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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Laser light phone. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur A lot has been said about the laser pointer and its use in various designs on the pages of the magazine. For example, a photo shooting range, a sniper simulator, and security devices were offered. Today, readers will be able to get acquainted with a light telephone based on such a pointer, which was developed in the radio engineering circle of the Genichsky regional station of young technicians under the guidance of the author of the article, Vasily Georgievich Solonenko. This light telephone was developed for demonstration purposes, but it can also be used for communication between points separated by up to 100 m. Of course, each point must have a transmitter and receiver. First, about the transmitter. A diagram of one of its variants is shown in Fig. 1. Since the supply voltage of the battery supplied with the pointer is 4,5 V, and the current consumed by the pointer is about 35 mA, the modulating stage is made on a single transistor.
But to increase the signal level from the BM1 dynamic microphone, another amplification stage is needed. The result is a two-stage amplifier, which allows you to get the amplitude modulation of the laser beam when talking in front of a microphone. The sound vibrations converted by the microphone into electric current are fed through the isolation capacitor C1 to the base of the transistor VT1 of the first amplification stage. The amplified signal is taken from the load resistor R2 and fed through the capacitor C2 to the base of the transistor VT2 of the second amplifying stage. Its load is a laser pointer. The changing collector current of this transistor leads to a change in the brightness of the laser beam. Capacitor C3 prevents the transmitter from being excited due to parasitic coupling through the power supply. The details of this version of the transmitter are mounted on a board (Fig. 2) made of one-sided foil fiberglass.
The transmitter can be simplified (Figure 3) by using an electret microphone. The sound signal, converted by the BM1 microphone, is isolated at the resistor R1 and fed through the capacitor C1 to the base of the transistor VT1 of the only amplification stage. The collector current of the transistor modulates the laser beam of the pointer.
For this version of the transmitter, the parts are placed on a printed circuit board, the drawing of which is shown in Fig. 4.
Now about the receiver. After numerous experiments on choosing a photo sensor, I had to stop at a powerful transistor with a sawn off hat. It was used to convert the light energy of a laser beam into electrical energy and was connected to the input of an amplifier through a decoupling capacitor, like a microphone. This method allows you to use any 3H amplifier with a microphone input as a photodetector without modification. The specified photosensor develops an EMF sufficient to listen to the transmitter signal on high-resistance headphones at a distance of up to 2 m without an amplifier. Moreover, a faulty transistor can be used as a photosensor if it has at least one junction intact. The photodetector uses a three-stage amplifier (Fig. 5).
The light energy of the laser beam is converted by the photosensor VT1 into an electrical signal, which is fed through the decoupling capacitor C1 to the base of the transistor VT2 of the first amplifying stage. The amplified signal is removed from the cascade load (resistor R2) and fed through the capacitor C2 to the input of the second cascade, made on the transistor VT3. From its load (resistor R4), the signal is fed through the capacitor C3 to the input of the third stage, in which the transistor VT4 operates. A dynamic microphone was used for the BF1 headphones as it provided a higher sound quality. Capacitor C4 shunts the load at higher frequencies and prevents self-excitation of the amplifier. Since the receiver is intended for speech reproduction, it is advisable to raise the lower limit of the passband frequency to 300 Hz by reducing the capacitances of the coupling capacitors. This significantly reduces interference from light sources (powered by a 50 Hz mains), which degrades reception quality. The details of the receiver are mounted on a printed circuit board (Fig. 6) made of one-sided foil fiberglass. Like other boards, this one is made by cutting insulating tracks.
