Sound transmission via IR channel. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sound transmission via IR channel. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / infrared technology

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To listen to TV programs on headphones, it is not at all necessary to pull a wired communication line from the TV. The proposed wireless system will help out (it was already described in the article by V. Gushchin, I. Fostyak "Broadcasting on IR rays" in "Radio", 1986, No. 1, p. 27), using infrared (IR) radiation of LEDs. In other words, a low-power transmitter is installed on the TV that emits infrared waves modulated by sound frequency, and a receiver is attached to the headphones, which captures these waves and extracts a modulating signal from them.

The scheme of the set-top box-transmitter to the tube TV is shown in fig. 1. To power the transmitter, a half-wave and diode rectifier VD1 was used, an alternating voltage to which is supplied from the filament winding of the power transformer (T1) of the TV through the SA1.1 contacts of the operation mode switch. The rectified voltage is smoothed by an oxide capacitor C1 and fed to a current stabilizer made on a transistor VT1. As a reference element in the stabilizer, the HL1 LED is used, which simultaneously serves as an indicator of the transmitter being turned on.

IR sound transmission
Fig. 1.

A chain of four series-connected diodes (HL2-HL5) is connected to the current stabilizer - sources of radiation in the IR range. The current through them is chosen to be 50 mA, while the diodes emit about half of the maximum power.

The audio frequency voltage is supplied to the LEDs from the secondary winding of the transformer (T2) of the 3Ch amplifier through the capacitor C2. At the same time, in time with 3H oscillations, the power emitted by the LEDs changes. This is how IR radiation is modulated.

The transmitter starts to work when the moving contacts of the switch are in the lower position according to the diagram. The dynamic head of the TV is turned off in this case.

The receiver circuit is shown in fig. 2. It has one VD1 LED that receives IR radiation. It is connected to a 3H amplifier assembled on the DA1 operational amplifier. From the output of the amplifier, the audio frequency signal is fed to the capsules BF1 and BF2 of the headphones connected in series. The sound volume is regulated by a variable resistor R5. The receiver is powered by a GB1 battery ("Krona").

IR sound transmission
Fig. 2.

The transmitter can use the KT208 transistor with letter indices A-M, as well as any transistors of the MP25, MP26 series. Rectifier diode - any of the D226, D7, KD103 series; HL 1 LED - any of the AL 102 series; the rest of the LEDs - any of the AL 107 series. Oxide capacitors - K50-24 or similar, resistors - MLT-0,125, MLT-0,25. Switch (it is installed on the back of the TV) - MT1, P2K.

The receiver can use operational amplifiers K140UD6, K140UD7, any LED from the AL 107 series, capacitors K53-1, K50-6 (C1, C2), KLS, KM (C3), fixed resistors MLT-0,125, MLT-0,25, variable resistor - SP3-3v (it is with a SA1 power switch), headphones TON-1, TON-2, TEG-1 (in any version, the capsules are connected in series).

Details of the transmitter and receiver are mounted on printed circuit boards. Boards are installed in cases of suitable dimensions. Opposite the LEDs, holes are drilled in the front walls of the cases. The transmitter's LEDs should be placed horizontally and directed towards the viewer. A similar position should be occupied by the receiver's LED, but directed towards the transmitter.

The transmitter power and receiver sensitivity are sufficient for reliable reception at any point in a relatively large room. Moreover, the receiver confidently receives not only direct IR radiation from the transmitter's LEDs, but also reflected from the ceiling, walls, and various objects. Since the emitting and receiving LEDs have a narrow radiation pattern, for uniform reception it is desirable to orient the emitting diodes in different directions, and direct one or two to the ceiling or wall. Then, regardless of the position of the head, the volume of the sound will be the same.

For a small room, the number of emitting LEDs can be reduced and experimentally selected as much as needed for reliable reception. The transmitter circuit does not change.

The transmitter can also be used with transistor TVs that do not have a filament winding. In this case, the modulation scheme remains the same, only the capacitance of the capacitor C2 is increased to 470 μF (at a nominal voltage of 15 V) and a constant resistor MLT-0,5 with a resistance of 2 Ohm is connected in series with the capacitor, and the emitting LEDs are connected to the power source through a quenching resistor with such a resistance that at a current of 40 ... 50 mA on four LEDs a voltage drop of approximately 5,2 V is provided (1,3 V each). Depending on the voltage drop across the resistor, the required rated power dissipation of the resistor is determined. Parts of the current stabilizer with such a power supply, of course, will not be needed.

If the transmitter is difficult to connect to the TV rectifier, it is powered from a separate stabilized unit with an output voltage of 9 ... 15 V at a current of at least 50 mA.

It should be added that such a communication system can be equipped not only with a TV, but also with other devices, for example, a radio receiver, a tape recorder.

Author: I. Nechaev, Kursk; Publication: cxem.net

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