PULSED MODULATED LASER

  Pulse Modulated Laser Transmitter Schematic  

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The ground line going to the mike is simply the shielding on the michrophone cable.  You must use a condenser mike.  I tried several dynamic michrophones and impedance matching transformers but did not have any luck.  Radio Shack still sells the condensor Mike.

The string of diodes (1N4001) in parallel to the Laser shunts excessive current around the laser.   These were found to be necessary after burning out a couple of laser pointer diodes.  The diodes are forward biased with approximately .6 - .7 voltage drop across their junction.  5 diodes adds up to approximately 3 volts which is the maximum rating of the Laser Diode.  Safe at last!

 

  Pulse Modulated Laser Transmitter Printed Circuit Board  

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The condenser Mike can be seen at the upper left corner of the circuit board.  Usually this would be extended several feet via shielded cable.   The fidelity of this circuit is quite remarkable.  We listened to the local FM station retransmitted through the Laser Transmitter and decoded back to audio at the receiver end.  Very impressive . . .   Until the laser burned out.

 

  Pulse Modulated Laser Receiver Schemati

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  Pulse Modulated Laser Receiver Circuit Board 

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Boards are powered by two 9volt alkaline batteries. Circuits are a compilation of circuits found in one of the Radio Shack Engineering Notebooks, "Handbook for Integrated Circuit Applicatons". The laser diode "D1" is an inexpensive laser pointer pen available at most hardware stores, K-Mart, Wall Mart, etc The transmitter board replaces the batteries used in the pen. Range up to 1/2 mile at night. Sound quality is excellent.

Use this transmitter/receiver at the ARRL Field Day to earn 100 points as a "Unique" form of communications.

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