Bug. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Microphones, radio microphones

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Among novice radio amateurs, miniature micropower radio transmitters (in the common people - "bugs") are very popular. After all, if a beginner "solderer" heard his voice from the radio in the next room, then this means that he has become a real radio amateur.

The scheme of my "bug" is shown in Fig. 1a. The printed circuit board for it is designed according to the size of a matchbox, so there are no problems with the case.

Fig.1. a) Schematic diagram of the bug; b) Colpitz generator on a bipolar transistor; c) Quartz stabilization

A microphone amplifier is assembled on transistor VT1, VT2 forms a master high-frequency oscillator. Its frequency depends on the inductance L1 and the capacitance between the drain-gate electrodes VT2. Through R5, the initial offset is applied to the VD2 varicap.

The load of the microphone amplifier is R6, with the help of C4 high-frequency components are shorted to the case. To increase the sound volume, increase the capacitance of capacitors C1 and C3. In the absence of a field effect transistor, you can use the so-called Kolpitz generator on a bipolar transistor (Fig. 1b). It is only necessary to add dashed elements to create bias at the base of the transistor. The frequency of this generator is less stable over time, so it is advisable to connect a quartz resonator in series with C' (Fig. 1c), while R' must be increased to 30 kOhm, and R "should be removed.

A power amplifier with a gain of about 3 ... 1 is assembled on the VT5 transistor (Fig. 7a). It may not be, then the antenna should be connected to the source (or collector) VT2, but the transmitter power will decrease and will be about 1 ... 2 mW.

Capacitor C11 is needed to load the output stage when using a short antenna (shorter than 0,5 m). Its capacity should be as small as possible and is determined as follows. Tune the receiver until the noise disappears and a specific whistle appears ("microphone effect"). Then briefly touch the area of ​​\u2b\u11bterminals VTXNUMX with a finger. Noise should appear and disappear in the receiver. If it has not disappeared, increase CXNUMX.

When setting up, do not touch the body of the C11 directly with your fingers. It is better to fix it in a foam plastic or other dielectric plate, so that only the leads stick out, and hold on to this plate. When using a bipolar transistor generator, a capacitor may not be needed at all.

The printed circuit board of the "bug" on a field-effect transistor is shown in Fig. 2, on a bipolar one - in Fig. 3.

Fig.2. FET bug circuit board

Fig.3. Bipolar transistor circuit board

With a common wire ("-" power supply), it is best to fill all the empty spaces on the board, and when using double-sided fiberglass (which is better), also connect the foil on the inside with it. The antenna wire should be reinforced with a tinned wire tie. Without this, in case of an accidental jerk for the antenna, it comes off along with the platform.

The board is installed in a matchbox with the parts up. Before that, in three places at the bottom of the side of the box, you need to scroll through the holes for the power and antenna wires. The antenna and power wires are bent so that they pass between the bottom of the box and the lid. It is advisable to paste over the outer side of the cover with foil, and then drill holes in it for the LED and microphone, which should protrude slightly.

Details. VT2 - any field high-frequency (KP302, KPZ0Z, KP307, KP364), VT3 - KT368. You can use KT3102, but then all parameters will be greatly underestimated. L1 and L2 contain 5 ... 6 (FM) or 8 ... 9 (VHF) turns of wire d0,5 mm, which are wound on a temporary mandrel d5 mm.

When setting up, they roughly tune to the frequency using L1, and with the help of L2 - more accurately (according to the maximum sound volume in the receiver). Varicap VD2 - any capacitance of 5 ... 15 pF, instead of it you can solder 1 ... 3 parallel-connected silicon diodes. By the way, KD409 diodes differ from varicaps in that they have two convex points on the body, while varicaps have one. Microphone - MKE332, but better - from imported tape recorders. Antenna - a piece of copper wire 30 ... 70 cm with a diameter of at least 1 mm (so as not to bend). Capacitors C1, C3, C10, C12 - preferably film, C9 - any electrolytic, the larger its capacity, the better (minimum - 100 microfarads). Resistor R5 - MLT-0,1 or imported with the same case dimensions (MLT-0,125 will not fit into the board). The LED together with R1 can be removed. The current consumption of the "bug" is 20 ... 30 mA, the range in open areas is up to 100 m.

Author: A.Koldunov

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