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You are here: Home / Transmitter Circuits / 10 Simple FM Transmitter Circuits Explained

10 Simple FM Transmitter Circuits Explained

Last Updated on January 14, 2021 by Swagatam 67 Comments

An FM transmitter circuit is a high frequency wireless device which is able to transmit voice signals into atmosphere so that it can be received by a corresponding FM receiver circuit for reproducing the voice signals in a loudspeaker.

Here we’ll discuss how to build small FM transmitter circuits using 10 different methods, one that consists of wire link from the transmitter to the receiver, and the other which is completely wireless and can be used to eavesdrop a particular conversation over a range of about 30 meters, over an ordinary FM radio.

All the FM transmitter circuits presented below are significantly powerful, hard to trace in their hidden positions, and equipped for grasping even the weakest of whispers in the vicinity. Moreover the designs are capable of transmitting the picked information upto radial distances exceeding 2 kms.

The above extraordinary capabilities have forced the legal authorities to enforce stringent laws against the use of these transmitters without permission, so before you make and use one of these make sure you have all the legal formalities completed.


Interested to learn how to detect these hidden Spy transmitters? The details can be found in this bug detector article.


Wireless Design:

I will begin with a transmitter which I have actually built numerous number of times and tested it thoroughly. Subsequently I am going to discuss more such designs which were selected from other websites online.

The sent signals can be received over any standard FM radio, tuned accurately to the respective frequency.

The above shown wireless FM transmitter circuit is basically a small RF transmitter built around a single transistor.

The circuit functions quite like a Colpitts oscillator incorporating a tank circuit for the generation of the required oscillations.

The frequency mainly depends on the positioning and the values of the inductor, C1, C2 and C3. The coil turn distance and diameter may be manipulated a little for optimizing best response over the FM receiver.

A small antenna in the form of a 3 inches wire may be attached at the shown point for making the “bug” highly responsive and generate distortion free signals.

Circuit Diagram

Parts List

  • R1 = 3k3,
  • R2 = 100K,
  • R3 = 470 Ohms
  • C1 = 10 pF,C2 = 27 pF
  • C3 = 27pF,
  • C4 = 102 disc
  • C5 = 10uF/10V,
  • Mic = condenser mic
  • T1 = BC547
  • L1 = 3 to 4 turns of 22SWG super enamel copper wire, 5 to 7 mm diameter, air corePlease refer the scanned image of the prototype for getting an idea regarding the coil dimensions.

Now let's discuss a few FM transmitter circuits which can be built using different configurations and features.

One Transistor Design

You might have already come across a host of these extremely basic one transistor FM transmitter circuits, however these may incorporate certain drawbacks as mentioned below:

  • No substantial transmitting range.
  • No enhanced sensitivity range
  • Use 1.5V for operating which render limited capabilities.

Among the first in the line, which is probably the simplest is shown in the following circuit diagram.

Surprisingly it does not employ a MIC, rather the antenna coil itself performs a dual function of detecting sound vibrations and also transmitting it into the atmosphere.

The design is void of a frequency determining stage and thus does not come under tuned transmitter circuits (we’ll discuss about these later on in the article).

Circuit Operation

The following single transistor FM spy circuit may be understood as follows:

When switched ON, the capacitor 22n inhibits the transistor from switching until it gets charged. A soon as this happens the transistor switches ON via the 47k resistor forcing the pulse through the inductor which feeds back a negative pulse to the base of the transistor discharging the 22n capacitor.

This switches OFF the transistor until 22n yet again charges fully. The procedures take place rapidly generating a frequency across the coil which is transmitted as carrier waves through the connected antenna.

In the course if the coil is subjected to an external vibrational pulse, it’s forced to mount the above explained carrier waves in the air and could be received and retrieved over a standard FM radio positioned and tuned at the same frequency nearby.

The circuit may be expected to work at around 90MHz frequency band.

Using Tuned Circuit

The second example below shows another single transistor FM spy circuit that incorporates a tuned circuit or a frequency determining stage in it.

In the original prototype the coil was created by etching a spiral track layout on the PCB itself, however for optimal gain and performance such etched antenna coil must be avoided and the traditional wire wound type of coil must be employed.

