There’s something rather futuristic about talking ‘over’ a laser beam, which is what this inexpensive project allows. It will easily give a communication distance of several hundred metres, and with a parabolic light reflector, up to several kilometres. It transmits high quality audio and the link is virtually impossible for anyone else to tap into.
As before, there are two sections: the transmitter board and the receiver board, both powered by a separate 9V battery or a fixed voltage power supply, depending on your needs. The transmitter board has an electret microphone module at one end, and the laser diode at the other end. The electronics modulates the intensity of the laser beam according to the output of the microphone. The laser diode has an inbuilt collimating lens, and is simply a module that connects to the transmitter board. The previous design required brackets for the laser diode assembly.
The receiver uses a photodiode as the receiving element, and the onboard amplifier powers a small 4-36 ohm speaker. This board is therefore a high gain amplifier with a basic audio output stage.
But what about results – are they better? Sure. Because this design uses a higher power (and visible) laser beam, the range is improved, and alignment is easier and not all that critical, especially over a few hundred metres. The quality of sound transmit ted by the link is quite surprising.
As a simple test, we set up the prototype with the transmitter microphone near a radio. The received sound was clear and seemed to cover the full audio bandwidth. We haven’t tried feeding an audio signal directly to the transmitter, but that will undoubtedly give even better results.
So clearly, this project is ideal for setting up a speech channel between two areas, say adjacent houses, or offices on opposite sides of the street. Or you could use it as a link between the work shop and the house. For duplex (two way) communication, you’ll obviously need two laser ‘channels’.
An important feature of transmission by laser beam is privacy. Because a laser beam is intentionally narrow, it’s virtually impossible for someone to tap into the link without you knowing. If someone intercepts the beam, the link is broken, signalling the interception. Fibre-optic cables also have high security, as it’s very difficult to splice into the cable without breaking the link. However it’s theoretically possible; so for the highest security, you probably can’t beat a line-of-sight laser beam.
You can also use an infrared laser, as in the previous project. While this gives even better security, as you can’t see the laser beam without special IR sensitive equipment, it also makes alignment more difficult. (An IR laser diode is available for the project; see end of article for details.
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