A short range FM transmitter allows you to wirelessly stream audio from a portable device like an MP3 player to a regular FM radio. It plugs into the headphone jack of the audio source and broadcasts the audio signal within the FM radio frequency band.


Nearby FM radios can then tune into the transmission and play the audio through better quality speakers than the portable device itself. This provides a convenient way to enjoy mobile music through a home or car stereo without being tethered by cables.


Due to their low power, most FM transmitters only broadcast up to about 100-300 feet away depending on reception conditions. They transmit on standard FM frequencies between 87.5 and 108 MHz that radios are designed to pick up.


In this circuit project, we will build a device that collects audio input through an aux cable. It will then broadcast the audio wirelessly within the FM band. A nearby FM radio tuned to the corresponding frequency can receive and play the transmission. This allows wireless streaming of audio to radios in a short range around the circuit.


Step 1: Required Parts

Transistors

2N3904 - 2 

Resistors 

100k Ω - 1

100Ω - 1

1M Ω - 1

1k Ω - 1

10k Ω - 3

Inductors

0.1µH inductor (Air coil)

Capacitors 

0.1µF - 2

40 pf trimmer - 1

4.7 pF - 1

10pF - 1

Other

Antenna

9V battery and clip 

PCB or breadboard for building the circuit 


Step 2: Building the circuit


The PCB layout file is attached so you can directly etch the circuit board. A Fritzing file of the design is also included.

Once the PCB is ready, insert the components according to the circuit diagram and solder them into place.

Now we need to construct the inductor coil. Use either 18 or 22 gauge copper wire. For 18 gauge wire, wind 4-5 turns around a 1/4 inch form. For 22 gauge, wind 8-10 turns at 1/4 inch. Solder the completed inductor onto the PCB.

If you have an antenna available, solder it to the board. Alternatively, use 8-10cm of hookup wire as the antenna.

I included a 3.5mm female audio jack for flexibility. It allows easy connection of microphones, audio players and other devices to provide input. Plugging in a mic will capture audio to broadcast over FM frequencies for nearby radios to receive. This could even be used as a basic spy audio transmitter.

fm transmitter fritzing file.fzz


Step 3: Making PCB

For a visual guide on etching PCBs using markers, check out this instructive YouTube video. In the video titled "How to make a PCB using Marker" on the channel Fritzing FX, the creator demonstrates the simple process step-by-step.



In this project, a permanent marker acts as the protective coating for etching the PCB.



Step 4: Tuning Transmitter

Tuning the transmitter to the desired frequency takes some patience, as it can be a time-consuming process to get it just right.

Varying the trimmer capacitor allows you to change the transmission frequency broadcast by the circuit. Slowly adjust the trimmer and listen closely for any distortion in the radio reception.

When you hear distortion, narrow in on that area by making smaller adjustments to the trimmer. Continuing to tune should bring the transmitter and receiver into alignment, at which point the audio from the radio should sound clear.

Once you have matched the frequencies through tuning, the FM transmitter build is complete. For a detailed visual demonstration of the entire construction process including tuning, check out the video link provided below.

The tuning steps are rephrased with a bit more clarity on how to iteratively hone in on the optimal frequency match through distortion detection. Emphasis is also placed on tuning taking patience to maintain interest while conveying its importance.

Step 5: Construction and Testing