Description:
Fiber Optic Voice Communications
Audio/Speech is pulse frequency
modulated and transmitted via fiber optic cabling to receiver and
subsequently demodulated and amplified for output.
Basic Operation:
555 oscillates at a center
frequency determined by R5 & C2 (around 40kHz). Low-level audio from
headset microphone passes through the microphone input of J1 and is
amplified by the 741 OpAmp (gain adjusted by R1). Amplified audio is
passed into the modulation input of the 555. The pulse frequency
modulated signal is output on pin 3 of the 555 and is used to drive a
laser diode (obtained from a 10BASE-FO Ethernet card.) Signal travels
via optical fiber to second transceiver. Signal is received by second
receiver’s photo-transistor(also from 10BASE-FO Ethernet card) and
again amplified by a 741 Op-Amp. Signal is then passed to the phase
comparator input of the 565 PLL (tuned via R8 & C7). The demodulated
audio is tapped at pin 7 of the 565 and passed to the 386 for final
amplification (gain controlled by R7). Output is routed to speaker pin
of 3.5mm jack and finally to the speaker of the headset.
Some fine-tuning is required in
order to match the transmitter to the receiver.
Notes:
The addition of a MUX/DMUX (multiplexer/demultiplexer)
in front of the 555 and 565 will facilitate the use of multiple signals.
(this aspect currently under development…for info, email michael.e.pierce@unforgettable.com)
This system is currently used as
an intercom system from front gate to front door with the exception of a
dedicated mic-speaker setup (the hands-free headset option was chosen
for development and experimentation). The multiplexed version is being
considered for a whole-house system. While this may seem impractical
with the current pricing of FO cabling, it is actually the first in a
series of designs which will (hopefully) end in a full digital pulse
code modulated system capable of multiple concurrent video transmissions
and I would rather not reroute fiber in-wall after construction.
Parts List:
|
Part Label |
Part Description |
Quantity |
Substitution |
|
R1, R2 |
100k Ohm, ¼ watt |
2 |
|
|
R3, R4 |
5.6k Ohm, ¼ watt |
2 |
|
|
R5 |
100k Ohm Variable, ¼ watt |
1 |
|
|
R6 |
10k Ohm, ¼ watt |
1 |
|
|
R7 |
10k Ohm Variable, ¼ watt |
1 |
|
|
R8 |
3.9k Ohm, ¼ watt |
1 |
|
|
R9 |
1M Ohm, ¼ watt |
1 |
|
|
R10 |
1k Ohm, ¼ watt |
1 |
|
|
C1, C4 |
.1 uF, non-Polarized |
2 |
|
|
C2 |
470 pF, non-Polarized |
1 |
|
|
C3 |
10 uF, Polarized |
1 |
|
|
C5 |
.001uF, non-Polarized |
1 |
|
|
C6 |
.047 uF, non-Polarized |
1 |
|
|
C7 |
.02 uF, non-Polarized |
1 |
|
|
C8 |
10 uF, non-Polarized |
1 |
|
|
C10 |
100 uF, Polarized |
1 |
|
|
741a, 741b |
741 OpAmp |
2 |
|
|
7555 |
7555 CMOS Timer IC |
1 |
555 |
|
565 |
565 PLL |
1 |
|
|
386 |
386 OpAmp |
1 |
|
|
J1 |
3.5mm Stereo Jack |
1 |
|
|
LD-T |
Laser Diode Transmitter |
1 |
|
|
LD-R |
Laser Diode Receiver |
1 |
|
|
HF1 (not shown) |
Hands-Free Headset w/ 3.5mm jack |
1 |
|
|
FO1 (not shown) |
Optical Fiber Cabling |
VAR |
|
|
As a personal guideline, all components are 5% or better
tolerance. |
|
LD-T & LD-R were obtained from a non-functional Fiber
network card. |
|