Incorporating Q Factor

Below's another circuit you would like to know about. The circuit basically makes use of the “Q factor” of the tank network achieved from the coil and the capacitor for generating a relatively high voltage. This stepped up potential attributes the circuit with a rather longer range of transmission.

For an improved performance make sure the coil and the capacitor are positioned as close as possible. Insert the coil leads as deep down the PCB as possible in order to make it tightly hugging the PCB. C2 value could be tweaked for achieving even better response from the circuit.

Preferably a 10pF could be tried. The coil is made of 5 turns of 1mm thick super enamelled copper wire, with 7mm diameter.

Better Saturation Capability

The next FM transmitter design is a bit different than the above types. Fundamentally the design could be classified as a common emitter type, unlike the others which are rather common base types with their design.

The circuit employs an inductor at its base which adds a better saturation capability to the device which in turn allows the transistor to respond in a much healthier way.

Adjustable Coil Slug

The next design in the list is much superior to its previous counterparts since it uses a slug based variable inductor.

This enables the transmitter to be tuned by adjusting the slug core using a screwdriver. In this configuration we can see the coil being attached to the collector of the transistor which allows a massive 200 meters range to the design, with a current that may be not more than 5mA.

The MIC stage is isolated from the base with the help of a 1u capacitor and the gain of the mic could be well tweaked by adjusting the series 22k resistor.

This circuit could be rated as the best as far range is concerned however it may lack stability which could be improved, we’ll learn how in the following explanation.

Improved Stability

The stability of the above circuit could be improved by tapping the antenna from one top turn of the coil as shown in the following figure.

This actually enhances the response of the circuits due to a couple of reasons. The antenna gets aloof from the collector of the transistor allowing it to function freely without unnecessary loading, and the slipping of the antenna to the top further allows the relevant side of the coil to get a higher stepped up voltage induced across itself and also the coil generating a higher concentration of transmission power on the antenna.

Although this enhancement may not actual increase the range of the device, it makes sure that the circuit does not get rattled when hand held, or when the grip is encircled close over the circuit inside its enclosure.

Transmitting Music

If you want your tiny FM transmitter circuit to transmit music instead of spying or eavesdropping, you would probably find the following design interesting.

The proposed FM transmitter will allow combining a stereo input simultaneously from the source so that the info contained inside both the channels get into the air for an optimal reception.

The design configuration is quite identical to the one that’s discussed above so does not need much of an explanation.

Analyzing a Two Transistor Spy Circuit

Adding a transistor stage to the above discussed single transistor FM transmitters could enable the designs with extreme sensitivity.

An electret MIC itself has a built in FET which makes it very efficient and makes it a stand alone vibration amplifier device. Adding another transistor stage with it enhances the sensitivity of the device to overwhelming limits.

As may be witnessed in the following diagram, the involvement of an extra transistor stage adds up to the gain of the MIC making the entire unit highly sensitive such that it now picks even the sound as low as a pin dropping on the floor .

The extra transistor prevents excessive loading of the MIC thereby ensuring better efficiency to the sensitivity.

Five things that that make the circuit extremely good with it reception are:

  1. The use of a fix capacitor in the tank circuit along with a adjustable trimmer.
  2. A low value coupling capacitor with the MIC sufficient to handle the capacitive reactance of the MIC which may be around 4k at 3kHz.
  3. A 1u coupler is included between the oscillator and the audio amplifier in order to make up for the low impedance rendered by the 47k base resistor.
  4. The coil used is wound practically using super enameled copper wire which ensures higher efficiency than PCB etched type of coil.
  5. The entire circuit could be compactly constructed over a small sized PCB for acquiring better stability and a drift free frequency response.

IC 741 Transmitter Using Wire Connection

In the above section we earned about wireless FM transmitter, if you are also interested to know how to make wired transmitter, in which voice could be transmitted through wires into a loudspeaker, then the following design may help

The IC 741 if configured as a non inverting amplifier which performs the function of a pre-amplifier stage.

The gain of this IC 741 preamplifier stage may be varied as desired, using the pot across its input and output pin outs.

The gain setting is used to set the sensitivity of the amplifier and is set to maximum so that even low volume speech conversation may be picked through it.

