DIY digital radio microphone circuit diagram. Simple stable radio microphone
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, this scheme was developed and assembled. Even if you turn the device in your hands, twist and unwind antenna - frequency doesn't go away at all. How to achieve stability will be discussed below.
So, the 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 across 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 a common output, which is why such a cascade is called a cascade with a 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 already generated by VT2, and modulated by the LF signal from VT1, goes to 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 blocks for 2 lithium tablet batteries of the CR2032 type. 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 schemes“bugs” 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 from the 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. Coil out too thin wire, which does not have a body 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 the 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 |
This is perhaps the most popular simple and widespread radio bug or radio microphone circuit. A minimum of parts and a minimum of time are required to build this little thing. Thanks to the use of a microphone from Chinese products, the sensitivity of this device is very high. This bug is not fussy to manufacture and is not demanding on the power source. Of course, along with the obvious advantages, this circuit also has disadvantages, the main one, in my opinion, is the large frequency shift when the power supply changes, but when this radio microphone is powered by batteries, this parameter is not critical.
This radio bug works according to a capacitive three-ton circuit. The oscillatory circuit is tuned to a frequency of 90 MHz. But you can easily select any frequency from 30 to 120 MHz.
Transistor KT660B. The reel is a frame with a diameter of 7mm, see the rest in the photo.
The transistor can be any, even low-frequency.
If the parts are in good working order, the bug starts working immediately. You just need to select the desired frequency.
Determining the operation of a bug without a receiver is very simple. To do this, you need to measure the current consumption, and then short-circuit the oscillating circuit; if the current consumption has changed, then the device is working.
The antenna is connected to the collector of the transistor; this will burn a piece of wire up to a meter long. It is better to connect the antenna through a 10 - 15 pF capacitor.
I forgot to draw, the power is connected to the capacitor C1, the upper terminal according to the plus circuit. Power supply 1.5 - 15 volts.
I don’t know how useful this can be and whether it is necessary household, but it can be used as a wireless “nanny” or an additional alarm system for a car. Its range is sufficient to hear a child waking up in the next room or a car roaring from the opposite side of the house. The entire modernization will consist of adding one amplification stage to increase the sensitivity of the microphone, and with a slight movement of the hand you can increase the sensitivity of the receiver and thereby increase the radio communication range.
I'll tell you how I modified the wireless karaoke microphone model ODEON SD -410. By the way, this is a very convenient model, since it is equipped with a full-fledged superheterodyne receiver, which allows you to receive an FM microphone transmitter outside the FM range, which is completely crowded with radio stations (88 -108). The CD 1191ACB (CXA 1691BM, its analogue) receiver chip itself allows direct connection of a headphone or dynamic head with a resistance of 8 Ohms, which makes it possible to mobile use the receiving box, slightly smaller than a pack of cigarettes, and not carry a music center with you.
Microphone design consists of a dynamic head with a winding resistance of 600 Ohms, an audio amplifier and a parametric (soft) high frequency (116 MHz) oscillator. The low-voltage power supply of one element equal to 1.5 V is converted into a voltage of up to 5 V thanks to a simple voltage converter made on a single transistor.
For me, the hardest part was disassembling the microphone housing. It was hard to guess that the circuit board rests on the switch, and the switch body is screwed to the cylindrical microphone body.
The whole job consists of installing an additional audio amplifier board into the gap in the wires coming from the microphone. Thus, the input and output wires, including the ground or negative wires, are known. All that remains is to connect the power, or find the output of the voltage converter from which the radio microphone itself operates, since the product itself is powered by a 1.5-volt battery. The converter voltage is 4.6 V, from this point it supplies the additional audio amplifier stage.
Transistor T1 – VS850
Resistors: R f – 1 kOhm, R k - 10 kOhm, R bq – 910 k, R e – 330 Ohm.
Capacitors: Wed -0.1 µF, St - 470 pF, Sf - 4.7 µF.
Audio amplifier has a gain of K = 20, you will even hear the baby’s breathing. The amplifier is made on one transistor. Thanks to two negative feedbacks and low current mode, it has low level noise. The cascade includes the simplest correction of the amplitude frequency response; these are separating capacitors Cp, which provide a cutoff in the low-frequency region, and a capacitor Sv, which provides a cutoff in the high frequencies, thus only the spectrum of the speech signal is isolated.
A correctly assembled amplifier, when connected, will immediately manifest itself as the howling of the speakers of a music center, thanks to the increased sensitivity of the radio microphone, which will lead to an increase in positive acoustic feedback, which can be eliminated by moving the microphone to another room or using headphones.
