The FM transmitter base with one transistor is an oscillator with a titrating circuit to which the microphone and antenna are directly connected. In the theoretical articles on radio transmitters and oscillators we have described in detail the principles of operation of all kinds of transmitters and oscillators. Therefore, here we will limit ourselves only to issues specific to the simplest "mini" transmitters with a single transistor, i.e. transmitters with the smallest number of elements (such as the smaller dimensions of the circuit), where we want to achieve as little electricity consumption as possible with greater stability and stability. Unfortunately, as we will see, in such transmitters, we will at one and the same time be at the expense of the other.

Choice of transistors

The frequency of the oscillator at the transmitter with one transistor depends on the inductance coils and capacitors connected in parallel coil in the oscillating circuit. In practice, the required capacity is 2-15pF for oscillators working in the FM area, however, unless the value of the capacitors in the oscillating circuit, there is also included and value of the internal (hidden) capacity used transistors, which is 2-20 pF. Accordingly, internal capacity transistors must be taken into account in circuits that operate over 100 MHz, because the size and all necessary capacitors in the oscillating circuit, while the low frequency he can not ignore. There transistors much smaller internal capacity, however, the simplest transmitter mentioned capacity is needed to achieve modulation.

This graph shows how the collector capacity changes versus the collector base voltage change. The graph is for the Philips 5 GHz transistor BFR92. It can be seen that the collector capacity has a small value 0,7 - 0,95 pF, while the emitter capacity is about 0,5 pF. For transistors of general application BC 547 these values ​​are Cc = 1,5 pF and Ce = 11 pF. From this it can be seen how the audio signal on the base affects the Cc and changes the frequency of the oscillator.

Frequency modulation of the transmitter in the simplest single transistor achieved in the following way: voltage audio signal from the microphone is supplied to the base of the transistor. This causes the transistor works at various points of their performance curves (see article Radio transmitters, Selection of working point) Produces different collector currents and voltages, and what is interesting, different internal resistances and capacities that cause change in the amplitude and frequency of oscillations themselves. There was thus obtained component FM and AM signals. Since the transistor in the oscillator works more like a switch (see article: Transistors), Means that the PM component is very small. In any case, the FM receiver does not respond to the small AM component.

This is probably the simplest transmitter that can be made. It consists of only four elements (the photo printer does not have to be fitted), it can be powered with a small lithium battery (1,5-3V) and can therefore be made of very small dimensions. The frequency can be changed by changing the capacitor value from 0,1 to 10 ¾F. Instead of the 2N 2222 transistor, some similar type may be used, but the performance of the transmitter will vary depending on the transistor used. Transmitter coil consists of 20 to 30 winding thin coiled copper wires, wound on non-magnetic body diameter 3 to 6 mm. The transistor emitter is connected to the coil output, which is made on the 1 / 3 of its total length. With such a coil the transmitter signal can be found on the VHF area (on the classic FM receiver). By increasing the number of coil curves, RF frequencies can be lowered up to SV area (classic AM radio). Antenna is an insulated wire length from 15 to 30 cm. The transmitter elements are not critical, however, several experiments with their values ​​are needed in order to achieve the optimum performance of the device.
If a thermistor, a phototransistor or some other variable resistance is connected to the 47K resistor, depending on the change in these resistances, the tone tone of the transmitter signal will also be changed.

Let's look at the elements that negatively affect the operation of the simplest transmitter with the LC titrating circuit, and then we will see in a few practical examples how their impact can be reduced.


Since the modulation depends on the internal capacity of the transistor, and the oscillator frequency of the coil and the condenser in the oscillating circuit, the very frequency stability and modulation quality will directly depend on the stability of these elements. This primarily refers to the temperature stability, ie, retention of permanent value with changes in ambient temperature or elements (eg. Heating during operation). The temperature of the transistor also changes and its internal capacity, so that Cc and will depend on the temperature. Also temperature will to some extent affect the size capacity of capacitors in oscillating circuit. Therefore, care should be taken to ensure a constant temperature environment when setting up and operation of the transmitter, and to use the quality elements with high temperature stability.


As noted above, the change of working point of transistor changes and its capacitive resistance. If it is intended for use with the rectifier transformer, its noise can directly modulate the oscillator. However, if you use battery power, the frequency of the oscillator will change how it changes (decreases) the battery voltage due to wear ( "Frequency Pushing"). What is the supply voltage of the oscillator is higher, it will flow through its elements strengthens current and they will be more heat, which directly affects the stability of the oscillator.