The mic at the input transforms sound vibrations into minute electrical pulses, which is further amplified by the IC 741 to suitable levels before applying it to the output amplifier stage consisting of a standard push-pull stage. This push pull stage is made using a couple of high gain transistors 187/ 188.

Here, the signal received from the 741 output is suitably amplified so that it finally becomes audible over the speaker.

For the 741 circuit, the speaker is only positioned and used as the receiver and may be placed in some other room, where the eavesdropping may be intended to be carried out.

The linking of the speaker from the amplifier circuit may be done through wire connections, preferably by using thin wires and escorting the entire length up to the speaker in some hidden way, probably by laying it under the carpet or across the corners of the room.

For the wireless spy transmitter circuit everything becomes pretty simple and you just have to hide the transmitter circuit in some suitable place, like under the table, couch, sofa etc.

Parts List

  • R1 = 10K,
  • R2 = 10k,
  • R3, R4 = 27K,
  • R5 = 1.5 M,
  • C1 = 104,
  • C2 = 220uF/25V,
  • T1 = 188,
  • T2 = 187,
  • MIC = electret mic,
  • IC1 = 741,Power = 9 volt battery
  • Headphone = 64 Ohms, or a small speaker of 8 Ohms, 2 inches

Morse Code Transmitter

Morse code transmitter

This morse transmitter circuit can be used for transmitting morse codes by tapping the switch associated with R3.

The transmitter will be able to send the signal thousands of miles away which could be received by all VHF, UHF band receivers over a suitable station.




Previous: Making a Regulated 9V Battery Eliminator Circuit
Next: Switching OFF Lights in a Periodic Sequence

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

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Comments

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  1. Search Related Posts for Commenting

  2. Jignesh says

    I don’t have questions as of now but I would like to say a big thank you for answering all the questions. I frequently surf the web for high power FM transmitter circuit ideas. There are too many but no one answers all the questions. Thanks again.
    Regards,
    Jignesh.

    Reply
    • Swagatam says

      You are most welcome, and I appreciate your thoughts!

      Reply
  3. Bytepirate says

    Hello all

    i plan on making a 3v tuned circuit transmitter like above. only i need it to beep instead of actual audio.
    if i was to link out the mic then apply power on and off would that give me beeps on the reciever device.
    i cant actually find the info online so any advice would be of great help.

    Reply
    • Swagatam says

      Hello, you can replace the MIC with a beeping 3V oscillator circuit.

      Reply
  4. xhiwah says

    THANKS
    What do you mean by
    R1=3K3 On the first transimiter circuit
    And what is a condesor mic

    Reply
    • Swagatam says

      R1 = 3.3K

      condenser MIC = electret mic

      Reply
  5. Davis says

    The fourth circuit i made but when I touch the antenna then it gives clear sound when I talk, but when I remove the hand from antenna it stops to work and can’t hear any talk. My master SWAGATAM how can I eliminate that and it works perfect without first touching antenna. Thanks

    Reply
    • Swagatam says

      make sure you have connected a 6 inches long wire as the antenna, an also try slightly stretching the coil, so it expands by 1mm or 2 mm.

      Reply
    • Carlos says

      Hello good evening, I am a faithful follower of the website. I want to know if you have a transmitter and receiver RF circuit. I need to control a relay … when the transmitter is close the relay is energized, but when it goes away more or less 5 to 10 meters the relay of the receiver is disconnected. Thanks a lot.

      Reply
      • Swagatam says

        Hi, thanks, you can probably try implementing this design:

        https://www.homemade-circuits.com/how-to-make-fm-remote-control-using-fm/

        or may be this:

        https://www.homemade-circuits.com/make-this-simple-fm-radio-circuit-using/

        Reply
  6. Lawal Ayobami says

    Hello Mr Swag, I bought a BC547 transistor but C547B W73 was written on it.. Is is correct sir or the components seller just sold to me the wrong transistor,, thanks.

    Reply
    • Swagatam says

      Hi Lawal, It looks OK to me, you can use it!

      Reply
      • Lawal Ayobami says

        Thanks for the swift reply sir,
        Now I’m new in transistor ebc identifying, is the C547B side the “emitter, and the middle pin the ” base” ? Thanks.

