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.
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.
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