Antenna connects directly to the coil of the titanium circuit or through the capacitor (because of the simplicity of manufacture we do not incorporate the degree of separation or the additional output filters). Thus, the antenna actually becomes part of the titanium circuit, and any movement of the antenna or movement around it causes a slight frequency change ("Frequency Pulling"). Some electronic circuits use this effect to detect movement or metal. In the case of an oscillator with antenna crystal, it will have the slightest influence on the titanium circuit.


Sometimes the microphone does not provide enough audio input to modulate the oscillator (because of the simplicity of the process we do not incorporate the degree of the microphone preamplifier). In that case, even though we receive a strong and constant signal on the receiver, due to poor modulation, the sound will be weak and with a lot of noise. For our transmitter we will in most cases use a small electrodynamic or condenser "Electret" microphone, which can be found in numerous cheap electronic devices and toys. It should be known that they can be of different sensitivity, and for constant capacitors, it is necessary to provide constant DC power (current consumption of min. 1 mA), so the choice of microphone will have a significant effect on transmitter performance (see the articles Electroacoustic Transducers and Electret Microphones). However, in most cases, the transmitter sensitivity with only one transistor (with a directly connected microphone at the input) is not enough to be used efficiently and illegally as a mini 'Buzz', since satisfactory modulation will only be achieved if it is directly spoken to the microphone. These transmitters are best used when mounting the headphone jack on the AF (NF) input instead of the microphone, and put them all together with the battery into a suitable plastic box. Then the device can be used as a small wireless signal transmitter of a device with an NF headphone jack, where the transmitter is simply plugged in.

Output power

With a single transistor can be achieved output power 1-5 mW or stable and high-quality range 10-20 meters. This is due to the limited supply voltage, limited modulation, and huge losses at the junction with the antenna. However, by choosing different values ​​of the elements can reach over 200 meters with ordinary battery-powered (1,5 - 9 V) and the antenna wire around 60 cm, or if the field strength to be unstable, and the output frequency will "walk". There you are to some extent we can help by using the receiver with automatic frequency control (AFC) where the receiver adapt to these changes. If we have the ability to connect the transmitter to a stronger power source (batteries, rectifiers) which can provide electricity from a hundred milliamps, with a single transistor, in ideal conditions (line of sight) can reach and range of several kilometers. However, strong currents and more heating means, and thus the greater the instability.

Transmitters with LC oscillator

Transmitters whose resonant circuit consists of a coil and capacitor (LC) are the cheapest and easiest to produce. When they are extremely easy to change the output frequency in a relatively wide range (see the article Oscillators - Calculation of the Titration Circuit). The frequency change is achieved by changing the capacity of the condenser (by installing a variable capacitor) or by changing the inductance of the coil in a resonant circuit (installation of adjustable ferrite core or simple stretching or contraction bends the coil). However, we have already mentioned that there are many factors that will affect the unwanted change the value elements in the LC circuit and the LC oscillator is regarded as one of the most unstable. In systems where needed high accuracy frequency used crystal oscillators or in various ways further stabilized LC oscillator (exactly the right intensity feedback and filtered supply voltage to prevent heating of the transistor, the use of polystyrene (stirofleks) capacitors small losses or air mechanically stable variable capacitors, oscillators encapsulation for protection from external high frequency noise and by filling the coil to prevent the coil set mechanical displacements etc.).

However, since this oscillator will be the only transmitter level, its high stability is not necessary because it will not affect the performance of other stages, and due to low power supply there will be no overheating. Probably there will be a bit of "frequency freezing" in the work, but with simple operations it can be reduced to a minimum: by installing the device in an antenna housing in the form of a hard-fixed, non-flexible wire, a coil-shaped version printed on a plate instead of a stand alone ensures its mechanical stability (See article: Budget coil printed on the plate), and using the AFC function at reception, this phenomenon will be completely eliminated.

Pictured above are shown three common performance mini-transmitter with one transistor which works as a modulator and an oscillator. In the first example, the coil is made as a detached plate (see article: Designing and Marking Coil, Coil, Toroid, Gimmick Condenser) in the second example the coil is printed on the plate itself, and in the third example, the frequency setting is done with a built-in variable capacitor. The coil is made of copper insulating wire thickness 1 mm (used for transformers and electric motors), and contains 6-8 threaded motors on the 5 mm diameter body, which, after rolling and processing the ends of the solder coil (removing the protective seal from the ends ) is pulled out of the coil and remains a self-tapping coil that is directly applied to the tile. The cone can also be threaded to the bolt of a certain diameter, and then we will get a coil with equal spacing between the bend. This coil in combination with the capacity of 4-20 pF is used for the titanium circuit at all of these transmitters for the UKV radio diffusion range 88-108 MHz. By collecting or stretching the coil of the coil and monitoring the signal on the receiver, we will very roughly adjust the transmitter to the UKV range (by stretching the coil frequency the frequency decreases). Using a ferrite core or variable capacitor, we can fine-tune the frequency fre- quently (increasing frequency capacity decreases).

