Here’s one of the easiest, simplest and the most popular FM transmitter for amateurs and electronics beginners. With this DIY kit, you can transmit your voice or audio over an ordinary FM radio within the FM broadcast band. It is a DIY kit designed by Sagar Sapkota. You can buy this kit from BuildCircuit Store. The best thing about this transmitter is that you don’t have to make your own inductor for this kit. Making an inductor is bit hard for amateurs. Besides, this kit is not only capable of transmitting voice using microphone but also transmitting music from your music player.
You can use this FM transmitter/microphone in two ways:
a. Audio transmission using electret microphone: You can transmit your voice to FM radio with the electret microphone. You can also place the transmitter circuit close to any loud speaker so that the microphone picks up the sound and transmits to the tuned FM radio.
b. Audio transmission connecting speaker stereo jack to audio source: For hearing better sound, you may connect it to your computer, ipod or mp3 player using a speaker jack. The audio of your music players get transmitted to your nearby radio.
Frequency Range: 88-108MHZ (Frequency can be changed by moving the inductor coil).
Operating voltage: 1. 5V-9V
Transmission range: More than 30 meters in open area. The transmission depends upon the length of antenna used. I used just a simple 10cm long wire. The signal can be transmitted even far with longer antenna of length around 50cm.
Working of the kit:
Electret microphone (MIC) converts the natural sound signal into the electrical signal. The capacitor C2 is coupled to the base of transistor Q1- S9018 and whenever there is signal from the microphone, the junction capacitance of the transistor changes that contributes to change in oscillation frequency.
Resistor R1(2.2K) is a microphone MIC bias point resistance, usually value between 2K-5. 6K is preferred, R2 provides biasing to the base of transistor. C4 and inductor L (5.5 turns coil) make up the oscillator tuning circuit, you can change the value of C4 and L to change the transmission frequency.
Formula for frequency calculation:
You can calculate the required frequency on this page.
Testing of the kit:
Find an FM radio, turn on the power and volume, adjust the frequency around 88 MHz. Connect the kit to the power supply board, align towards the radio and use a screwdriver to adjust the oscillation coil L until the radio catches the signal. Then slowly increase the distance between the microphone and radio, while properly adjusting the radio (or handset) volume, tuning knob until the clearest sound is heard.
Important posts related to this article:
- How to make this project on a stripboard– posted in March 2014.
- How to connect electret microphone.
- How to make an inductor coil for FM transmitter.
- How to make an FM transmitter- easy tutorial.
- Assembly tutorial for FM transmitter kit.
- Flickr images