FM (88-108MHz) are used in radio broadcasting. We can build simple FM transmitter and utilize the FM radio receiver to receive our transmitted audio. And these FM transmitters in frequency range of 88-108MHz are always called bug 🙁
FM transmitter can be easily built using very few components. Here are 2 example circuits that using 1.5V or 3V of power supply. the 1st circuit is using a single transistor whilst the second circuit ( more stable) using 3 transistors which includes audio amplifying and buffer stage from oscillator. Please make the size/ form factor of the transmitter as small as possible to get the best out from your transmitter.
This simple FM transmitter can used to make your speaker wireless. Just eliminate the microphone part and get an audio jack to connect the audio signal from your computer to the C3 coupling audio capacitor directly; and you are transmitting the audio/songs from the computer. Use a radio receiver to tune into the frequency transmitted.
You can tune your radio into a favorite radio station and listen to the radio when your computer is off, or your transmitter is off. This is the interesting part; when you turn ON your transmitter and start transmitting song from computer, it will overlap the radio station and make the radio output the song from computer only because your transmitter is nearer to the radio and thus giving stronger signal than radio stations. Hence it can automatically switch between the radio station and the audio from computer.
This transmitter is battery powered. it’s wasteful, since we can get the power from usb if we want to use it at transmitting audio from computer. so we can further improve it by getting the power from usb 5V. However, this will make the transmitter full with wires: wires from audio jack, wires from usb to power the transmitter, wire of antenna. We can probably further this project by adding an USB audio chip. Now we can get the power from usb and too the audio from usb!! here is the link, someone had built usb audio DAC. We can integrate the USB DAC to our transmitter, and eliminate all the wires!
be cautious. transmitting FM are illegal if the power emitted from the transmitter is over a certain limit.