sound via laser light

The transmission distance can be increased greatly by replacing the transmitter`s red LED with a cheap red laser diode purchased from a $2 Dollar Shop . The circuit design makes a compromise between laser brightness and sound quality and loudness. This is because the laser needs to be bright enough so that it is easy to locate the beam when posi
sound via laser light - schematic

tioning the receiver and for students to see that the laser beam is really there and carrying` the sound. Do not apply any more than 5 volts to the laser diode. I find that 4. 5 volts from 3 AA cells works well. At that voltage the laser draws about 30 mA allowing a cheap general purpose NPN bipolar transistor (such as the BC548) to work well in modulating the current through the laser. The laser casing needs to be removed. I cut carefully along the metal casing using a fine hacksaw and then prise it off the laser barrel with long-nosed pliers. The photo alongside shows the laser barrel inserted in a black plastic pen casing which is itself mounted in a small wooden block glued to the larger block. This is particularly useful when a remote control is shone` at the receiver. Then the oscilloscope can display the different digital patterns when each of the remote control keys are pressed.

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