[م .نبيل زبن]

Super simple receivers


Using a super sensitive piezoelectric earphone (see our catalog)* you can make a laser voice receiver that doesn't need any expensive amplifiers or power source. Just connect it to a small solar cell* as shown below



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Also in our catalog* we have tiny silicon solar cells that you can attach to a piezoelectric earphone with simple transparent tape* instead of soldering (which can be difficult to do on silicon solar cells).



a solar cell is too expensive or fragile* a cadmium-sulfide photoresistor can also be used. The earphone wires are connected across the photoresistor* and the battery is also connected across the same wires. The battery* the earphone* and the photoresistor are in parallel. A 220 ohm resistor is placed in series with the battery* to reduce power consumption* and prevent heating of the photoresistor



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Either of these earphone approaches has the nice feature of making the communication private. Only you can hear what is coming over the secret laser link.



How does it do that?


In all of the laser communicators on this page* the laser light is amplitude modulated. This simply means that the amount of light the laser emits varies over time.
To understand what is going on* it helps to consider how a loudspeaker makes sound. A loudspeaker is a paper cone attached to a coil of wire that sits in a magnetic field from a strong permanent magnet.
When an electric current flows in the loudspeaker coil* the coil becomes an electromagnet* and it moves toward or away from the permanent magnet. As it moves* the paper cone pushes on the air around it* compressing the air in front of it* and expanding the air behind it. Waves of compressed and expanded air travel to your ear* and cause your eardrum to move in time to the movements of the paper cone.
The laser communicator adds two components to the loudspeaker concept. We take the electrical signal that goes to the loudspeaker* and connect it instead to the laser* so the laser gets brighter and dimmer as the electric current varies. The second component is the receiver* which converts the light back into an electric current. This current varies in time with the first current* because the amount of light that it receives is varying in time.
This second electric current is used to move the paper cone of a loudspeaker* just as before. However* now the loudspeaker can be quite a distance away from the original electric current* without any wires connecting the two.