Several transistor radios described in this article do not require battery power. They use high-frequency signals that are transmitted to the air by radio stations, and are rectified to obtain DC power, which is used as a power source for the radio. However, this kind of radio can only receive broadcasts of local close-range high-power radio stations; in addition, it must also use an outdoor antenna and a good ground wire; the earphone should use a coil impedance of not less than 2 kΩ.
L1 and C2 form the input circuit of the radio
The first circuit of Figure 1 operates as follows: L1, C2 form the input loop of the radio. A high-frequency voltage is obtained at the center tap and the ground of the coil L1, and is applied between the base and the emitter of the transistor T1 for detection; the low-frequency signal voltage obtained after the detection is amplified by the transistor. For low frequency signals, a transistor is an amplifier in which the load (headphone) is connected to the collector circuit. In addition, the high-frequency voltage across the input circuit is rectified by the diode Д1 to obtain a DC voltage, which serves as a DC power supply for the collector circuit of the transistor. Because the frequency of the rectified voltage is very high, the rectified filter only needs to use a capacitor with a capacity of 0.1 microfarad.
Voltage doubler rectifier circuit
The difference between the second circuit shown in FIG. 2 and the former circuit is that a voltage doubler rectifier circuit composed of C3, Д1, Д2, and C4 is used here to increase the DC supply voltage, thereby increasing the amplification effect of the transistor, so that Sounds a little.
In the third circuit of Fig. 3, the high-frequency voltage is directly applied between the base and the emitter for rectification, and the DC voltage obtained at the resistor R1 after rectification is used as a power source for the collector circuit. Transistor OC-813 can be replaced by П15, П401, П402, etc.; crystal diode 0A625 can be replaced by any type of point contact type crystal diode.
In the first few circuits, the transistor is used for both detection and amplification. Next, another experimentally similar circuit (Fig. 4) will be introduced for reference during trial production.
In the circuit of Figure 4, a single diode is added for detection. In addition, two input loops are used, one (L2C4) for high frequency signals and the other (L1C2) for power.
The two loop coils each have a number of taps to select the most suitable position. The data and requirements of each part are described below. Antenna – Г-type antenna, the horizontal part is 8 meters, and the vertical part (the distance between the horizontal part and the roof) is about 4 meters. Use 0.23 mm (equivalent to No. 34). The enameled wire is twisted into five strands. Grounding device – welded with a 18-gauge galvanized iron wire on a piece of 150 × 150 mm 2 copper, buried 1 m deep underground. Coils L1, L2 – A badminton paper tube with a diameter of 65 mm is placed in paraffin and boiled as a bobbin. L1 is tapped around 70, 20, 35, 50 with a 0.45 mm (26 gauge) enameled wire. L2 is also the same number of turns with the same number of wires, but taps at the 15th, 20th, 25th, and 30th turns (the number is counted from the ground)
C4, C2 – Single-connected variable capacitors with a general air medium and a capacity range of 16 to 360 picofarads. There is no need to double-connect here because it is difficult to adjust the tuning circuit synchronization.
Д1, Д2, T1——The crystal diode is made of domestic Д1B type. It is better to use a forward resistance of about 500 ohms and a reverse resistance of 100 kilo ohms or more. The multimeter range is measured in the (R × 100) or (R × 1K) range during the test. The transistor T1 is made of a domestic П6 type or a 2G100 type.
C1, C3, C5 – C1, C3 use paper capacitors with a withstand voltage of 400 volts. C5 uses a 3-volt, 10 microfarad ultra-small electrolytic capacitor.
R1, R2 – use 1/2 watts of thin film resistors. R2 is about 100 kΩ, and it is necessary to choose the most suitable resistance according to the specific transistor used.
Headphones – the line coil impedance is not less than 2 kohms.
The size of the wooden case is 210×140×140 mm3. The bottom plate is placed at a height of about 30 mm from the bottom of the wooden shell. Place L1, L2, C2, C4 on the bottom plate. Other parts are welded to the terminal frame under the bottom plate. See Figure 5 for how the parts are arranged on the terminal block.