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Freespace Communication Circuits, Page 3
(Light Beam Communication Circuits)
Circuits designed by David Johnson, P.E. Last Updated on:
Thursday, December 01, 2011 07:15 AM The contents & graphics
of Discovercircuits.com are copyright protected.
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AIR TRANSPARENCY
MONITOR, XENON FLASH RECEIVER
I designed this circuit many years ago to monitor the quality of a mile long
column of air for future optical communications experiments. The transmitter
system (circuit 72 below) uses a powerful xenon flash in conjunction with a
large 12 inch fresnel lens at the transmitter end and a matching 12 inch lens
with a PIN photo diode at the receiver. The receiver system was connected to a
weather station and a computer to collect the changes in intensity of the light
flashes under different weather conditions. It has the potential for a 30+ mile
range. I have also used this system to conduct cloud bounce experiments.
AIR TRANSPARENCY MONITOR,
XENON FLASH TRANSMITTER
This is the matching transmitter for the above receiver. The transmitter launches
powerful 1000-watt light pulses that last about 20 microseconds.
Audio AMP + 3kHz Filter
This circuit is the audio amp section for a complete optical transmitter. The
circuit amplifies and filters the voice audio signals from an electret microphone.
The circuit is described in more detail in the transmitter circuit section of Dave
Johnson's Handbook of Optical Through the Air Communications.
FET INPUT HIGH SPEED LIGHT
DETECTOR
This circuit is yet another design that converts current from a PIN photo diode to a
voltage. It has a bandwidth that extends beyond 50MHz.
Laser Light Detector, 10mhz
to 20mhz
This circuit was originally designed to detect laser light pulses for an optical
Ethernet communications system. It has good ambient light immunity....
LASER/LED LIGHT OUTPUT INTENSITY METER
This circuit uses a large 1cm X 1cm silicon PIN photo diode and a transimpedance
amplifier to measure the light power output of infrared and visible LEDs and laser
diodes. It can be modified to produce almost any milliwatts to volts scale factor.
It can be connected to either a multi-meter or an oscilloscope
LIGHT DETECTORS WITH AMIBIENT LIGHT COMPENSATION
These circuits were taken from a few application notes on infrared remote control
devices. They use a current compensation method to separate the modulated light
pulses from ambient light. They appear to have limited bandwidth and may only work
at the 30KHz to 50KHz frequencies often used by TV and VCR remotes. I have not yet
tested the circuits.
Light
to Frequency Converter
This circuit uses a CMOS version of the classic 555 timer, to form a light intensity
to frequency converter. A small PIN photo diode is used as the light detector.
The pulses produced are short, so in some applications you may want to stretch them
or feed them through a flip/flop to produce a square wave signal. Although the
circuit shown is designed for a 5v supply, it could operate from almost any voltage
from 3v to 15v.
LINE POWERED XENON FLASH TRANSMITTER
This line powered xenon flash circuit drives a small camera type flash tube. t
has an optical isolator to allow the flash to be safely triggered from some remote
device. A flash rate of 2Hz is possible with the circuit.
Low Power 40KHz Light Receiver
Drawing only 100uA, this circuit provides high sensitivity with excellent ambient
light immunity.
Low power 100KHz
Light Receiver
By starving a high speed logic inverter for current,
this circuit can produce a sensitive 100KHz light receiver circuit, which is immune
to ambient light, but only drawing 100 microamps from a 3 volt supply.
