Make Your Own FM Transmitter For Broadcasting

By Joy Mukherji

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This is one of the simplest FM transmitters that can be built. It does not require any tuning, and transmit frequency is crystal locked to 90MHz. It eliminates frequency drift common to LC-based FM transmitters. This design is best suited for broadcasting voice or music up to a distance of fifty metres depending on the antenna used.

Circuit and working

The simple FM transmitter is built around low-power audio amplifier using LM386 (IC1), transistor PN2222A (T1), 30MHz crystal (XTAL1), varactor diode 1SV149 (D1) and a few other components. Inductor L1 is a three-turn coil made with 20SWG wire that has 8mm diameter with half-turn taping.

Fig. 1: Circuit diagram of the simple FM transmitter
Fig. 1: Circuit diagram of the simple FM transmitter

Tank circuit at the collector of T1 comprising inductor L1 and capacitor C5 is tuned to three times the crystal frequency, or 90MHz. Third harmonic of 90MHz is boosted and coupled to a wire antenna via capacitor C4 for transmission that can be picked up by an FM receiver thirty to fifty metres away.

The audio amplifier formed around IC1 has a gain of twenty. Its frequency modulates the oscillator by varying the capacitance of varactor diode D1, which is connected in series with XTAL1 via resistor R2. Potmeter VR1 sets frequency deviation and should be adjusted for best sounding audio and low distortion. The transmitter accepts audio input from such external devices as a computer, music player or cellphone. Use an 83cm-long piece of wire for the antenna.

Construction and testing

A PCB layout for the simple FM transmitter is shown in Fig. 2 and its components layout in Fig. 3. After assembling the circuit on the PCB or veroboard, enclose it in a suitable plastic box. Use a shielded cable for input audio connection for hum-free signals.

Fig. 2: Actual-size PCB layout for the simple FM transmitter
Fig. 2: PCB layout for the simple FM transmitter
Fig. 3: Components layout for the PCB
Fig. 3: Components layout for the PCB

For downloading PCB and component layout PDFs: Click here

Audio quality of the transmitter is decent, and fidelity is good enough for music. Connect 12V DC regulated power supply across connector CON1.

Connect the audio input at JACK1 from a computer, music player or cellphone. You can listen to the audio on an FM radio by tuning to 90MHz frequency band.


Joy Mukherji is an electronics hobbyist who likes designing radio frequency circuits

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21 COMMENTS

    • Yes. Replace C5 accordingly for resonance at 3rd or 4th harmonic of 27MHz (81 and 108MHz). Replace C5 with a trimmer cap and tune for maximum range and best sound.

      If ISV149 is hard to get then try using an IN4007 or several of them in parallel.

        • Please don’t make ignorant comments. The varactor diode is properly reversed biased by the LM386. It’s not a wrong circuit but good design since the audio fidelity is excellent and even music sounds good. ISV149 is able to pull the XTAL frequency a lot but if it’s not available then try other Varicap diode in its place or an IN4007. These rectifier diode make good varicaps.

  1. can i replace 30mhz with 32 mhz and L1 with fixed 1uh inductor and c5 2.7pf to receive at 96mhz. Also tell me more about 1sv149 alternative.

    • Yes you can do that. ISV149 have high fidelity sound and music sounded awesome too. You can just use an IN4007 diode in place of varactor. I’ve one more circuit of crystal tx. Email me [email protected]

    • Excellent idea. Using a large value coil and low value capacitor will raise the Q of tuned circuit. I used a 2n3904 and its collector to emitter capacitance is 4.5p so your 2.7p would be added to it giving you a total value of 7.2pf. To resonate that on 96MHz you would need a coil of 0.381uH. Hope this helps.

    • No you cannot. The circuit is not suitable for driving an external VHF amplifier either as its output contains a lot of harmonic energy. But my other project crystal locked fm transmitter has a pure signal so you can connect it to a 2n3866 amplifier for more power. I’ve developed a circuit where 2 parallel PN2222As are delivering half a watt of power on the FM band. Will publish it soon.

  2. what are the regulation and guidelines regarding use of mini fm transmitter in India. In other countries these are allowed by law upto certain field strength.

    • No transmissions are allowed without valid license. But there is no enforcement of the law. One is free to run 1KW of RF power at 100MHz and unless you’re interfering with local FM channels there should be no problem at all. Spain is also full of FM pirate radio stations but authorities don’t care. In USA it’s a different story thou
      since the fines can be hefty ($50,000).

  3. Sir good cercuit design. Whats the range of this transmitter. Sir from wher e i can get complete pcb with component. Thanks

  4. Hi nice circuit I have a couple of questions, in the circuit diagram VR1 is 10K in parts list VR1 is 100K, which is correct? also in the circuit diagram R1 And R5 have the letter E after the value what is this?

    • You can use VR1 with 10K or 100K but during testing we have used 100K. The letter E for R1 and R5 simply means ohm. That is, R1=330E=330-Ohm and R5-150E=150-ohm.

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