PHOTOCONDUCTIVE
CELL
Engr. Raymond J. Pidor, PECE
PHOTOCONDUCTIVE CELL
It changes resistance in response to a change in
light intensity (the formal term is illumination)
Is also referred to as photo resistor.
Also known as light dependent resistor (LDR).
Photoconductive cells can be used for many of the
same purposes that photovoltaic cells are used,
except, of course, that they cannot act as energy
sources.
Photoconductive cells are preferred to
photovoltaic cells when very sensitive response to
changing light conditions is needed.
APPLICATIONS
Analog Applications
Camera Exposure Control
Auto Slide Focus - dual cell
Photocopy Machines - density of toner
Colorimetric Test Equipment
Densitometer
Electronic Scales - dual cell
Automatic Gain Control - modulated light source
Automated Rear View Mirror
Digital Applications
Automatic Headlight Dimmer
Night Light Control
Oil Burner Flame Out
Street Light Control
Absence / Presence (beam breaker)
Position Sensor
TYPICAL APPLICATION CIRCUITS
TYPICAL APPLICATION CIRCUITS
TYPICAL APPLICATION CIRCUITS
OPTICAL COUPLING AND ISOLATION:
PHOTOTRANSISTORS, LIGHT-EMITTING
DIODES
The LED-phototransistor combination has some
important advantages over the bulbphotoconductive cell combination:
A LED has an extremely long life compared to a bulb
of any kind. A LED will emit light forever if operated
at the proper current; an incandescent bulb does well
if it lasts 10,000 h.
A LED can withstand the mechanical vibrations and
shocks in industrial environments much better than a
filament lamp, thus providing greater reliability.
The LED and phototransistor have faster response
speed than a lamp and photoconductive cell. This may
be an advantage for certain high-frequency switching
applications.