Soundtrack - without wires. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Soundtrack - without wires. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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When the soundtrack of television programs interferes with others, you have to use headphones that are included instead of the dynamic head of the TV. There is a more radical way - to connect a frame from a wire stretched along the plinth of the room to the TV instead of a head, and listen to transmissions on a miniature AF signal receiver mounted on headphones.

To get rid of the wires connected to the TV, the author of the published article offers another option - the transmission of 3H signals over the air. By the way, this option will allow you to equip a radio, tape recorder, or any other sound source with a silent listening system.

It is best to implement the proposed system in the VHF band, where it is easy to make a micropower VHF transmitter, and receive its signals on an existing small-sized VHF receiver or player with a VHF band.

Soundtrack - without wires

The diagram of a micropower VHF transmitter with frequency modulation is shown in fig. 1. It is a single-ended multivibrator with an L1C4 frequency setting circuit. This scheme is well known to radio amateurs involved in the design of transceiver equipment; it is used in the design of smooth range master oscillators (GPA).

A distinctive feature of generators based on this circuit is the ability to operate at high frequencies at very low currents and voltages.

One galvanic cell with a voltage of 1,5 V is used as a power source for the transmitter. The direct current through the transistors is set by resistors R3, R4. A change in this current leads to a slight change in the frequency of the generated oscillations, therefore, in order to obtain frequency modulation, a 1-hour modulating voltage from the signal source is supplied to the current-setting circuit from resistor R1 through capacitor C2 and resistor R3. The deviation value is regulated by a variable resistor R1, and since it is combined with the SA1 switch, the transmitter is also turned off by it.

The transmitter power is low - less than 1 mW, so it will not create any interference with the radio reception of neighbors, at the same time it is enough for confident reception of its signal within the room. With a supply voltage of 1,5 V, the current consumption is 0,42 mA, which indicates high efficiency and the ability to work from one battery for a long time. The normal deviation is obtained when a 3H signal with an amplitude of 100 mV is applied, the input impedance is 10 kOhm, so you can connect the transmitter to the line output of a TV, tape recorder, radio, etc.

The transmitter remains operational when the supply voltage is reduced to 0,8 ... 0,9 V, i.e. almost until the battery is completely depleted, and the current consumed in this case is 0,07 ... 0,1 mA. A piece of copper wire with a diameter of about 1 mm and a length of 20 cm to 1 m is used as an antenna. The longer the antenna, the larger the zone of reliable reception of the transmitter signal.

All parts of the device are placed on a printed circuit board made of double-sided foil fiberglass. The parts are installed on one side, and the second is left metallized and connected to a common wire along the circuit board in several places. The battery is attached to the same side.

In the device, you can use a variable resistor SPZ-4, SPZ-3 with a power switch, fixed resistors - MLT, S2-33, oxide capacitor - K50, K53, tuning - KT4-25, the rest - KM, KLS, KD. For the VHF-2 range (88 ... 108 MHz), the L1 coil contains seven turns of PEV-2 0,8 wire wound on a mandrel with a diameter of 3,5 mm with a tap from the middle.

Adjust the transmitter in the following sequence. First, choose the range in which it will work - it depends on the receiver used. Then the receiver is tuned to the part of the range that is most free from broadcast radio stations and interference. The transmitter is connected to the line output of a tape recorder or TV and the capacitor C4 tunes it to the frequency of the receiver. Resistor R1 sets the most acceptable frequency deviation, which provides normal volume and minimal distortion. If the volume is not enough even with the upper position of the resistor R1 slider according to the diagram, it will be necessary to reduce the resistance of the resistor R2.

Then they check the coverage area and, if it turns out to be small, increase the length of the antenna to 0,7 m.

The transmitter is placed in a housing (possibly plastic) of suitable dimensions, and a shielded wire with an appropriate connector is used to connect to a TV, tape recorder or other signal source.

If you do not have a small-sized VHF receiver, it is easy to make it using domestic K174XA34, KXA058 microcircuits or their imported counterparts. Several variants of such receivers were described in [1-3].

Literature

1. Gerasimov N. Dual-band VHF receiver. - Radio, 1994, No. 8, p. 6-8.
2. Makarov D. VHF receiver-pack "MARLBORO". - Radio, 1995, No. 10, p. 41-43.
3. Semenov B. A simple VHF receiver. - Radio, 1996, No. 10, p.22,23.

Author: I. Nechaev, Kursk; Publication: N. Bolshakov, rf.atnn.ru

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