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fm radio receiver

FM Radio Receiver

This simple fm radio receiver circuit consists of a regenerative rf stage, TR1, followed by a two of three-stage audio amplifier, TR2 to TR4. In some areas 3 stages of audio amplification may not be necessary, in which case TR3 and its associated components can be omitted and the free end of capacitor C5 connected to the collector of TR2.

Radio Receiver Circuit Diagram

fm radio receiver circuit diagram

The critical part of the fm radio receiver is the first stage, TR1/VC1, where the wirings must be kept as short as possible. Coil L1 is formed by winding 8 turns of 1mm (20 swg) enamelled copper wire on a 6 mm diameter former, which is then removed. After that L1 should be stretched carefully and evenly to a length of about 13mm.

Transistors List
TR1 = BF199
TR2 = TR3 = TR4 = BC547

Video presentation and photos of the working radio receiver

by Aleksandar

The tunning capacitor VC1 is one of the two fm sections of a miniature fm transistor radio with built-in trimmers (VC2). The “earthy” end (moving vanes and spindle) is connected to the 22pF capacitor C1. The value of the rf choke L2 is not critical, anything from 1µH to 10µH being suitable.

The output is suitable for ordinary earphones connected in series to provide an impedance of 64Ω.

Tuning-in the fm radio receiver

To operate the radio receiver, potentiometer VR1 must first be advanced slowly (towards the end of the track connected to battery positive) until, at about the half-way point, a sudden slight increase in background noise will be heard, indicating the onset of oscillation. It then should be backed off, very slowly, until oscillation just stops; it then should be possible to tune in some stations.

The correct frequency range of 87 MHz to 108 MHz can be obtained by adjusting VC2 at the high frequency (108 MHz) and slightly stretching or squeezing together the turns of coil L1 at the end (87 MHz).

316 Comments

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  • vinitmeena011

    Sir I need to complete this project within this month
    I already tried for a circuit bt that wasn’t working.so plz send me appropriate circuit with complete details.

  • shuchi_p6

    Sir please send me the schematic of your circuit with the aidio amplifier joined to it URGENT

    • vinitmeena011

      If u have appropriate info regarding this fm reciever plz share it with me.Its urgent

  • dimitri. K.

    Thanks for the circuit i was happy to see it working!
    To contribute to the thread and the project.
    1. BF494 works ok. No BF199 here in Greece.
    2. If you try simple little variable capacitor and having problems on tuning (tune is lost when you remove screwdriver from VC1) it happened to me as well and didnt manage to resolve it.

    To really enjoy the project just find an ordinary Fm radio that is broken and salvage the tuning capacitor from it. (most of the times its a 20pf VC). Only analog radios have the tuning capacitor not the digital ones that you push button to tune to next station and then push again to tune to the next etc.
    The tuning variable capacitor consists of 2 or 4 caps and 2 or 4 trimmers. Just use the 20pf one and trim it to establish the fm frequency range.
    Dont give up if you fail once, RF projects are tricky but very addictive… Im done with the 555 and the blinking leds…

  • Siyam

    You should be able to adjust VC2, the high-pass filter (rectifies low-frequency signals and allows high ones to pass), to allow lower frequencies through. For example, a 100pF capacitor used with a 32-ohm resistor passes frequencies higher than ~50MHz. This http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php calculator makes figuring out filters easy.
    In other words, maybe try a higher value for VC2. I hope that helped.

  • amol

    How do I tune it to receive signals before 87 MHz ?

  • mrproteindna

    It works! 🙂
    Have a great time constructing this one. Here is my video contribution: https://www.youtube.com/watch?v=6c4zOPvlZvQ
    Cheers! Greets from Serbia 😉

    • mrproteindna

      Yes, actually I did. Well, there was disturbance in the oscillator with 9 V transistor battery, I guessed, but if you have nice oscilloscope, then you should try several experiments, and kindly inform us about voltage/current characteristics of this superb circuit. 🙂

    • Rum bum bum

      Have you tried to increase the supply voltage?

    • mrproteindna

      Thanks Alex 🙂

    • Aleksandar

      Good job! Great build. 🙂

  • TomD

    Thank so much for the response. So if I understand correctly, if I were to get say a 5pF – 322pF and a 3.6pF – 68pF it just widens the range of frequencies I can tune the resonance to? Because I was thinking about having the L1 specified for FM and then putting in a switch that could change the path to a different coil in parallel to L1 for AM or some other range. So depending on what I wanted, I could toggle between them with a switch at the node between L1,C1, and C2?

    • Aleksandar

      5pF – 322pF and a 3.6pF – 68pF in parallel would add up making a single variable capacitor in which you’d use the larger one for main tuning and the smaller one for fine tunning. Yeah basically you can expand the frequency range or shift the frequency band. Band switching with the switch between the coils in the tank circuit should work. imho your logic is sound 🙂 Try it, test it, document it and post it here so we can see what you’ve done. Pics and videos would be nice. Make a demonstration video or something like that 🙂

  • TomD

    I appear to be a couple years late with this post but I am building this circuit. I am struggling to understand the variable capacitors. I have read and reread the comments. I understand how to use them as far as tuning but as far as why we are using two and their ranges are kind of confusing me. I have all the components required except for VC1 and VC2. I can purchase a 5 pF to 108 pF or 5 pF to 68 pF and a bunch of other variations. Is the range critical as long as it gets down to 5 pF?

    In a comment earlier you mention that theoretically its equivalent to a 25 pF capacitor. Can i just get that instead? I love to tinker and plan on doing so but obviously the tuning circuit has to work so i wanted to stick to the planning.

    I look forward to your response, any help/direction would be much appreciated. And if it is easier, I can provide my email for more efficient communication. Thank you for your time and consideration of this issue.

    • Aleksandar

      5 pF to 68 pF sounds about right.
      Here’s the thing.
      VC1 and VC2 are both part of the LC tank circuit that allows you to change/set frequency of the radio.

      VC1 for eg. can be the MAIN tunning capacitor while VC2 would be FINE TUNE capacitor.

      So for eg. VC1 could be 5-68pF while VC2 could be something smaller. You could use larger value variable capacitors in series with small fixed value to get a small value variable capacitor.

      Use the Thompson formula to calculate the LC circuit frequency and use the Parallel and Series capacitor connections formula to calculate the possible values of capacitors using different combinations of values to get what you need. You might have the appropriate part in your junkbox without even knowing it.

      Happy tinkering 🙂

  • Shelton Burrup

    Next time I read a blog, Hopefully it won’t fail me just as much as this particular one. I mean, Yes, it was my choice to read, but I genuinely thought you’d have something helpful to say. All I hear is a bunch of complaining about something you could possibly fix if you weren’t too busy searching for attention.

    gagfap.com

  • StellarRat

    Here are some suggestions for future improvements if you want to experiment. I think using a couple of varactors and small pots to tune the radio would make the circuit more user friendly. It will eliminate a lot of hands/tools capacitance problems and make the radio far less “touchy” to tune. You’ll need to change your circuit to run on 9v to do this. You can still run the RF on 1.5v by using 1.5v voltage regular IC. You can eliminate all of the audio amp transistors with an LM386 IC and get a lot more volume, probably enough to drive a small speaker. Alternately, a high speed OP AMP could be used if you want to just run headphones with a high sound quality, OPA227 maybe?.)

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