As an antenna for these transmitters can be used any copper wire length about 50 cm. It connects to the end of the first bend coils. If you reduce the binding site of the antenna coil, we will increase the stability of the oscillator but this will also reduce the radiated power, ie, the range of the transmitter. Because of the reasons already mentioned, it is good to use a firm (the fixed) performance antennas, because its movement or bending affects the output frequency of the transmitter. It is necessary to experiment with different lengths, until we get the maximum range. Reliable operation is achieved by the length of the antenna of 50 cm or less. Of course that longer antenna we can achieve higher range, but if you overdo it, the circle can begin to oscillate and performance will plummet. It is best to take a standard telescopic antenicu, what is the most radio-receiver (not flexible and easy it is changing length). The range of the transmitter is at least 100 meters, but it can be reduced or increased by changing the resistor value on the emitter transistors Re. The table shows how to change the range of the transmitter according to the emitter resistance:

Emitter Resistor
Electricity consumption
Approximate range
100 E
30 mA
500 meters
220 E
11 mA
120 meters
330 E
8 mA
100 meters
1 K
5 mA
50 meters

The microphone used is a condenser (Electret) that is powered by a Rmic resistor using DC. In our case this resistor has the value 4K7. However, for a good performance of most electret microphones, the current required is greater than 1mA, so reducing the value of this resistor can significantly increase the microphone sensitivity. If you want to use a dynamic microphone or if you want to connect the output of an audio device to the FM transmitter input, remove the Rmic resistor and replace the electrolytic capacitor polarity of 1 uF. Of course, as already explained, the sensitivity of the microphone without amplifiers is not great, so you can not expect to hear a whisper on the 5 meters of the microphone. It is necessary to speak directly to a microphone, as with any other normal microphone.

The values ​​of the other components in these transmitters are not critical. The resistor tolerance tolerance can be from -50% to + 200%. Capacitors used for signal switching and separation can be any between 470pF and 100uF and instead of the 1nF capacitor to which the microphone is connected can be placed any one between 270pF to 2n2 (how much of the capacitor value is too high then the high frequencies are missing in modulation ). The transistor is also not critical and can be installed by most NPN general purpose transistors (BC 107, BC108, BC109, 2N3564, 2N5225, 2N2222 ...)

For these three examples scheme transmitters worth the same as in the previous scheme. The first diagram shows a transmitter with a minimum number of elements that can be directly linked condenser microphone, and if the terminals "Signal IN" We want to connect another source NF audio signal, it is necessary to remove the resistor rmica. Then the input signal to the base of transistors may lead through a capacitor of about 0,1 uF.

The transmitter displayed in the middle is fed from the 3V battery voltage (one small lithium battery), which is achieved by reducing the value of the Rmic and Re resistor. Also, to improve performance and increase the transmitter range, a bypass capacitor of 1n can be installed (shown in the diagram shown).

In the third example we have a scheme that is stable transmitter operating in the 60-110 MHz. Voltage connected from 12 V while consuming 2 mA. Thanks coil L2 sufficient antenna length 15 cm. transmitter output power is 5 mW. Here is information about making coils for the transmitter:

L1 - 6 windings of 0,8 to 1 mm wavelength, widened on wire thickness, on body 4 x 0,75 x 10 mm with ferrite core U-31; the output on the second thread from the top (from the point of supply + 12V)
L2 - the plastic body without a core diameter 4 mm, winding bend to bend the wire diameter to 0,4 0,6 mm with a length of up to 8 10 mm
L3 - damper 1 mH

We see that the biggest disadvantage of the transducers described so far is the low level of the input signal from the microphone causing poor signal modulation. Modulation can be significantly improved with the help of capacitive (varikap) diodes. The capacity of these diodes depends on the connected inverse voltage, so the modulation is achieved by superimposing the alternating component of the input audio signal to the inverse voltage varicap diode (See article: varicap in skilled receivers). Below we look at several transmitter schemes with varikap diodes.

