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  1. #1

    [Moved] FM Transmitter with Single Transistor

    Fm Transmitter
    I was looking for a project to make for fun when i found this FM transmitter, i got curious because it is a best project for fun as well as you tube, it is a simple FM transmitter but i make it it the simplest. I replaced the variable capacitor with a fixed one, re-calculated its frequency to be able to tune it only by the coil instead of the variable capacitor.
    This circuit design is unstable because its resonant frequency drifts off if you get your hands close to the antenna or the circuit, the length of the antenna affects the resonant frequency so you should fix it on a fix place so that its frequency remain constant.
    Note: It is a very simple transmitter but its range can be 2 km if u attach long antenna with it.

    Circuit
    single FM Transmitter

    Components Required
    R1 : 100ohm resistor
    R2 : 4.7 ohms resistor
    C1 : 1nF ceramic capacitor
    C2 : 47pf ceramic capacitor
    Q : bc2655,2n222 or 2N3094
    L : 212nH (5 turns of 15 gauge copper wire)
    Mic : Condenser mic
    Bat : 3v battery and its holder
    Ant : Ideally a piece of wire, 1/4 or 1/2 of the wavelength of the frequency if you want more range then use roof antenna and adjust frequency to increase and decrease the space between inducter coil.
    Note
    6v battery can be used to increase the range of this transmitter
    Circuit Diagram

    https://envirementalb.com/simple-fm-transmitter

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  2. #2
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    Re: [Moved] FM Transmitter with Single Transistor

    The mic is an electret type, not a simple condenser type that needs a 48V supply and an impedance converter.
    The simplicity of this transmitter causes performance problems and it has errors that will cause it to sound awful:
    1) It is missing a voltage regulator so the radio frequency will change as the battery voltage runs down.
    2) It is missing an RF amplifier/buffer so of course the frequency changes if something moves towards or away from the antenna.
    3) The 100nF capacitor kills audio frequencies above 533Hz. My FM transmitter uses only 470pF.
    4) There is no mic preamp so it will not sound loud enough.
    5) It is missing pre-emphasis (audio high frequencies boost) that all FM radio stations have then the de-emphasis used in all FM radios will cause received high audio frequencies to sound muffled.
    6) It is missing a power supply decoupling (bypass) capacitor.

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  3. #3
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    Re: [Moved] FM Transmitter with Single Transistor

    If the mic is an electret type then the 100nF cap should go from the mic to the base of the xtor with another resistor going from the base to the + supply ( say 100k )

    the 4k7 goes directly to the electret mic.

    For a capacitive type mic ( condenser ) a different ckt is needed to get the best out of that

    a 1uF cap across the supply is a good idea

    the freq won't change with the battery volts - but it will move with "hand effect" and aerial loading effect ...

    good luck experimenting - there are a host of slightly better ckts on the net ...

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    Re: [Moved] FM Transmitter with Single Transistor

    The transistor oscillator produces FM caused by the transistor capacitance changes which is the radio frequency changing up and down by the audio voltage swing. But the battery voltage running down also causes the transistor oscillator frequency to change which again is caused by the transistor capacitance change.

    I fixed a similar circuit that had a mic preamp transistor that was saturated when the battery was new and was cutoff when the battery voltage was running down. The oscillator radio frequency also changed when the battery voltage was running down so I fixed these problems by adding a low dropout voltage regulator.
    I fixed the radio frequency change that was caused by something moving towards or away from the antenna by adding an RF amplifier/buffer to isolate the oscillator from capacitance changes at the antenna.
    I made it sound great by adding worldwide pre-emphasis.
    I built it soldered together on stripboard. It is here:

  5. #5
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    Re: [Moved] FM Transmitter with Single Transistor

    for the simple ckt it would be better to take the aerial off a tap point on the RF inductor -

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