In the design of the light telephone, oxide capacitors of the K50-16 series can be used, the rest - K73-17, KM-5, KM-6. Resistors - MLT, VS or other appropriate power. In the first version of the transmitter, instead of the MP26B transistor, it is permissible to use any of the MP40-MP42 series, we will replace the 2T603A transistor with KT603, KT608 with any letter index. The same transistor can be installed in the second version of the transmitter, but with a current transfer coefficient of at least 150, otherwise it will not be possible to obtain the desired modulation depth. In the second version of the transmitter, an electret microphone CZN-15E was used. In the receiver, in place of the photosensor, transistors of the KT803, KT808, KT827, KD617 series (by TESLA) were tested. The best results were shown by KD617. Receiver transistors can be indicated on the series diagram with any letter index. In place of BF1, except for MDM-7, you can use headphones from the player, as well as any electromagnetic phones or capsules with a resistance of 50-150 Ohm, for example, TK-67, TA-56. The power source in the transmitters and the receiver is a battery composed of four D-0,26 batteries connected in series. Setting up the receiver begins with setting half the supply voltage on the collectors of transistors VT2, VT3 by selecting resistors R1, R3, respectively. When establishing the third stage, a milliammeter is turned on in the collector circuit of the transistor VT4 and a current of 5 mA is set by selecting the resistor R10. When establishing the first version of the transmitter, first set half the supply voltage at the collector of the transistor VT1 by selecting the resistor R1. Then, by placing the receiver and transmitter at a distance of 10 ... 15 m from each other, by selecting the resistor R3, the maximum brightness of the laser beam is achieved with a good quality of the received signal. Similar results are achieved when setting up the second version of the transmitter by selecting the resistor R2. Unfortunately, laser pointers have a large spread in parameters, so the resistance of the resistor that regulates the brightness of the beam can differ significantly from that indicated in the diagram. Structurally, the light telephone is made in the form of a handset with a stand (Fig. 7).
In the body of the tube there is a transmitter board and a power source with a switch, and in the stand there is a photo sensor, a receiver board with a switch and a laser pointer. The handset can be connected to the cradle with a four-wire cable through a connector (not shown in the diagram). The photo sensor is placed in a cylindrical glass (box from filmstrips) to protect against side illumination. The design of the light telephone was developed for educational and demonstration purposes, therefore, the photo sensor and laser do not have a stationary mount, but are located in the stand for the handset. Since it is difficult to find horizontal surfaces located at the same height during demonstrations of the operation of a light telephone, a simple device for moving the laser beam in a vertical plane is used to align the laser with the receiver’s photosensor (Fig. 8).
It consists of a frame 2, glued from polystyrene with a P647 or P650 solvent and fixedly attached to the wall of the case 4. The frame is located at the rear end of the pointer 1, the front part of which, with the help of a cone nozzle, abuts against the hole in the front wall of the case. The pointer is spring-loaded from below by a semi-compressed spring 8, and is held from above by a threaded pin 7. To move the stud, a nut 3 is fused into the upper part of the frame, and a handle 5 is put on the outside of the stud. By turning the handle, you can move the back of the pointer in a vertical plane, which leads to the movement of the laser beam. The receiver power switch 6 and connector 9 are fixed on the front wall of the case. To communicate via a light telephone, it is necessary to install its stand vertically (Fig. 9).
By moving the stand in the horizontal plane, align the laser beam with the photo sensor of the receiver of another communication point, and in the vertical plane correct the position of the beam with knob 5 (Fig. 8). During the tests of the light telephone, connections were made through a beam reflected from window glass, as well as from polished furniture. In both cases, the quality of communication remained high. Focusing lenses can be used to increase the communication range. In our design, a focusing lens from the Ogonyok filmoscope was put on the diameter of the light-protective tube. Author: V.Solonenko, Genichesk, Kherson region, Ukraine
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Comments on the article: Victor Excellent, simple and effective scheme. Well done! Emelyan Valeryanovich Useful, clear article, without superfluous words. It is even easier to modulate the beam without connecting a pointer to the circuit, that is, through reflection from a mirror film stretched over a tube. We speak into the receiver, the mirror film moves and changes the strength of the reflection of the beam. It is even possible to transmit the voice with the modulation of the "sunbeam".
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