In the first example, the transmitter scheme reaches up to a maximum of 200 m. The BA 121 varicose diode changes its capacity to the NF rhythm of the microphone. The capacitor serially connected to the diode serves to disconnect DC voltage DC. The coil with a parallel condenser forms an oscillating circuit that determines at which frequency the transmitter - oscillator will work. The emitter resistor determines the working point of the oscillator and its value depends on the transistor used. The coil is made of a 6 winding Cu-wire wire of 0,8 mm thickness on the body of 5 mm, power supply - 2.5 winding from the cold end, antenna - 3.5 winding winding from the cold end. Right now we see another transmitter scheme, also powered by 1,5 V and with a range of up to 200 m DC preamps via a resistor of 12K. And here, modulation is performed over a varikap diode, which also determines the frequency of the transmitter with the serial-connected capacitor. Through a transistor-connected resistor, a DC bias is obtained, and it determines the working point of the transistor. Resulting coupling, as with most other examples, was achieved through a capacitor connected between the emitter and the transistor collector. The throttle prevents the transition of the HF signal to the ground and is made to cut a diameter of about 3 mm winding about 70 cm of wire mesh thickness about 0,2 mm. Instead of a varistor of the BA121 diode, a replacement circuit can be used with the transistor BC 107 (or similar) scheme. The titanium circle coil makes 7 windings of silver-plated 0,8 mm wires, wound to the 5 mm diameter body with an adjustable ferrite core in the center. The output of the antenna is on the 0,5 - 1 winding from the cold end.

In the middle we have a diagram of the transmitter power 10 - 20 mW and consumption around 10 mA. For it is necessary to provide a power source of 9-15 V, and then the range of the transmitter can be up to 500 m, depending on the quality of the receiver which is listening and from the antenna to the transmitter. It is best that the antenna is vertical and in most cases this is a piece of wire thickness 2-3 mm. The working point of the oscillator and here determines emitter resistor. Muffler and the coil is completely the same as in the previous example. The last scheme transmitters with low frequency transistor BC548. Transistors from the BC series are typically used in NF audio devices, or the data obtained for this transistor (and most other BC transistors) he can operate in the frequency range of 200-250 MHz, and that is enough to operate in the VHF spectrum. Capacitive diode is used here in order to better FM modulation. The coil L1 consists of 4 bend Cu insulated wire diameter 0,3 mm, wound on a non-ferromagnetic body with a copy to the end of the first turns of the coil. Capacitors connected to the power supply should be mounted as close to the top of the coil L1. The range of the transmitter is about 200 m, however, and here the output power can be increased by reducing the value of the resistor broadcasting organization. But one should not exaggerate, because if the value falls below the 22 E oscillator may stop working. The power source is an ordinary battery of 9V, which together with the transmitter can fit in a pack of cigarettes. In order to ensure better stability, it is recommended that the entire assembly placed in appropriate metal housing.

It's time to name another, bigger problem that occurs in the transmitter with one transistor. In fact, with the primary frequency transmitter will radiate and unwanted harmonic frequencies that will "contaminate" a part of the frequency spectrum (could interfere with radio and TV receivers), and with it will be on them to spend part of the total output power. Radiation of unwanted harmonics can be blocked by installing a filter (see Articles: Filters), But we'll deal with more complex transmitter. You can use FET instead of a transistor can reduce impurities harmonics thanks to the linear transmission characteristic of the FET at the output (source), and he also its inherently high input impedance is very little affected by the resonant circuit of the oscillator and thus ensure better the stability of the operating frequency. Therefore, we will give a few practical scheme of the transmitter with a FET transistor and diode varicap for modulation, as well as the highest quality transmitters that can be compiled with a single transistor and a few passive components.

In the first example on the transmitter scheme with the Hartley oscillator, in which the feedback is achieved through the lower coils. The transmitter has a low power consumption and can be powered by a small battery type 12V VR22: EL 12 23 (the length of the battery is 30 mm and diameter 10 mm). With such a battery and appropriate derived plate with printed coil transmitter can easily "squeeze" in felt pen, a fountain pen or the like. But that at the same time keep their features in writing. Printed Circuit Board dimension 55x10 mm. Below is the transmitter range 300 m with cheap and easy BF245. The coil transmitter contains 6 winding Cu-nail wire thickness 0,8 mm body diameter 5 mm x - 0,5 coil of cold end, y - 1,5 coil of cold end. At the end of the scheme is somewhat more powerful transmitter with a FET transistor, power around 300 mW, however, must provide that the power supply can provide the current required to power the same. Consumption is about 70 mA. Oscillations are excited over circuit sours FET with a copy of the coil L. Control of the transmitter can be carried out as an additional image. If everything is correct, the bulb should light up. The range of the transmitter can be very big - to open up to several kilometers. Data for coils: Muffler hanging on a piece of the outer casing diameter of about 3 mm on which winding around 70 cm Cu-nail wire thickness of about 0,2 mm; Inductor L has 6 coils Cu-nail wire thickness 0,8 mm body diameter 5 mm x - 0,5 coil of cold end, y - 1,5 coil of cold end.

