Applicaton of Photo-diodes

There are a large number of applications of photo-diodes. However, we shall five two
applications of photo-diodes by way of illustration.

(i) Alarm circuit using photo-diode.

Fig.1 shows the use of
photo-diode in an alarm system. Light from al light source is
allowed to fall on a photo-diode fitted in the doorway. The reverse
current I R will continue to flow so long as the light beam is not
broken. If a person passes through the door, light beam is broken
and the reverse current drops to the dark current level. As a result,
an alarm is sounded.

(ii) Counter circuit using photo-diode. A photo-diode may be use
to count items on a conveyer belt. Fig. 2 shows a photo-diode
circuit used in a system that counts objects as Applicaton of Photo-diodes they pass by on a
conveyor. In this circuit, a source of light sends a concentrated
beam of light across a conveyor to a photo-diode. As the object
passes, the light beam is broken, I R drops to the dark current level
and the count is increase by one.

Example 1.

From the reverse current-Illumination curve for a
photo-diode shown in Fig.3, determine the dark resistance,
Assume a reverse-biased voltage of 10 V.

Solution.
The current that flows when the incident light is removed from the photo-diode is
called dark current. The resistance of a photo-diode corresponding to dark current is
called dark resistance. From the curve shown in Fig.3, it is clear that for zero
illumination, the reverse current is 50µA.
Dark current, Ir = 50 µA = 50 A
Reverse voltage, V_R = 10 V

Example 2. A photo-diode is expose to light with an illumination of 2.5
mW/cm 2 . If the sensitivity of the photo-diode for the given conditions is 37.4 µA/mW/cm 2 ,
find the reverse current through the device.
Solution.

Reverse current = Sensitivity Illumination
or ,I _R = m E = 37.4 2.5 = 93.5 µA

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