbs.