To listen to TV programs on the headphones, not necessarily pull away from the TV wired communication line. Help out the proposed wireless system (it is already explained in article V. Gushchina, I. Fotka "Broadcast on the IR rays" in "Radio", 1986, No. 1, p. 27), using infrared (And) radiation of the LEDs. In other words, the TV set low-power transmitter that emits the modulated audio frequency infrared waves, and with head phones attached receiver catching those waves and produce from them the modulating signal.
Scheme prefix-tube transmitter to the TV is shown in Fig. 1. To power the transmitter used half-wave rectifier and the diode VD1, the AC voltage which is supplied with filament winding of the power transformer (T1) on the TV through the contacts SA1.1 switch the operation mode. The rectified voltage is smoothed oxide capacitor C1 and is supplied to the current regulator, made on the transistor VT1. As a supporting element in the stabilizer HL1 led is used, which simultaneously serves as the indicator switch on the transmitter.
Fig. 1.
The current stabilizer connected chain of four series-connected diodes (HL2-HL5) - sources of radiation in the infrared range. The current through them is selected equal to 50 mA, while the diodes emit about half the maximum power.
The audio frequency voltage is supplied to the LEDs from the secondary winding of the transformer (T2) of the amplifier 3H through the capacitor C2. Thus in time with the oscillations 3H emitted by the LEDs is changed power. So a modulation of the IR radiation.
The transmitter starts to operate when the moving contacts of the switch are in the lower circuit on the situation. Dynamic head is the TV in this case off.
The receiver circuit is shown in Fig. 2. In it - one led VD1, perceive infrared radiation. It is connected to the amplifier 3H, assembled by operational amplifier DA1. From the amplifier output audio signal is supplied to the primers BF1 and BF2 headphones that are connected in series. The sound volume adjust the variable resistor R5. Powered the receiver from the battery GB1 ("Crown").
Fig. 2.
The transmitter can be used transistor CT with an alphabetic index A-M, as well as any transistors series MP, MP. Rectifier diode - any of the series D226, D7, CD; led HL 1 - any of a series of AL 102; the remaining LEDs - any of a series of AL 107. Oxide capacitors - C50-24 or similar, resistors - MLT-0,125, MLT-0,25. Switch (install on the rear of the TV) - MT, P2K.
In the receiver it is possible to use operational amplifiers COD, COD, any led from the AL 107, capacitors K53-1, K50-6 (C1, C2), KLS, KM (C3), fixed resistors MLT-0,125, MLT-0,25, variable resistor - CP3-3V (he with the power switch SA1), head phones TONE-1, TONE-2, TAG-1 (in any case, the fuses are connected in-series).
Details of the transmitter and receiver are mounted on printed circuit boards. Boards installed in the buildings of suitable size. Opposite the LEDs in the front walls of drilled hole. The LEDs of the transmitter should be placed horizontally and be directed toward the viewer. A similar provision should occupy the light emitting diode, but is pointing toward the transmitter.
Transmitter power and receiver sensitivity is sufficient for reliable reception anywhere in the relatively large room. And confidently receiver receives not only the direct infrared radiation from the LEDs of the transmitter, and the reflected light from the ceiling, walls, various items. Since the emitting and receiving LEDs have a narrow focus, for uniform reception preferably light emitting diodes oriented in different directions, and one or two to send on the ceiling or wall. Then regardless of the head position the volume will be the same.
For a small room, the number of emitting LEDs can be reduced and experimentally to pick up as much as needed for reliable reception. Diagram of the transmitter is not changed.
The transmitter can be used with transistor TVs that have no filament winding. In this case, the modulation scheme remains the same, only the capacitance of the capacitor C2 is increased to 470 UF (at nominal voltage 15 V) and in series with the capacitor includes a constant resistor MLT-0,5 resistance 2 Ohm, and emitting LEDs connected to a power source through the quenching resistor with too much resistance so that when current of 40...50 mA on the four LEDs are provided to drop the voltage to about 5.2 V (1.3 V each). Depending on the incident on the resistor voltage to determine the desired nominal power dissipation of the resistor. Details of current stabilizer with this power, of course, is not required.
If the transmitter is difficult to connect to the rectifier of the TV, it is fed from a separate stable block with an output voltage 9...15 V at a current of not less than 50 mA.
It should be added that such a communication system can be equipped not only television, but also other devices such as a radio, tape recorder.
Author: I. Nechaev, Kursk; Publication: www.cxem.net