laser phone. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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laser phone. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The advantages of the proposed self-made transceiver on the LU are due to the peculiarities of the optical range in which it operates. Firstly, the device is simpler compared to conventional radio analogues. Secondly, the optical communication line is protected from outside listening - it is more difficult for a "stranger" to infiltrate through his receiver, being outside the beam.

Thirdly, a fairly wide area is outlined where a transceiver with a laser pointer can be used: from wireless communication or, say, transmitting the sound of TV programs to covert monitoring of the acoustic environment of a protected object, listening to calls in the depths of an apartment, a country house, at the front door .

The self-made device I developed, somewhat similar to the well-known amateur light telephone, consists of two parts: a transmitter and a receiver. The radiation of the LU serves as a carrier of information, while the rest is provided by simple electronic units.

In particular, the main unit of the transmitter is a fairly powerful audio frequency amplifier, assembled on a DA1 chip and several attachments that provide the device with a normal operating mode. Information is supplied to the sensitive input 1 of the microcircuit through the capacitor C2 - electrical vibrations of the sound frequency from the electret microphone BM1, the DC supply of which is provided by a two-resistor voltage divider. Moreover, as R2, a "trimmer" is used, which allows you to adjust the level of the audio frequency input signal supplied to the amplifier.

A laser pointer A6 is connected to the output of the amplifier through resistor R1 with its internal contact. Its metal case has a galvanic contact with the "plus" of the power supply. The function of protecting the pointer from overvoltage is performed by the zener diode VD1.

Schematic diagrams of the transmitter (a) and receiver (b), as well as sketches of circuit boards (c) and (d), allowing you to organize an amateur communication line using a laser pointer (click to enlarge)

As you know, the working body of the LU - a laser or a powerful special light-emitting diode - has one-sided conductivity. Given this, in addition to the modulating signal, an electric voltage of some intermediate level, set by the resistor R6, is applied to it. As a result, there is only an increase or decrease in the glow of the pointer. That is, the beam sent by the transmitter and then received by the phototransistor VT1 contains, along with the information, a constant component necessary for the correct operation of the light telephone.

The constant component of the light signal that comes from the transmitter keeps VT1 ajar, and the variable makes it open a little more or a little less than the average level. Accordingly, the voltage on the collector of the phototransistor will fluctuate, but in time with the sound signals from the transmitter microphone.

The receiver amplifier is no different from the one discussed earlier. The electrical signal containing the transmitted information, the level of which is determined by the "trimmer" R2 of the receiver, enters the capacitor C2, which cuts off the constant component, and the variable is supplied to the input of the DA1 microcircuit, as in the transmitter. Only at the output of the amplifier, instead of a laser pointer, there is a dynamic head BA1, which converts the 3H electrical signal into sound vibrations identical to those captured by the transmitter microphone.

In all experiments with a laser pointer, extreme caution is required. In particular, in order to achieve stable operation of the light telephone, even accidental contact of the beam with the eyes should be excluded. If it is necessary to mask the optical connection, the light emitter of the transmitter and the phototransistor of the receiver must be placed in a hood, tube or covered with a visor.

For the configuration of both blocks, fixed resistors MYAT with a power of 0,125 to 0,5 W, trimmers SP4-1a and equally common inexpensive capacitors K73-24 (C2, C4) and K50-6 (all others) are acceptable. If an increased sensitivity of the MKE-332 microphone is desired, its modification with the letter index V or G at the end of the name should be taken. The dynamic head can be of the type 0.5GDSH-9; for more discreet listening, a capsule from a handset or an ear microphone with a voice coil resistance of about 30-100 ohms is suitable.

Electricity sources can be three series-connected galvanic batteries of the 3R12 type or network adapters with an output voltage of 12 V. In the latter case, it is recommended to install a power switch in the circuit connecting this adapter with household 220 V.

Mechanical fastening and electrical connections of radio elements in the manufacture of both electrical circuits - on printed or split circuit boards made of plastic foiled on one side with a thickness of 1-1,5 mm. The laser pointer and the phototransistor are placed on the walls of the respective blocks or taken out of the receiver (transmitter) case. If both parts of the light telephone are supposed to be placed permanently, then it is advisable to provide brackets or stands with sockets that allow you to quickly and unambiguously set the desired optical axis "emitter - phototransistor".

Author: Yu.Prokoptsev

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