You can further increase the sensitivity of the microphone by reducing the resistance of the resistor R e to 50 Ohms. The microphone itself has a narrow directional pattern and this feature can also be taken into account by directing it towards a stroller or other sound source.
Receiver design. This is a superheterodyne receiver with one frequency conversion, with an intermediate frequency of 10.7 MHz, with a fixed tuning to one receiving frequency. Tuning to the microphone frequency is carried out by a variable capacitor and is held by the APCG (automatic adjustment of the local oscillator frequency). The supply voltage from one element is also converted to a voltage of 3 V.
 |
| Photo 4. Disassembled receiver. |
When I started to practically study the receiver, I was very surprised. The current consumption was 65 mA! Two empty spaces for electrolytic capacitors caught my eye. I filled the slots with 220 uF ratings and the consumption dropped to 19 mA. What a savings!
When connecting the measuring generator and oscilloscope, I was confused by the sensitivity, only 50 microvolts. I took a toothpick and, with a slight movement of my hand, tried to move the input coil, compressing and unclenching the turns at the level of a weak signal from the generator, achieving maximum sound with minimal noise at the output, and thus additional adjustment of the circuit improved the sensitivity of the receiver to 5 microvolts.
The 27th pin of the microcircuit goes through an electrolytic capacitor, then through a divider of two resistors to the plug (microphone input of the music center). Directly after the capacitor you can install a headphone socket. The best results were obtained when connecting telephone coils in series. It’s even better if the resistance of the telephone heads is not 15 -17 Ohms, but 33 Ohms. This is due to the low-current stabilizer, which does not maintain voltage as consumption increases.
Tests of the modified device showed a reliable radio communication range of up to 50 meters in line of sight. The room provides radio communication up to 15 meters, taking into account two solid reinforced concrete walls located between the receiver and the radio microphone.
You can stop there for now; it’s quite enough for a radio nanny. True, this is not the limit for this set.
To increase the communication range, the shortened antenna of the radio microphone can be replaced with a stranded copper wire 65 centimeters long (a quarter of the wavelength). It is advisable to replace the filament antenna of the receiver with a stranded copper wire of a larger cross-section and the same length as the antenna of the radio microphone.
You can install a volume control in the receiver.
It’s better to get rid of the converter completely, the whistling and beating will disappear, and power the receiver (pin 26 of the microcircuit) from two elements, thus increasing the voltage to 3 volts or from one telephone battery with a voltage of 3.7 volts. In this case, you can use a speaker with a resistance of 8 ohms, increasing the capacitance of the coupling capacitor to 470 µF.
The sensitivity of the receiver can be improved to 1 microvolt by adding a high-frequency resonant amplifier. Cm.
I have compiled instructions for you on how to assemble a bug with your own hands. This bug circuit is not difficult to assemble, consists of accessible parts and is powered by 9 volts from the crown. The range is 200 or more, it all depends on the transistor used. I found the circuit on an American website, it’s fully working and effective, verified!
Bug Scheme
Parts List:Resistors:
Capacitors:
Transistors:
Miscellaneous:
|
Location of parts on the board:
To get started, download the archive with the signet and make a beetle board. Then solder all the parts into place as shown in the photo above. The archive can be downloaded from the link at the end of the article.
Coil manufacturing:
Now you need to make a coil. To do this, take a bolt and wind 7-8 turns along the thread copper wire, with a diameter of 0.5-0.7 mm, then twist the finished coil from the bolt and solder it to the board.
The coil is in its place and our beetle is almost ready, all that remains is to figure out the power supply. For ease of use of the beetle, I suggest installing it directly on the battery (crown). To do this, we need two crowns, one can be taken as a used one, from it we will need to remove the power mark and solder the wires from the board to it. See below for how to do this. The second crown will power our circuit and serve as a stand for the beetle.
Installing the power stamp:
Well, take a glue gun or glue for it and glue the stamp to the board. Our bug is ready!
Setting up a radio bug:
To tune the beetle, take the receiver and tune it to a frequency in the range of 87-108 MHz. Place the beetle on the crown without touching the coil, use a screwdriver to slowly turn the trimmer capacitor until you hear feedback from the radio receiver in the form of a tone sound signal. By the way, the beetle can also be caught on the radio mobile phone, even my auto search finds it, so try this option first. When tuning, the beetle and the receiver should be close, and once you tune in to the sound, move them away from each other. That's it, the beetle is completely ready and set up!
Answer
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry"s standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five http://jquery2dotnet.com/ centuries , but also the leap into electronic typesetting, remaining essentially unchanged. It was popularized in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
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. Ready scheme The bug is placed in a small plastic case. If the dimensions are not too tight, use a mini-pen or AA battery to power the bug. 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.