Elenco Fiber Optics Voice and Data Kit - Manual (Page 4)

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SCHEMATIC DIAGRAM
TRANSMITTER
There are 5 main components in the transmitter
(see Figure 1A). They are:
a) Power supply (9V battery)
b) Microphone (MIC)
c) Op-amp LM741, (the driver)
d) NPN transistor 2N3904, and
e) Transmitter LED
The microphone picks up your voice signal and
converts it into a voltage signal. The strength of this
voltage signal depends upon the pitch and loudness
of your voice. This signal is then ac-coupled through
C1 and R2 to the input pin 2 of the LM741 op-amp
for amplification.
The gain of the op-amp LM741 depends on the ratio
of R6 to R2, which is equal to 100k/10k = 10. Hence,
the voice signal coming from the microphone will be
amplified 10 times by this op-amp, and the amplified
signal will appear at the output of the op-amp.
At 0 Hz (DC) the impedance of C1 is infinite. The
amplifier then acts as a voltage follower. A voltage
follower is an op-amp in which the output voltage is
equal to the input voltage. In our case, the output
voltage at pin 6 is equal to the input voltage at pin 3
and pin 2 which is about 4.5V. This 4.5V at the input
pins is due to the effect of resistors R4 and R5
which act as a voltage divider. This constant DC
voltage helps keep the NPN transistor (2N3904) on
all the time.
The function of the NPN transistor (2N3904) is
similar to that of a valve, it controls the flow of the
current through the LED. The flow of this current will
depend on the base voltage of the transistor. This
base voltage in turn depends on the loudness and
pitch of your voice. Thus, the light intensity of this
LED will vary as you speak into the microphone.
This encoded light signal will then be transmitted to
the receiver through a fiber optic cable.
The LED (D1) acts as an ON/OFF indicator. It will
also indicate the state of the battery. If the LED
becomes dim, the battery is weak and should be
replaced. C2 filters out any noise that comes
through the voltage divider. C3 helps in stabilizing
the op-amp. It will also reduce any high frequency
noise generated in the transmitter. When S2 is
closed (toward the LED D2), C4 is placed into the
circuit and the op-amp will oscillate at about 1kHz.
As a result, you will hear a shrill noise from the
speaker in the receiver.
Figure 1A