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Circuits Designed by Dave Johnson, P.E. : |
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10MHz TO 20MHz LASER LIGHT DETECTOR
This circuit was originally designed to detect laser light pulses for an optical
Ethernet communications system. It has good ambient light immunity.
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1uS LIGHT PULSE RECEIVER PLUS POST AMP
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 my
Handbook of Optical Through the Air Communications. Note: The LF357 op amp
is no longer available, this circuit is for reference only.
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30KHZ LIGHT RECEIVER AMP
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.
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30KHZ LIGHT RECEIVER AMP
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.
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40KHz LASER BURST DETECTOR
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.
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40KHz Light Detector with Sunlight Immunity
NEW
The circuit below was designed to turn on an external 12v relay, whenever it detects
light from a nearby LED light source, modulated at 40KHz to 50KHz. This
circuit was originally designed to operate from a fast moving vehicle. The light
transmitter was positioned at a stationary position, while the matching receiver was
mounted on the vehicle. The circuit has high ambient light immunity and in
most cases, can operate in direct sunlight.
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40KHz Light Detector with High Ambient Light Immunity
This circuit is designed for detecting infrared light modulated at around 40KHz.
It’s feedback scheme cancels much of the DC component from ambient light. It’s
conversion factor is about 100 millivolts per microwatt of 900nm light.
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40KHz LIGHT RECEIVER IS IMMUNE TO AMBIENT LIGHT
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.
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40KHz MODULATED LIGHT DETECTOR
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.
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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.
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BROAD BAND 2MHz OPTICAL FIBER RECEIVER
If you need more sensitivity than the above circuit this circuit provides about ten
times more gain. It too is designed around an inexpensive plastic optical fiber
detector.
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BROAD BAND 50MHz OPTICAL FIBER RECEIVER VERSION A
If the above circuit it still too slow, you can try this circuit. What it lacks in
sensitivity it makes up for in speed. The circuit attaches a plastic fiber optic PIN
photodiode assembly to a small box containing a small 3v battery and a standard BNC
plug. When the box is plugged into to the input of an oscilloscope with a 50-ohm
termination resistor, it can detect light pulse frequencies beyond 100MHz.
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BROAD BAND 5MHz OPTICAL FIBER RECEIVER
This circuit is a simple broad band light detector that uses a very inexpensive IC
and a PIN photodiode that is packaged for use with plastic optical fibers. It has a
bandwidth from 1KHz to over 5MHz. It is great for experimenting with various
modulated light sources.
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CASCODE LIGHT RECEIVER CIRCUIT
This page provides a detailed explanation of how the modified cascode light receiver
circuit operates. The cascode technique in conjunction with an inductive load
provides very high current to voltage conversion as well as very high speed.
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Clear Fluid Detector -2
Using an ultra low power technique, this circuit detects when a clear fluid is
inside a piece of clear plastic tubing. It can be used for fluid level control and
monitoring.
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Clear Fluid Detector
This circuit detects when a clear fluid is inside a piece of clear plastic tubing.
It can be used for fluid level control and monitoring.
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