Make this Simple FM Radio Circuit Using a Single Transistor

When it comes to making an FM receiver it's always thought to be a complex design, however the one transistor simple FM receiver circuit explained here simply shows that it isn't the case after all. Here a single transistor acts as a receiver, demodulator, amplifier to constitute a wonderful tiny FM radio.

Simple FM Radio Prototype using a single transistor

 

Image Courtesy: Elektor Electronics

It's basically based on a superregenerative audion receiver circuit where the use of minimal components becomes the main feature of the unit.

However fewer components also means a few compromises involved, here the receiver requires a large metal base for grounding the unwanted signals, and for keeping the noise factor to the lowest, and also this system would work only in places where the reception is rather strong and thus may not be suitable in areas where the signal strength is thinner.

How the One Transistor FM Radio Receiver Works

As mentioned above, the circuit is basically a single transistor superregenerative RF oscillator with a constant amplitude.
Here we have tried to enhance the design such that the amplitude becomes considerably magnified in order to turn OFF the transistor completely during the oscillations.

This called for an increase in the feedback capacitor and also to use a transistor specifically designed for handling extreme high frequency ranges such as a BF494.

Further modifications include an inductor with the emitter of the transistor, and a capacitor across the emitter resistor of the transistor.

Due to this the transistor is switched ON as soon as the base emitter voltage of the transistor falls significantly, resulting in an abrupt cut off in the oscillations.

However this prompts the emitter capacitor to discharge, allowing the collector current to yet again resume its flow, initiating a fresh cycle of oscillation.

The above happening forces the circuit to flip flop between two situations, oscillator OFF and oscillator ON, resulting a sawtooth frequency of about 50kHz at the output.

Each time the circuit flips across the above ON/OFF states, results in a significant stepping up of the amplitude which in turn constitutes greater amplification of the received signals. The procedure also gives rise to noise but only as long as a station is not being detected.

 

Simple FM Radio circuit using a single transistor

The above design has one drawback, though. The output received from the above circuit would have greater content of sawtooth noise compared to the actual FM reception.

A smart technique can be seen employed in the following single transistor FM radio circuit to attribute better efficiency to this simple design.

Here we pull out the emitter capacitor C5 ground link and connect it with the output.

This results in a fall in the collector voltage as the collector current rises, which in turn forces the emitter voltage to rise, prompting the emitter capacitor to negate the situation at the output.

This enforcement results in making the sawtooth effect on the received signal practically to zero, thus presenting an FM audio with much reduced background noise.

 

one transistor FM Radio circuit using noise reduction

Single Transistor Radio with Audio Amplifier

To make the above circuit self-contained, an additional transistor stage may be introduced for enabling the radio to play the music loudly over a small loudspeaker.

The circuit is self explanatory, just the inclusion of a general purpose BC559 transistor along with a few inexpensive passive components can be witnessed in the design.

 

Simple FM Radio circuit using a single transistor and amplifier with loudspeaker

How to Make the Inductors

The involved coils or the inductors are very simply to wind.

L1 which is the oscillator coil is an air cored inductor, meaning no core is required, wire is super enamelled type, 0.8mm in thickness, diameter of 8mm, with five turns.

L2 is wound over R6 itself using 0.2mm super enameled copper wire with 20 turns.

How to Set Up the Circuit

  1. Initially when the circuit is switched ON, the output will be accompanied with substantial  background noise which will gradually tend to disappear on detection of am FM station.
  2. This may be done by carefully tuning C2 with the help of an insulated screwdriver.
  3. Try to keep the tuning at the edge of the band of the particular FM station, with some practice and patience this would get easier with time.
  4. Once tuned, the circuit would respond to that reception every time its switched without the need for further alignment.
  5. As indicated at the beginning of the article, the circuit should be installed over a wide circular meta plate, preferably a solderable material, and all the ground of the circuit soldered on this plate.
  6. This is important to keep the circuit stable and avoid drifting away of the received stations and also for cancelling unwanted noise.
  7. The antenna in the proposed single transistor FM radio receiver circuit is not crucial and in fact should be kept as small as possible, a 10cm wire would be just enough.

Remember, the circuit also acts a like an effective transmitter circuit, therefore keeping the antenna size bigger would mean transmitting noise across the ether and disturbing your neighbors radio reception.

The upside being that the design can also used as a walkie talkie within a small radial distance....more on this next time.

 



151 thoughts on “Make this Simple FM Radio Circuit Using a Single Transistor

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Good Day Sir, I'm just kinda confused regarding caps C1,C2, C3,C4? what kind of caps are they? Cant wait to try this one. Thanks A lot!

