Freespace or Light Beam Communication Circuits
The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode. Using an unique inductive feedback network, the circuit provides high sensitivity under high ambient light conditions. It is a great circuit when you want to extend the range of an optical remote control transmitter. 40KHz LIGHT RECEIVER IS IMMUNE TO AMBIENT LIGHT (16) (40krvr1)
If you want even more sensitivity than the above circuit, try this design. When used with a one centimeter square photodiode, you can achieve a range of several hundred feet with a standard TV or VCR remote control module.
1uS LIGHT PULSE RECEIVER PLUS POST AMP (27) (500krvr3)
This circuit is designed to detect very weak light pulses
lasting 1uS. It uses a tuned LC feedback network to provide high
sensitivity while giving high ambient light immunity. A post
voltage amplifier is included with a gain of about X20. The
circuit is described in more detail in the receiver
section of Dave Johnson's
Handbook
of Optical Through the Air Communications. (this link is
off-site)
This circuit is designed to detect the narrow 1uS pulses produced by the above amplifier circuit. The clean logic type pulses produced by the discriminator are then sent to a frequency to voltage converter. The circuit is designed to process a pulse frequency of 10KHz that is frequency modulated by voice audio signals. The circuit is described in more detail in the receiver circuit section of Dave Johnson's Handbook of Optical Through the Air Communications.
VOLTAGE TO FREQUENCY CONVERTER + 1uS LED PULSE DRIVER (34)
(voicex2)
This circuit receives the signal from the above amplifier
and launches powerful 1uS infrared light pulses from a low cost
LED that are frequency modulated by the audio information. The
10KHz center frequency of the pulse stream is low enough so a
standard infrared LED can emit ten times more light than
conventional long pulse techniques. The circuit is described in
more detail in the transmitter section
of Dave Johnson's
Handbook
of Optical Through the Air Communications. (this link is
off-site)
This circuit is designed to be placed directly in front of a standard TV or VCR remote. The exiting light pulses produced by the circuit match the pulses from the remote but are about 10 times more powerful. Using the device, the remote can operate a TV or VCR over three times the normal distance.
MICRO POWER 40KHz BURST LASER DIODE DRIVER (36) (40klrl)
Some laser tag or simulated combat games can use this
circuit to send short bursts of modulated laser light at the
opponent's vest, equipped with a matching light receiver. The
circuit operates from three 1.5v cells (4.5v) that should
provide enough energy for about 200,000 shots.
This 9v battery powered circuit is designed for remote control flash needs. A charge control circuit turns off the high voltage generator when the photoflash capacitor is fully charged. A neon lamp is included to indicate when the system is ready to flash.
SIMPLE NITROGEN SPARK GENERATOR (43) (spark4)
Nitrogen or air sparks are very powerful light sources that
produce flashes that last only a few nanoseconds. This line
powered circuit generates a continuous series of very small
sparks across electrodes with a 0.05 inch gap.
This line powered xenon flash circuit drives a small camera type flash tube. It 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.
20MHz VCSEL 3mW LASER TEST CIRCUIT
(58) (20mhzlsr)
This circuit takes advantage of some new vertical cavity
surface emitting lasers (VCSEL) that don’t require light output
control circuits. The circuit shows how to drive the device from
a single high speed CMOS IC. The circuit can easily be modified
to transmit signals from kilohertz to about 50MHz.
This circuit uses a unique cascode amplifier circuit to convert the current from a PIN photo diode to a current without any feedback network. It is very stable and very sensitive. The circuit shown has the potential for a conversion factor of 10 volts per microwatt at 900nm. I included a simple JFET post-amplifier with a gain of about 20.
40KHz LASER BURST DETECTOR (60) (40krvr3)
This circuit was originally designed to detect weak flashed
of laser light bounced off of a fabric video projection screen.
It was used as part of a firearm training system. It generates a
100mS output pulse whenever it detects a 3ms to 5ms laser burst,
modulated at 40KHz. It is very sensitive and could be modified
for long range laser communications.
This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity.
PULSED LED TEST CIRCUIT (63) (testled)
This circuit is designed to test visible and infrared LEDs
in pulsed mode operations. It can drive the LED with peak
currents in excess of 10 amps. A light detector nearby can
monitor the response time and intensity of the LED under test.
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.
AIR TRANSPARENCY MONITOR, XENON FLASH RECEIVER (70)
(airmon0)
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.
This is page two of the receiver circuit above.
AIR TRANSPARENCY MONITOR, XENON FLASH TRANSMITTER (72)
(airmon2)
This is the matching transmitter for the above receiver. The
transmitter launches powerful 1000-watt light pulses that last
about 20 microseconds.
5W FLUORESCENT LAMP INTENSITY MODULATOR (75) (5wlamp)
The circuit was designed to experiment with using small
fluorescent lamps as a broad pattern source of modulated light.
The circuit hits the small lamp with narrow 1us pulses at a rate
of 10KHz. Each pulse launches about 10 watts of visible light.
The lamp starting method is a bit crude but the circuit does
work.
LASER/LED LIGHT OUTPUT INTENSITY METER (80)
(laserpwr)
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 (83)
(40krvr4)
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.
I designed this test the concept of using light techniques to send identification data instead of RF. A more detailed discussion on this scheme can be found in the Imagineered new products section.
WIDE BAND ZERO CROSS DETECTOR (109) (zerocross1)
This circuit was designed to convert a low amplitude 40KHz
signal into a clean square wave signal. It will work with
inputs as small as 5mv peak to peak or as large as 3 volts peak
to peak. The input frequency can range from a few kilohertz to
about 150KHz.
40KHZ LED TEST SIGNAL GENERATOR (111) (40khzref2)
This 40KHz crystal controlled oscillator circuit drives an
infrared LED with powerful 40ma pulses. The circuit can be used
to test optical communications circuits, designed to receive
40KHz modulated light signals.
This circuit uses a small 2.5mm square photo diode in conjunction with a 100mH coil to detect the short light flashes from a xenon lamp. The coil makes the circuit immune to normal room lights. Its 10mv sensitivity can detect light flashes from a range of over 100 feet. Reflections from a room’s walls and ceiling is usually enough to trigger the circuit. The entire circuit draws only 3 microamps from a 6 to 9 volt battery.
30KHZ LIGHT RECEIVER AMP (121) (30khzamp1) 40KHZ LIGHT RECEIVER AMP
(122) (40khzamp1)
This circuit uses NPN darlington transistor to amplify the
signal produced from short light flashes, as detected by a PIN
photo diode. The circuit draws only about 330uA from a 6v
battery.
This circuit is similar to number 121 but provides more gain and
operates up to 40KHz. However it draws more power supply
current.