Powerful DIY radio microphone circuit diagram. Simple radio microphones. Methods for increasing the stability of radio microphones
Good afternoon to all Radio Amateurs. First, I want to express my deep gratitude to its inhabitants. This is where I learned how to solder and use a multimeter and much more. It all started when at work, while rummaging through a friend’s box, I found an old car tape recorder, and the idea immediately came to assemble the bug, since it had almost all the necessary parts.
The next day I took with me a soldering iron and all sorts of little things like rosin, a circuit board, an RF detector and additional parts. I removed all the radio components I needed from the car radio board.
Everything was done as in the circuit, except for transistor T1 and C5, instead of KT315 I put C9014 and instead of C5 (15pF) I put 20 pF.
I desoldered, soldered, cut, threw, wrapped, cleaned the board with white alcohol and that’s it, it’s time to turn it on. And bam, I connect the battery (9V, “CROWN”), and the result is zero. There is no consumption, the detector does not show, pain, anxiety, sadness... what to do!? I decided to take a closer look at the board, but it turns out I connected the winding to the negative line)).
I connected it correctly and the radio microphone immediately started working. The current consumption was 9-10 mA, after a while the cartoon began to show 8.50 mA, although the beetle works as before. I thought the battery was dead - no, everything is fine. My multimeter is lying a little. In general, I'll experiment. The food is the famous Crohn's.
The winding was made from 0.8 mm copper wire and contained a coil of 6 turns.
About the microphone: I took it out of some phone. You can check the functionality with a multimeter. Typically its resistance is around 1-2 kOhm. If you blow on it, the resistance should change.
And here is the reading from the RF detector:
The antenna was made from stranded wire about 40 cm long. Below you can see a photo of the finished Radio Microphone (bug). Also attached. The noise you can hear in the recording is the noise coming from the computer's CPU cooler. Can you already imagine the sensitivity of the microphone?)) The frequency was caught at 82.00 MHz. But to be honest, the frequency often “floats”. That is, if you turn off the power and plug it in again, the frequency goes either to 83 MHz or to 81 MHz. But he definitely won’t go far - you’ll find it)).
Radio microphones are used both for concerts and for important conversations in a closed room. For conversations in a closed room, it is necessary to carefully disguise this “bug” from prying eyes, and therefore must have a small size and a simple circuit.
The diagram of the simplest radio microphone is shown in Fig. 1.
The radio microphone operates on the FM band (approximately 96 MHz). In the diagram in Fig. 1, a piece of wire 37 cm long is used as an antenna. A 3 V lithium “pill” (CR2032, CR2025, etc.) can be used as a power source. Coil L1 contains 6 turns of PEV or PEL wire 0.5 mm; it can be wound on the core of a helium pen with a diameter of 4 - 5 mm. Electret microphone.
The radio microphone is configured using a radio broadcast receiver with an FM band tuned to a frequency of ~96 MHz (in an area free from broadcast stations). By squeezing and stretching the turns of coil L1, the frequency is captured by the radio receiver at the maximum signal. The setup is complete. If necessary, fix the coil turns with glue or paraffin.
The diagram of a radio microphone with an additional microphone amplifier is shown in Fig. 2.
In this circuit, coil L1 contains 5+5 turns of PEV 0.5 wire on a mandrel with a diameter of 3 mm.
The diagram of the radio microphone on K174PS1 for the range 88 - 108 MHz is shown in Fig. 3.
In the diagram in Fig. 3 an electret microphone is used. Coils L1 and L2 are frameless, have 5 turns each. Winding is done with 0.2 - 0.5 mm wire on a mandrel with a diameter of 3.5 mm.
The transmitter is tuned using tuning capacitor C6, and capacitor C8 is used to adjust the maximum output power.
A micropower radio microphone for the range 66-100 MHz, without inductors, built on the digital K155LA3 is shown in Fig. 4.
In this circuit, adjustment to the required frequency is carried out by resistor R2. For stable operation of the radio microphone when the supply voltage changes, voltage is applied to transistors VT1, VT2 and zener diode VD1. A rod about 1 m long made of thick copper wire or a telescopic antenna from radio receivers is suitable as an antenna.
