Miniature radio. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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A reliable companion on the road and on vacation - a miniature radio that will allow you to listen to the latest news or your favorite music programs. Two such structures are described in the proposed article.

A diagram of one of the options for a miniature receiver is shown in fig. 1. It is designed to work in the ranges of long (LW) and medium (MW) waves. They are tuned to radio stations in each band with a variable capacitor C1. When receiving medium-wave radio stations (switch SA1 is in the "CB" position), the oscillatory circuit is formed by the coil L1 of the magnetic antenna WA1 and this capacitor. On the long-wave range (switch SA1 in the position shown in the diagram), the oscillatory circuit is composed of series-connected coils L1, L2 of the magnetic antenna and capacitor C1.

The signal selected by the circuit is fed to an RF amplifier assembled on a field-effect transistor VT1. Due to the high input resistance of the transistor, it was possible to connect it directly to the oscillatory circuit, eliminating the traditional coupling coil for such input circuits. The gain of the cascade on the field-effect transistor, and hence the volume of the sound of the received transmission, is regulated by a variable resistor R2, combined with the SA2 power switch.

From the output of the amplifier (from the load resistor R1), the RF signal enters through the capacitor C2 to the second stage, made on the transistor VT2. It combines the functions of a detector and a pre-amplifier for AF signals. The subsequent amplification of these signals is carried out by a cascade on the transistor VT3. The signal removed from its load (resistor R6) is fed through a matching stage on the transistor VT4 (this is an emitter follower) to the load - stereo headphones included in the XS1 connector. A small direct current (less than 1 mA) flowing through the phones does not affect the sound quality and is not dangerous for the phones themselves.

The receiver is powered by a GB1 battery, composed of two D-0,06 disk batteries connected in series. In silent mode, the receiver consumes no more than 2 mA, and in maximum volume mode - no more than 2,5 mA.

It is advisable to select a transistor VT1 with an initial drain current of not more than 1 mA. Then the drain voltage will be approximately 1 V. If a transistor with an initial drain current of 0,6 ... 0,8 mA comes across, it is possible to slightly increase the resistance of the resistor R1, which will lead to an increase in the gain of the cascade. Of course, a transistor with an initial drain current of up to 1,5 mA will do, but you will have to reduce the resistance of the resistor R1 and put up with a possible decrease in the gain of the cascade and an increase in the current consumed by the receiver.

In addition to those indicated in the diagram, transistors VT2, VT4 can be any of the KT315 series, and VT3 - KT350A or KT361 with any letter index. Variable resistor - any small-sized with a switch, the rest of the resistors - MLT-0,125. Capacitor C1 - from the Sokol transistor receiver, but any other small-sized one will do, capacitors C2, C3 are small-sized ceramic ones. The SA1 switch is small-sized, the XS1 connector is the same as that used, for example, in players, for connecting small-sized stereo headphones. Phones - any.

The receiver uses a magnetic antenna from the latest Sokol receiver. Its windings are wound on a flat rod measuring 4x12x62 mm from 400NN ferrite. The antenna coupling coil is not used. If you cannot get such an antenna, wind it yourself, for example, on a flat rod measuring 4x16x60 mm or on the one indicated above. Coil L1 should contain 83 turns of PEV-2 0,21 wire, and L2 - 250 turns of PEV-2 0,1 ... 0,15. Moreover, the L1 coil is wound turn to turn, and the turns of the L2 coil are placed in five sections 2...3 mm wide with a distance between sections of 3...4 mm, 50 turns in each section. The coils are placed at a small distance from each other.

The details of the receiver are mounted on a printed circuit board (Fig. 2) made of foil fiberglass with a thickness of 1,5 mm. The batteries are located in a compartment made of a foil fiberglass plate measuring 15x15 mm and a tinned copper wire with a diameter of 1 mm.

The appearance of the mounted receiver is shown (without housing) in fig. 3.

Setting up the receiver comes down to checking the voltage at the drain of the field-effect transistor (about 1 V at the top position of the resistor R2 according to the circuit) and correcting it (if it was not possible to measure the initial drain current) by selecting the resistor R1, as well as selecting the resistor R5 of such a resistance at which the current consumed by the receiver in silent mode will be about 2 mA.

The diagram of the second version of the receiver is shown in fig. 4. It uses the EKR1436UN1 chip in the AF amplifier. A feature of this microcircuit is the presence of anti-phase outputs (pins 5, 8), doubling the amplitude of the output signal, which is equivalent in relation to, say, the previous design, doubling the supply voltage. Moreover, in the absence of a signal at the input of the microcircuit (pin 3), the voltages at both outputs are equal and make up about half of the supply voltage, which allows you to connect the load directly, without an isolation capacitor. The microcircuit operates at a supply voltage of 2 ... 16 V, the current consumption for most microcircuits does not exceed 3,5 mA.

(click to enlarge)

The gain of the microcircuit is determined by the resistance of the resistors R6, R7 of the feedback circuit and is calculated by the formula Ku \u2d 7R6 / RXNUMX.

The receiver load can be either a small-sized BA1 dynamic head with a voice coil with a resistance of 8 ... 100 Ohms (the greater the head resistance, the less current consumed by the receiver), or stereo headphones included in the XS1 connector. With telephones, the receiver consumes no more than 4 mA in silent mode, and no more than 4,5 mA at maximum volume. Therefore, a battery of two disk batteries remains the power source. If the receiver will be used with a dynamic head, you will have to make a battery of more energy-intensive "finger" cells or the same batteries.

Additionally introduced oxide capacitors C3, C6, C8 - K50-16 or others; fixed resistors, capacitors and magnetic antenna are the same as in the previous design. Develop a printed circuit board drawing yourself, using the principle of the location of parts on the board of the previous receiver. Establishment comes down to practically selecting a resistor R7 of such a resistance that provides the required sound volume and receiver sensitivity. If the volume of the sound is not enough, you can increase the supply voltage to 4,5 and even up to 6 V. In this case, its dimensions will, of course, increase.

Author: D.Turchinsky, Moscow

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