Sound accompaniment via IR channel. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / infrared technology

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When someone from the family is sleeping, and you can't tear yourself away from an interesting TV show, you have to reduce the sound volume to a minimum, but this often does not help. At the same time, the problem is easily solved using headphones. But how to send a sound signal to them? The wire gets in the way and gets tangled underfoot. One option for wireless transmission of sound to phones is described in the proposed article.

When listening to TV sound, you can use an infrared (IR) system to transmit sound to and receive sound from your headphone device to avoid disturbing those around you. If amplitude modulation of IR radiation is used to transmit information, then it is possible to implement the simplest version of such a system.

On fig. 1 shows a diagram of a transmitter operating in the IR range. It is a two-stage AC amplifier operating in class A mode. The DC amplifier mode is set automatically by DC feedback through the R3R4 resistor chain. Capacitor C3 eliminates AC negative feedback. Several (up to eight to ten) IR emitting diodes are included in the collector circuit of the transistor VT1.

In the absence of an input sound signal, a quiescent current flows through the emitting diodes - approximately 40 ... 50 mA. When an audio signal is given, the current through the diodes will change, which means that the power emitted by them will also change - the radiation will be moduped.

The transmitter is powered by a stabilized power supply unit with a voltage of 10 ... 20 V and a current of up to 150 mA. The transmitter is turned on with the SA1 switch, if necessary, the HL1 LED is inserted into the device to indicate the switching mode. The transmitter input is connected to the linear output of the TV ("AUDIO" output) or to the "SCART" type socket, the modulation depth is set by the resistor R1. All parts of the device are mounted on a printed circuit board made of one-sided foil fiberglass, a sketch of which is shown in Fig. 2.

The holes for the leads of the parts in the board are not drilled, the installation is carried out from the side of the foil, imposing the leads of the parts on the corresponding contact pads. The mounted board is inserted into a plastic case of a suitable size. Emitting diodes must be positioned so that the radiation is directed in all directions, including the ceiling.

The following parts can be used in the transmitter: transistors VT1 - KT315, KT312, KT3102 with any letter indices, VT2 - KT815, KT817 with any letter indices, emitting diodes - AL107A, B, capacitors - K50-6, K50-35 or similar imported . Trimmer resistor - SPZ-19a. The adjustment comes down to setting the quiescent current through the emitting diodes by selecting the resistor R5. Resistor R1 sets the maximum modulation depth (at the loudest sound signal), at which distortion does not yet occur.

The receiver can be assembled according to the scheme shown in Fig. 3. It uses a specialized microcircuit UZCH DA1 with a photodetector (photodiode IR range) BL1 connected to its input. The receiver is powered by a 3 V battery, composed of two galvanic cells. Direct or scattered IR radiation from the transmitter hits the photodiode and generates an audio voltage that is amplified by the microcircuit. Headphones with a DC resistance of at least 1 ohms are connected to its output (jack XS50). The volume control is carried out by the resistor R1, combined with the power switch.

Most of the parts are placed on a printed circuit board made of one-sided foil fiberglass, a sketch of which is shown in fig. 4. Installation is carried out in the same way as described above. The board must be placed in a case of suitable sizes, and a hole should be made for the photodiode.

In the absence of a microcircuit, the receiver can be assembled on transistors according to the circuit shown in Fig. 5. Its basis is a three-stage ultrasonic frequency converter, made according to a scheme with a direct connection between the cascades. Thanks to the direct current feedback through the R1R5R6 circuit, the transistor mode is set automatically. The volume is controlled by resistor R2 by changing the depth of the OOS by the alternating voltage of audio frequencies. Headphones with a total DC resistance of at least 100 ohms can be connected to the receiver.

Most of the parts are placed on a printed circuit board made of one-sided foil fiberglass, a sketch of which is shown in Fig. 6. Installation is carried out in the manner described above. Establishing the receiver is reduced to setting (by selecting the resistor R6) the voltage on the collector of the transistor VT3 is about 0,8 ... 1 V.

In both receivers, the following parts can be used: a variable resistor with a power switch SDR-4v, fixed resistors - MLT, S2-33, polar oxide capacitors - K50, K52, non-polar - K10-17. The XS1 jacks must be suitable for the headphones used, and their connection is designed for single phones. If stereo telephones are used, then the socket contacts must be connected so that the telephones are connected in series.

When operating this system, be aware that it is subject to optical interference from lighting fixtures as well as the TV screen itself. At the same time, the background of an alternating current with a frequency of 100 Hz can be heard in the phones. To attenuate these interferences, you can try to install an IR filter from remote controls and reduce the capacitance of the input capacitor of the receivers (C2 - in Fig. 3 and 5), which will ensure the blockage of the frequency response of the receiver at low frequencies. You should also avoid direct light from lighting devices on the photodetector.

Author: I. Nechaev, Kursk

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