low power fm transmitter

Low Power FM Transmitter

This low power fm transmitter is designed to use an input from another sound source and transmits on the commercial FM band. This low power fm radio transmitter it is actually quite powerful…
The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver’s mute circuit to hear this).

When assembling the fm transmitter circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver – this will not alter the frequency.

Q1 is a conventional Colpitts oscillator design. The audio signal applied to the base of Q1 causes the frequency to change, as the transistor’s collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver.

The inductors are 9.5 turns of 1mm diameter enamelled copper wire. They are close wound on a 3mm diameter former, which is removed after the coils are wound.
The output is a low power of 100 mW, but for some of you this fm rf transmitter can delivers the desired power for broadcasting on your street or with a proper antenna you can cover a small neighborhood. If you need a power wireless fm transmitter use the above menu, you can find transmitters starting with low fm power up to high power fm transmitters.

Low power transmitter circuit diagram

low power fm transmitter

Small power fm transmitter PCB layout

low power transmitter pcb

low power fm transmitter parts layout


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  • Value of Inductors

    I want to make this transmitter. Can you please suggest the values of the inductors used in the circuit?

  • aidan makwenda

    How can i connect an input signal? And what is the maximum range in distance covered for that 100mW as an output

  • Okpara Scofield

    how much could i get the equipments needed??

  • Raj


    Can you please tell me if the above circuit will cover a range of 50 to 100 meters? If not then what will be the maximum range of this 100 mW FM transmitter?

    Thank you in advance,
    Kind regards,
    Raj Kumar Mukherji

  • Vincent

    It’s a great and simple circuit, but one thing I don’t understand:

    Why are there 2 grounds and why are C8 and C9 both connected to it directly from the supply voltage. Can’t you just add them together?

    • DJAE

      There aren’t two grounds, they are the same symbol (not ground/ earth/0v etc). They are used that way to minimize confusing lines all over the schematic, they make it simpler to understand. The caps c8&9 cannot be added together, one decouples the supply, and the other smooths and decouples the oscillator supply to keep the signal more stable. This should be placed physically close to L1 and c2.

      As for component values, try and keep as close as possible, but 10pf instead of 12 is OK, just stretch the parallel coil a tiny bit and it will compensate.

  • no

    yes i believe that is how it works. Also, does anyone know if the cap values in the circuit are critical? reason being i have plenty of 30pf/10pf/etc. caps and really don’t want to have to pay shipping for a couple odd values

  • vi

    Hi, How can I adjust it to an frequency? Just select an unused frequency on an reciever and turn C3 until I hear something?

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