Making a bug radio microphone attracts many, especially beginner radio amateurs. And most often they try to repeat them, considering them easier to manufacture. Yes, on the one hand this is true, but in terms of setup it is better to choose a three-stage radio microphone circuit, where each transistor has its own role: microphone amplifier, generator and RF amplifier. In this design, each cascade of the bug can be easily and conveniently configured individually. Of course, it will require 3 times more parts, but the characteristics (sensitivity, stability, radiation power) will also improve. It is on this principle that the Filin-3 radio microphone circuit works, which I found on the Internet and successfully repeated.Radio microphone details:
VT1 - KT3130B
VT2 - KT368A
VT3 - KT3126B
R1 - 12 kOhm
R2 - 300 kOhm
R3 - 4.7 kOhm
R5 - 20 kOhm
R6 - 200 Ohm
R7 - 200 Ohm
C1 - 100-300 pF
C2 - 0.03-0.1 µF
C3 - 0.03-0.1 µF
C4 - 500-1000 pF
C5 - 22 pF
C6 - 12 pF
C7 - 39 pF
C8 - 0.1-0.5 µF
Specifications transmitter:
Frequency: 88-108 MHz
Range from 100 to 1000 m - depending on antenna
Power supply 9V (Krona)
Output power 50 mW
Current consumption 25 mA
Microphone sensitivity 5 m
Microphone M1 type MKE-332 or any button microphone. The length of the antenna for a good range is 95 cm. The antenna should be positioned vertically and away from metal objects. Reducing the length and using a helical antenna will correspondingly reduce the range.
Coil L1 contains 6 turns of 0.4 mm wire on a mandrel with a diameter of 3 mm. We wind 2 turns, make a branch to R7 and finish the remaining 4 turns. Choke DR1 - 20 turns of 0.1 mm wire on a small ferrite ring 2x4x7. Any ready-made one with an inductance of 100 μH will do. I took it from a Chinese receiver.
The frequency of the device is adjusted by compressing and decompressing L1. Can be caught with any mobile phone with FM band.
Answer
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DIY radio microphone 150m
I present to your attention a circuit of a simple transmitter powered by a 1.5V galvanic element. The current consumption of the circuit is about 2 mA and the operating time is more than 24 hours. The range of the bug, depending on conditions, can be up to 150m.
Device diagram:
About work:
The master oscillator is assembled on a KT368 transistor, its operating mode is DC are set by resistor R1-47k. The oscillation frequency is set by a circuit in the base circuit of the transistor. This circuit includes coil L1, capacitor C3-15pf and the capacitance of the base-emitter circuit of the transistor, the collector circuit of which includes a circuit consisting of coil L2 and capacitors C6 and C7. Capacitor C5-3.3pf allows you to adjust the excitation level of the generator.
Setting:
When setting up the device, they achieve the maximum high-frequency signal by changing the inductance (compressing - stretching) of the coils L1 and L2. The finished bug circuit is placed in a small plastic case. If the dimensions are not too tight, use a mini-finger or AA battery. In this case, the scheme will work much longer, up to several months. For ease of operation, you can install a miniature power switch.
If you can’t find MKE-3, you can install any button microphone from a radiotelephone or mobile phone. It may be necessary to add a ULF cascade, but the increase in sensitivity will be significant.
I propose a circuit for a very stable radio microphone. The creation of this circuit was prompted by the need for a high-quality beetle, with a stable frequency that does not go away when a person approaches or the device moves. As a result, it was developed and assembled this scheme. Even if you turn the device in your hands, twist and untwist the antenna, the frequency does not go away at all. How to achieve stability will be discussed below.
So, distinctive qualities of this radio microphone:
- adjustable sound sensitivity
- extremely stable work
- adjustable power
Characteristics:
Power: 30-300mW
Supply voltage: 3-15V
Range: 70-140MHz
Description of the circuit operation
Through R1, power is supplied to the electret capsule, then with the help of C1, the useful signal is separated from the constant component of the power supply and goes to the base VT1. VT1 contains an ultrasonic sounder, which is necessary for pre-amplification of the signal from the microphone. An ordinary cascade with a common emitter, in which R3 sets the bias to the base, and R2 is the load. R4 limits the cascade current, which is necessary to adjust the cascade gain, and C4 shunts it with alternating current, that is, passing only the useful signal. R5 limits the current of the low-frequency part, and together with C2 acts as a G-filter that protects the circuit from self-excitation. Through C3, the signal goes to the VT2 base, on which the HHF is performed. R6 and R7 set the base bias, R8 limits the cascade current. C5 bypasses the base to general conclusion, for which such a cascade was called a cascade with common base. C7 creates feedback, and C8 bypasses R8, allowing the RF signal to pass freely. A parallel oscillatory circuit is assembled at L1 and C6, on which the generation frequency depends. Through C9, the HF signal has already been generated by VT2, and modulated by the LF signal from VT1, it reaches the VT3 base, on which the UHF is assembled. R9 and R10 set the offset based on VT3. R11 limits the cascade current and allows you to change output power devices. L2 and C10 form an oscillatory circuit similar and resonant to the HHF circuit. Capacitor C11 is a separation capacitor between the UHF and the antenna. C12 bypasses the circuit via HF, which prevents self-excitation at high frequencies.
Elements used and interchangeability
VT1-9014; VT2, VT3-9018.
L1, L2 - 6 turns of 0.5mm wire, on a frame with a diameter of 3mm.
Antenna - a piece of wire 20-60cm.