  3. Hi Swagatam,
    super circuit, adding a band-pass_filter to allow only a particular frequency and reject all others will reduce noise and give a smooth tuning. is it?

    -AND, i am making a wind charger setup to charge a 6V 4.2Ah battery. Can you provide some details on winding coils, turns & voltages.

    • Hi Max,

      Thanks very much!

      I am not very sure how that would be possible in this smallish design, will try to it figure it out.

      For the wind charger what kind of circuit are you using? If you specify some more details then may be i can help.

  4. Good day sir!!
    I have some questions and doubts.
    1. Can I change the L1 w/ inductor? And if possible what value?
    2. Where is the antenna?
    3. Can I use a 9V pp3 battery as power source?
    4. Can I connect the output to an earphone instead of amplifier?

    that's all thanks in advance!!!

    • Good day Achilles,
      Here are the answers:
      1) You can change but it's not recommended, moreover the given data is too straightforward to need a change.
      2)Antenna for the receiver is not crucial ad may not be used.
      3)yes will do.
      4) yes that would be OK.

  5. sir,

    Is it possible to sweep radio station by 555 timer wave genarator instead of trimer.,actually I am making a "ghost box". where i need to change fm station very fast without lock any station.this circuit is eligible for this method.

    • 555 with a 4017 IC can be used but will be a complex circuit.
      The above circuit is a crude FM receiver, will work only if the FM radio station is very near.

  6. Well, what can I say? All I get is a "humming" noise from the speaker. I built it ugly fashion, on a bit of copper-clad, everything in the air apart from grounds, where indicated, and everything as short as possible. I made the circuit third picture down from the top. I am not at all sure what that winding around the resistor is for, but I suspect it.

    I have never been able to get an FM receiver or even an oscillator to work. Ever. This one is no exception, and now I really do give up. In any case, thanks for the effort and your time posting this, it was fun to make, at least :D.

    • optimizing tuned circuits is never easy unless you are an expert in the field.

      As for the above circuit it's a tested design from elektor electronic engineers so definitely the design is OK.

      you'll need to attach a ground plate under the circuit with all the negative connections soldered to this plate…as shown in the top image.

      However the above circuit will be able to receive the signals only if the FM station is fairly close by, for weaker receptions the circuit will not respond.

      by the way be positive, keep trying I am sure you'll succeed some day:)

  7. what are the actual value of L1 and L2………….i need it emergency ………because its my project…………and i have to submit it very soon…………

  8. hare is only given the structure of L1 and L2…………but i need value……….please help me………….its designed by you………so u know it batter……….

    • No, it's not designed by me,

      to know the values you can quickly make the coils practically, and measure the values using an Inductance meter, it's very simple.

  9. those 3 circuits are a nightmare –they DO NOT WORK !—3 months sweat —just a hum !
    —-how do u get 9 volts at the base of bf 494 TRANSISTOR when u adjust the 22pf trim cap –
    then u get zero —turn more –u get 6-9 volts at the base again —madness !

  10. good day sir, i have something to clarify regarding with your comment above about this, "However the above circuit will be able to receive the signals only if the FM station is fairly close by, for weaker receptions the circuit will not respond."

    i have a transmitter and i want to use the circuit above as a receiver so that i can make a simple walkie talkie. i have set my transmitter at 89.8Mhz and i assure that there is no station that occupies this range, if i set the circuit above also at 89.8Mhz is it possible that i can transmit?if yes can it transmit within 20meters?

  11. Good day sir. I would like to ask the following:

    1. BF494 to 2SD756 ; BC559C to BC558B is these replacement ok?
    2. Is there anyway to boost the FM Signal (88-108 MHz) coming in to the circuit?
    3. How would I wire the Varicap and tune it using the trimmer screws? In my case I shorted the Oscillator and Amplifier pins then solder them on the PCB.
    4. I used 0.8mm for the L2, is that ok sir?

    Your site and works is a very big help to me as a student. Every time I have projects, this site is my first option and I even recommended this site to my classmates. Many thanks to you, for your ideas and help. More power and God Bless !

    • Thanks PSK,

      For selecting the transistor you'll have to refer to their datasheets and confirm if their transition frequency is above 200MHz, only those will work here.

      Boosting is not possible with this circuit.

      Varicap is not recommended for the above design so can't comment on it.

      The inductors must be wound exactly as per the data given in the article, others will not work correctly.

    • Thanks for the speedy reply sir.

      For the Transistors, I've checked them and fortunately they're good.

      Sorry for the misconception, I used Variable Cap. not a varicap. I shorted the A and the O.

      Oh I see. then I must replace them.