        Reply
        • Swagatam says

          yes with the printed side facing you, the right side pin is the emitter, middle is the base

          Reply
  7. Eshkariel says

    Regarding the ckt for transmitting music:
    Can I use it as a mono input by removing the capacitor/resistor from one of the input side?
    Does it need to change values for other components? Thanks. 🙂

    Reply
    • Swag says

      yes that’s possible, just remove one of the RC pairs from the input, no other changes would be required 🙂

      Reply
      • Eshkariel says

        Referring to Ckt. 3.3 Incorporating Q Factor:
        Can I use it to transmit music instead of mic? and if possible what changes should be made? Thanks

        Reply
        • Swag says

          yes definitely, all the shown designs can be used for transmitting music or a high frequency content simply by removing the MIC and connecting the relevant points with a music source…

          Reply
          • Eshkariel says

            Referring to the ckt with the fewest component..what AWG is the coil? Can I use 23 AWG? cause I got 1 here haha..Thanks a lot

            Reply
            • Swag says

              AWG is the US version for SWG which is the universal version for presenting wire thickness. you can 23 AWG, instead it may be better to check the thickness of the wire…ideally a 0.6 to 0.8 mm thickness works best as per my experience 🙂

  8. praveen kumar kulhari says

    what is trimmer
    can we use five pin air trimmer that we use in fm radio
    and sir you also tell me how to use five pin trimmer please

    Reply
  9. praveen kumar kulhari says

    hi sir please tell me every think about 3v transsmiter
    please send me evry detail in my email

    pkkulhari790@gmail.com

    Reply
    • Swagatam says

      praveen, you can try the following circuit, it has a PCB design and all the details regarding the connections.

      /blog/transmitter-circuit-etched-coil-pcb/

      Reply
  10. praveen kumar kulhari says

    sir please tell me full name of every component
    and its values

    Reply
  11. Tun says

    I am so sorry sir, thank you

    Reply
  12. Tun says

    Sir, i want to ask if i can make changes to the electrolitic capacitors in the last circuit, …by the increasing(not reducing) range(nf) of capacitors

    Reply
    • Swagatam says

      Tun, Please do not repeat your question more than once…your questions appears at least 25 to 30 times which becomes very disturbing while moderating.

      there's only one electrolytic capacitor in the last diagram and its value is 1uF…which can be increased

      Reply
  13. Davis Kakumba says

    thanks so much

    Reply
  14. Davis Kakumba says

    hi SWAGATAM , CONSTRUCT FOR ME A CIRCUIT OF SIMPLE FIELD STRENGTH METER which can measure the freguency radiated by transmit antenna. thanks

    Reply
    • Swagatam says

      Hi Davis, you can try the following circuit

      https://homemade-circuits.com/2011/12/how-to-make-non-contact-ac-mains-phase.html

      connect a frequency meter at the output for reading the frequency

      Reply
  15. Unknown says

    Hi,
    I want to design a simple tranmitter and receiver but for a very limited range(around 2-3 feets). And at the same time their operating frequencies should be configurable. Can I do that with the one transistor transmitter and receiver circuits that are shown. How can I configure the operating frequencies.

    Reply
  16. Suhail Halankar says

    At the receiver end buzz sound comes means sound is not clear when I use it with 9v adapter. I want to use this circuit with adapter do I have to do change in adapter circuit or in Tx circuit.

    Reply
    • Swagatam says

      try a 1000uF filter capacitor across the 9V input

      Reply
  17. Suhail Halankar says

    Sir I want to use 9v adapter for the 9v fm transmitter circuit. but when I connect it it makes buzzz sound how to clear the noise.

    Reply
    • Swagatam says

      what makes buzz sound?

      try 3V first and confirm the working and after that try 9V….or increase the 330 to 1K and test again

      Reply
  18. Davis Kakumba says

    sir for the first bug i used a wire 3cm for antenna but does not come out

    Reply
  19. Clockwerk Clockwerk says

    Sir you're second bug has an ot of 87.62mhz , i use 50p trimmer , so what coil should i use? What is the dimeter and how many turns ?