All resistors are 0.125-0.5W. Capacitors C1, C2, C3 and C4 are electrolytic, the rest are ceramic.
Power source: any voltage 3-15V, in my case 2 lithium tablets of CR2032 size.
VT1 can be replaced with a KT315, BC33740 transistor or almost any low-power NPN structure transistor with sufficient gain. VT2, VT3 can be replaced with a KT368 transistor, or any other low-power ones with a cutoff frequency of at least 200 MHz.
Settings
The setup comes down to setting the microphone sensitivity, setting the frequency and tuning the UHF circuit to resonance.
Using R4, it is necessary to adjust the sensitivity of the ULF cascade so that a close conversation does not cause overload, and the sensitivity is still sufficient to hear it within a room or apartment.
Using C6, a rough choice of frequency is made; for more precise adjustment, it is necessary to change the geometry of L1 by stretching the turns. Using C10, the UHF circuit must be adjusted to resonate with the carrier. The output power depends on the value of R11.
Assembly
In my assembly version, the device was assembled on double-sided foil fiberglass. On one side there is a direct surface-mount circuit, on the second there are pads for 2 lithium batteries tablets type CR2032. One of features - use key as a power switch. In order to activate the device, you need to insert the key into the connector; this was done for convenient and reliable activation.
The photo shows a beetle assembled and covered with a thermal tube, as well as a key. A piece of tin was soldered to the end of the antenna to make it easier to attach the end of the antenna.
You can download the printed circuit board in format below
Methods for increasing the stability of radio microphones
Many novice radio amateurs who decide to try simple and interesting bug circuits often cannot configure the circuit after assembly. And faced with a problem in best case scenario They bother you on the forums, and in the worst case, they abandon this idea. One of the most common problems in such designs is unstable operation and frequency drift.
First of all, we will consider the factors influencing the operation of the main frequency generator, on which the stability of the carrier depends. Most “bugs” are created using a three-point type HHF on a single transistor. Let's consider several factors influencing the stability of generation.
1. The case in which the antenna clings directly to the MHF and the influence of the antenna.
An antenna connected through a capacitor or inductive coupling directly to the MHF essentially becomes a receiver, and not just a transmitter, because its capacity, as well as its location in space and extraneous HF currents induced into it are transmitted to the MHF circuit and have a great effect on its operation. It's the same as connecting a source of interference to the HHF.
The solution to this problem is a simple UHF cascade, or a repeater, that is, a UHF with practically no gain, necessary only to limit the UHF from feedback with antenna. An example of the simplest low-power UHF is given below.
2. Oscillatory circuit.
The quality of the oscillating circuit coil also influences the stability of operation. A coil made of too thin wire, which does not have a housing and is not filled with anything, will change its geometry when there is a physical impact on the device, that is, during movements and other vibrations. A change in geometry will cause a change in inductance, which in turn will cause a change in frequency.
The solution to this problem is to glue the coils, wind them on a frame, and wind the coils with thicker wire.
3. Nutrition.
The operation of the device in general always depends on the power source. Over the course of their operation, batteries will change their voltage quite significantly, which will also be expressed by a gradual decrease in frequency.
The solution is to use stabilizers and circuit solutions that are not strongly dependent on the power source.
4. Shielding.
When metal or other electrically conductive objects approach, they affect the inductive and capacitive environment of the circuit. For example, metal shielding passing next to oscillatory circuit will affect its inductance, increasing it and decreasing the frequency. Permanent shielding with an unchangeable geometry that has a constant impact is not a problem; on the contrary, it protects the device from external influences. Otherwise, when the device is placed on a metal base, it may interfere with operation. The solution is to use shielding, using a thick plastic case that limits the minimum possible distance to the board.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| VT1 | Bipolar transistor | 9014 | 1 | KT315, BC33740 | To notepad | |
| VT2, VT3 | Bipolar transistor | 9018 | 2 | KT368 | To notepad | |
| C1 | 0.47 µF | 1 | To notepad | |||
| C2, C4 | Electrolytic capacitor | 10 µF | 2 | To notepad | ||
| C3 | Electrolytic capacitor | 1 µF | 1 | To notepad | ||
| C5 | Capacitor | 100 nF | 1 | To notepad | ||
| C6, C9-C11 | Trimmer capacitor | 35 pF | 4 | To notepad | ||
| C7 | Capacitor | 15 pF | 1 | To notepad | ||
| S8, S12 | Capacitor | 470 pF | 3 | To notepad | ||
| R1, R2, R5, R6, R9 | Resistor | 9.1 kOhm | 5 | To notepad | ||
| R3 | Resistor | 470 kOhm | 1 | To notepad | ||
| R4 | Trimmer resistor | 3 kOhm | 1 | To notepad | ||
| R7, R10 | Resistor | 3 kOhm | 2 | To notepad | ||
| R8 | Resistor |