Gore is a diagram of the transmitter with a crystal oscillator. In electrical terms tile crystals represents a high quality oscillator circuit containing inductance L, capacitance C and resistance R, and capacitance C (capacitance between the metal coating). Such crystalline properties have been used to construct crystal oscillators that are characterized by very high frequency stability. Changes in temperature and little mechanical earthquakes affecting the operation of the oscillator (changes are few parts per million), so that crystals are used where necessary to achieve a large frequency accuracy. In order to achieve even greater accuracy (most often required in measuring instruments), the crystals are embedded in the casings heated by the heaters to a precisely determined and precisely maintained temperature (e.g. the 70 ° C). With this quartz crystalline thermostat, it is possible to achieve (and maintain) frequency accuracy in a wide temperature and time interval (the accuracy of 10 to 100 is higher than that of quartz without thermostats). Since crystalline oscillators have a high degree of stability, it is often not necessary to use the degree of separation for their realization. Also, in the collector circuit we can set an oscillator circuit set to one of the harmonics and get the required frequency right away (see diagram). When making crystals, a special cutting technology can be applied so that the crystal oscillates on the third, fifth, seventh ... harmonica. Such crystals are called overtones, and if, for example, the overtones crystal oscillates to 96 MHz its base frequency is 19,2 MHz. In this scheme is just such a transmitter with overtone (5. overton) crystal oscillator (from the basic crystal frequency, which is 19,2 MHz, we get 5. Overton, 19,2 x = 5 96 MHz sufficient level and power). When the transmitter is not used varicap but the frequency modulated audio signal that is coming to the base of transistor. It has already been mentioned that the transmitter frequency will be stable as the oscillator itself, but connecting the antenna directly to the coil will cause frequency changes. But without the antenna this transmitter will have a range of meters. The coil has a 4 winding Cu-Lak wire diameter 1 mm, hull on body 10 mm, without core.

Construction of mini-transmitters

Given that this is a very simple schemes, it is easy to draw a circuit board, which will be of such a form that can be easily installed in a particular case. Considering that for every builder mini transmitter to have its own specific purpose, it is on a piece of universal circuit board first experiment with components and antennas, and when he found satisfactory results (range - consumption - stability - dimensions) make such a transmitter on board that will be adapted to the specific installation site, with the possible use of SMD-shelf components. Although the small number of components, which in addition can have great tolerance, there should be no problems with the operation of the transmitter, it will eventually give some tips in case we can not find the receiver signal our transmitter.

For starters, of course, it is necessary to check the power of the power supply itself and the power of the transmitter. If it is significantly different from the values ​​mentioned in the text, it is necessary to check wires and printed lines for short circuits or breaks (cold connection, balloons ...). If everything is okay, we'll still check to see if the oscillator actually works. It is easiest to determine this with these transmitters by tracking the current of the current through the circuit while the finger is touching the oscillator coil. If it changes, it means the oscillator works. Now you need to check the microphone. It is best for microphones, while we are still in the experiment phase, to connect an NF audio source (tone generator) to the input, the signal of which will be easier to recognize on the receiver. Now it is still easy to stretch or collect the coil, while the transmitter is not brought to the desired UKV area. In the end, it may happen that the transmitter works just at the frequency where a strong station is emitted, which then completely silences our signal. In this case, the receiver and the transmitter should be placed in a metal armor (farade's grille), but it is much easier to first set the receiver to a frequency where there is no signal, then switch on the transmitter, and by lightly stretching / collecting the coil (by adjusting the ferrite core inside the coil or by rotating the variable capacitor), then adjust the transmitter frequency. At the end of the setup, if there is such a possibility, it is good to turn on the AFC on the receiver.

NOTE: On DVD-1 The Electronic Encyclopaedia can find the calculated values ​​of the elements for all the transmitters described here, as well as the theoretical articles mentioned in the text.