Sensor - The Electronic Nose Artificial Olfaction Technology - page 61

2.9 Photo-conductive Sensor

It is a recent development in solid-state electronics. It is a special material when
exposed to light; alter their internal electrical resistance in proportion to the
amount of light falling on them [ 13 ].
Commonly sensed measurands:
The primary measurands sensed by the photoconductive sensor is light. There
are numerous secondary measurands like position, sound, motion, temperature,
flow, and force. Figure 2.19 shows the schematic of photoconductive sensor.
Phototransistor:
A phototransistor is a normal transistor in which the envelope enclosing the
junction is transparent to allow light to fall on the base emitter junction.
Photodiode:
Photodiodes are PN junction diodes designed specifically to harness the pho-
toelectric effect. There are many useful applications of this phenomenon, such as
solar cells, light detection/emission, and thus signal receiving/sending. As of their
fast response time, photodiodes are used as film sound track readers. They can also
be used as detectors of modulated light in optical communication systems and also
in switching circuits. Photodiodes may be biased and operated in two basic modes:
photovoltaic and photoconductive.
The advantages of phototransistor/diode:

• low power consumption,
• small size,
• immediate operation on switching on,
• low voltage operation and long life.

Fig. 2.19 Photoconductive sensor (photodiode/phototransistor)