Elenco Fiber Optics Voice and Data Kit - Manual (Page 18)

RECEIVER
Figure 3A
-17-
There are 4 main components in the receiver (refer
to Figure 3). They are:
a) Power Supply (9V battery)
b) Phototransistor LPT80A (the detector)
c) Audio op-amp LM-386
d) Speaker
The phototransistor Q1 (LPT80A) used in a
common-collector configuration has high current
gain. This transistor acts as a valve which controls
the flow of current to the potentiometer R1. The flow
of current is directly proportional to the intensity of
light striking the base. The more intense the light,
the more current will flow through transistor Q1. The
current will then be coupled to the audio amplifier
(LM386) through capacitor C1 for amplification.
The gain of the audio amplifier (LM386) is internally
set to 20. Hence, the voltage signal that is coupled
through C1 to input pin 2 will be amplified 20 times,
and will appear on the output of the op-amp (pin 5).
The above amplified voltage will then be coupled
through C4 to the speaker. The speaker then
converts this voltage into sound.
The LED (D1) acts as an ON/OFF indicator. It will
also indicate the state of the battery. If this LED
becomes dim, the battery is weak and should be
replaced. C2 filters out any noise at the power
supply (9V battery).