      Thanks a lot sir…

    • I did not understand what you meant by "sand the wires" please clarify…if it means scraping the ends then yes you must do it in order to solder it…..L2 is not critical, wind it tightly wound and adjust it the to the indicated value

  12. How do I modify his circuit for 3V operation? PP3s are rather expensive… Also, I can't find that specific rf BJT on digit key for one unit. ( minimum order qty. is like 10k) How do I choose a replacement?

    • modification of any kind can affect the circuit performance, so it's not advisable…since 9V was suggested in the original article, this voltage should be only tried…or perhaps you can add many 1.5V cells in series to achieve the same.

      you can try BC547 in place of the shown device

  13. Hi, can I use NTE229 as replacement for the BF494? I can't find that one in my country…If not, is there any other NPN I can use? Also, do you know the numeration of the wire in AWG? Is it 20 AWG and 32 AWG? Does it matter if I use wire a little bigger in diameter or tinier?

  14. Hi sir , thanks for all the effort. I have a few questions too. 1st: Can i use a BF495 transistor instead of BF494? because I was not able to find it. 2nd which circuit will i use the first or second circuit the one with two transistors? lastly: will an 8 ohm speaker do the job ?. Thanks again sir.

  15. HI. i am ATTEMPTING THIS CIRCUIT.HOWEVER I CANT FIND THE WIRE FOR INDUCTORS WHICH SATISFIES THE GIVEN DIMENSIONS. IS THERE ANY ALTERNATIVE TO WIRE WINDING THE INDUCTORS?

  16. OH AND COULD YOU ALSO TELL ME THAT IN THE PIC OF THE CIRCUIT ABOVE WHAT IS THAT BLACK BOX TYPE THING ON THE PLATE AND THAT GREENISH CYLINDER SOLDERED ONTO THE PLATE TOO

  17. I choosed to make cirquit number 2 (the one with reduced background noise) .I pluged in a breadboard and works great.But there is one big problem-the tuning does not work at all.I clearly hear that an FM statio is playing.The local electronics shop did not have the transistor BF494,so I put 2N3904.Is the problem from the transistor or something else.Do you have any ideas to solve the problem?

    Cheers

    Toni

    • 2N3904 can handle upto 270MHZ so that cannot be a problem…check C2 connections, if it's not correctly connected or the part is faulty then tuning can become impossible, additionally if the one of the stations is too close by then tuning other stations could become a lot difficult

  18. upon checking the output from the tank circuit, using a digital cro,no signal was visible. Also, i built the second circuit and did not get any output at all. The problem is i am not even getting a static. I have also tried out other FM receiver circuits using the 741 and lm 386 audio amp. I think the problem might lie in the tank circuit itself. I have wound the inductor myself 4 turns on a 0.8mm copper wire(used sandpaper on edges) and the calculated value is 0.12uH. The trimmer capacitor is 0 to 50pF, so it can be used to receive the normal FM stations. SO what else might be the problem??

    • I am sorry, it would be quite difficult for me to understand the fault without actually checking your design…all these circuits were tested by the original authors…the above design was built and tested by the elektor electronic engineers so definitely it has to work.

      please do not measure the inductance, it's not required, the number of turns and the diameter are what we have to consider and must be done exactly as mentioned in the article.

      these circuits are very basic and therefore the FM station must be quite close by for reception by these circuits, weak stations or far away station will not be detected by them.

  19. I am a student of class 12 and i don't have any idea for how to make a radio but i followed ur instructions and have nearly completed the circuit i just want to ask where should I connect the positive and negative terminals of the speaker

    • Wesley, please do it as explained below

      L1 which is the oscillator coil is an air cored inductor, meaning no core is required, wire is super enamelled type, 0.8mm in thickness, diameter of 8mm, with five turns.

      L2 is wound over R6 itself using 0.2mm super enameled copper wire with 20 turns.

  20. Sir, what is the function of R1 and c1? And the function of the polarised capacitor and the 1k ohm resistor in the top right corner? And the function of c4 connected to the base of t2?

  21. Hello sir, in relation to the third circuit above, will the signal go A) up into the emitter and out the collector or B) will it go down and around and through the collector and out the emitter of T1?

  22. Hi there sir 🙂
    I want to know Is this circuit working perfectly?. because I have a project assignment to make a FM receiver without using IC. Your replied would be help me a lot.

    Thank you sir.

    • Hi Hariz, this circuit was designed and tested by the Elektor electronic engineers, so this is a tested and a verified design, however this being a very small and a low tech FM receiver/transmitter circuit, the reception quality will be bad, and only the nearby strong stations will be received by this unit.



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