    Reply
    • Swagatam says

      you can use 8 turns, with 6mm diameter, using 0.5mm magnet wire

      Reply
  20. Clockwerk Clockwerk says

    Sir can I replace the trimmer cap to 50p and the 4.7 cap with 5p in the second bug?

    Reply
    • Swagatam says

      yes that would be fine…

      Reply
  21. Clockwerk Clockwerk says

    Very last question sir , how you do the coil in the third bug?

    Reply
    • Swagatam says

      it's the same as in the second design

      Reply
    • Clockwerk Clockwerk says

      Can i use 3v lithium cell in second bug instead of two double A ??

      Reply
    • Swagatam says

      yes you can…..

      Reply
    • Tovarisch Man says

      How about the first bug then?

      Reply
  22. Clockwerk Clockwerk says

    Hey can i just put the second bug together without ground,pcb,or breadboard. Just solder their pins together??

    Reply
    • Swagatam says

      you can try that, it should work

      Reply
    • Clockwerk Clockwerk says

      Last question sir , can i use 9v or 6v or 5v supply in the second bug instead of 3v ?

      Reply
    • Swagatam says

      yes you can, but only after replacing the emitter resistor with a relatively higher one, for example a 470 ohm or a 1K and so on.

      Reply
  23. Kuldeep Tripathi says

    Hello sir good evening, sir is the sound or voice transmitted by this circuit is catch by a mobile FM radio.

    Reply
    • Swagatam says

      yes it will catch but from a closer proximity not from greater distances….a normal radio will catch the signals from greater distances also.

      Reply
  24. Buddhi Saini says

    kay mai Transistor BC 547 ki jegh Bc548 c use le sakta hu kay

    Reply
    • Swagatam says

      yes it will do.

      Reply
  25. orrie says

    sir,
    last circuit me coil ke sath 39p cap fixed.var cap ke jaga trimer use kar sakte hey. trimer ka 3 pin hey.mene trimer kavi use nehi kiye.pin con.kesa hoga plz thora batanese acha hota. mujhe green trimer ka valu nehi pata.apki suggsation ke liye wait karege.

    Reply
    • Swagatam says

      yes you can try a 39pf trimmer in place of the 39pf cap in the last circuit.

      use the center and any one of the outer pins for the trimmer.

      you will have to confirm the value from the retailer, he will know better about it.

      Reply
  26. Mohamed Tarek Keshk says

    Hello sir.
    As iam a verry begginer I wanted to ask you couple questions
    In the verry first diagram what are 6t and 15t, also what is the thikness of the antenna and thanks for your great help

    Reply
    • Swagatam says

      hello Mohamed, those indicate the number of turns in the coil.

      antenna thickness is not critical, any ordinary flexible wire can be used as the antenna.

      Reply
  27. Tovarisch says

    How could we change the emitting frequency of the very first bug?

    Reply
    • Swagatam says

      by tweaking the 22nF value

      Reply
    • Tovarisch says

      I might sound a bit stupid here, but would it work the same if I used a variable capacitor?

      Reply
    • Swagatam says

      yes it will work.

      Reply
    • Tovarisch says

      Two last things, both regarding the coil:
      1) Are the dimensions of the coil (wire format, diameter and length) critical? Which dimensions would be optimal?
      2) I've thought of making a 21-turn coil, and solder an extra lead on the 6th turn. Would it be wise?

      Reply
    • Swagatam says

      The thickness is not critical, but the no. of turns and diameter is critical.

      the turn number must not be increased above 4 or 5

      Reply
    • james says

      What is the correct diameter to form the coil around? Pls and TY

      Reply
    • Swagatam says

      can be around 5mm in diameter

      Reply
  28. Saurabh Sinha says

    Hello sir…..
    Again i have question.
    {{for upper most curcuit}}

    Can i use old mobile's small ANTINA and "copper 22 Swg "wire to coil ?
    IS IT WORK?????
    AND can i use my NOKIA 310 for tunning. On which frequency ??

    Reply
    • Swagatam says

      Hello Saurabh,

      for antenna you can simply use a 6 inch long flexible wire, a mobile antenna is not required.

      22SWG will do for the coil.

      for tuning use an FM radio as it would have a better receiving power than a mobile phone.

      Reply


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