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Simple FM transmitter

Discussion in 'Electronic Projects' started by ElectroMaster, Jul 19, 2002.

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  1. circuitnoob

    circuitnoob New Member

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    I fixed a electret microphone to it but there seems to be no sound when i speak. is there something wrong with the circuit diagram or something wrong with my circuit....???
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It is a terrible circuit.
    Its preamp transistor is not biased correctly so might be saturated or cutoff which makes it not work.
    Its radio frequency could be anywhere and might not be in the FM broadcast band.
     
  3. whiz115

    whiz115 Member

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    can i use an 100nH axial inductor instead of an air coil for the LC tank?
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I never tried one and I didn't measure the inductance of my simple air coils but they work.
     
  6. sarathreddy

    sarathreddy New Member

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    it's interested.i want try this

     
  7. samadica

    samadica New Member

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    this circuits is really unstable,i have build it,it dosent works good!
     
    Last edited: Sep 15, 2009
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Layout and construction is critical at VHF, how did you build it?, and what on?.
     
  9. whiz115

    whiz115 Member

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    so.. you're one lucky fellow?
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    There are about 150 FM transmitter projects on the internet. The coil in them is almost the same. 5 to 9 turns of wire.
     
  11. aragupta

    aragupta New Member

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    i like this circuit but could you tell me what is the frequency of operation of this circuit . And what are the calculation related to this circuit..
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I don't know which FM transmitter circuit you are talking about.
    The FM broacast band in most of the world is 88MHz to 108MHz.
    These FM transmitters can be tuned in that band.

    Calculations are difficult because wiring capacitance affects them. The amount of wiring capacitance depends on how the circuit is built and anything conductive nearby.
     
  13. ANTlabs

    ANTlabs New Member

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    I don't understand how to make capacitor 6 and cant get the circuit to work. Pleas post the theory.
     
  14. lino_pm

    lino_pm New Member

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    thanks for registraion
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I don't know which circuit you built. If you made it on a breadboard then the high capacitance of all the wires and between all the conductive contacts make it not work. Make it on a compact pcb.

    What is "capacitor 6"?

    Most simple FM transmitters have a microphone preamp transistor, a Colpitts oscillator transistor and maybe an RF amplifier transistor. The Colpitts oscillator is a common-base transistor and the modulation at its base makes it conduct more and less. When it conducts more and less its capacitance changes that changes the frequency of the oscillator producing FM.
     
  16. ANTlabs

    ANTlabs New Member

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    I don't have much money, can you remove parts from a PCB.
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Electronic parts are very inexpensive.
    Use parts from a pcb if you can de-solder them and if they are the correct parts.
     
  18. ladybliss

    ladybliss New Member

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    Can you give me the specifications of this transmitter and the antenna system with transmitter line?
     
  19. dinhkli

    dinhkli New Member

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    i want to a design of Simple FM transmitter(by electroMaster). How calculate value of R1,R2,R3,R4,R5? help me? I'm doing project of circus. reply soon.
    mail: dinhkli245@yahoo.com
    thaks inf.
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your English is horrible.
    You need to know simple arithmatic and Ohm's Law to see how the first transistor is biased.

    R1 applies about 0.5mA to the JFET in the electret microphone.

    The circuit is too simple:
    1) Since the tuned circuit is connected to the antenna then the frequency will change if something moves toward the antenna or moves away.
    2) Since it does not have a voltage regulator then the frequency will also change as the battery runs down.
    3) It will sound awful without high audio frequencies because it does not have pre-emphasis like FM radio stations have.
     
  21. dinhkli

    dinhkli New Member

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    with R1 applies about 0.5mA. thus, R1=9/0,5=18k but result of R1 =1k.
    Can u guide the calculation R2,R3,R4,R5,R6?
    Thaks rely soon!
     
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