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Freespace Communication Circuits, Page 2
(Light Beam Communication Circuits)
Circuits designed by David Johnson, P.E. Last Updated on:
Thursday, December 01, 2011 07:14 AM The contents & graphics
of Discovercircuits.com are copyright protected.
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40KHz
Light Detector with Sunlight Immunity
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.
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.
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.
40KHz TV-VCR LIGHT SOURCE
REPEATER 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.
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 Dave Johnson's Handbook of Optical
Through the Air Communications. (this link is off-site)
