Stereo Wireless LCD Display FM Transmitter for iPod

The variable capacitor and your self-made inductor will vibrate at frequencies in the FM radio band (88 to 108 MHz). The electret microphone has a resistance that depends on how loudly you speak into it. This microphone is battery powered and according to the V=IR Ohm's Law, changes in resistance for fixed voltage will result in proportional changes in current. This current feeds into the base of the 2N2222 NPN transistor which is connected to your variable capacitor, inductor and antenna. The net effect is that depending on your variable capacitor's value, your voice will be modulated to transmit at a frequency between 88 and 108 MHz. If a nearby pocket FM radio is tuned to this frequency, you'll be heard when speaking into your transmitter.

The component values in the circuit are derived to better understand how this FM transmitter will work. The underlying math is rather simple and can be found in most undergraduate university physics textbooks.

An FM transmitter is a device that uses the voice or music audio signal voltage to modulate i.e. vary the frequency of the transmitter signal. FM means Frequency Modulation. As the amplitude of the voice or music changes the frequency of the FM transmitter changes correspondingly. The strength of the FM transmitter signal stays the same. Just the frequency changes. This is in contrast to an AM transmitter which uses the audio signal to vary the intensity or amplitude of the AM transmitter signal. AM means Amplitude Modulation. As the strength of the audio signal goes up and down the strength of the AM transmitter signal goes up and down correspondingly.

Why FM? Well, a circuit called a discriminator in the FM radio receiver is uded to recover the audio signal from the transmitted signal. The discriminator is pretty neat. It does not respond to the change in amplitude of the radio signal. It only responds to the change in the frequency. Since static is a change in amplitude or strength of the signal not the frequency the discriminator does not "hear" the static.

Because of this FM is ideal for transmitting high quality static free music and audio from your laptop to an FM receiver in your home or in your car. You can buy a small FM transmitter at Circuit City or Best Buy etc. that will plug into the audio output of your laptop and transmit the signal a short distance to your home or car FM receiver.

It's not wireless so there's no interference. Not to be confused with an FM transmitter. Basically it adds an AUX input to ANY radio. No searching for a clean channel.

It feeds your audio signal from a headphone jack (or RCA cables) by FM modulation, directly into the antenna input on your radio bypassing the antenna when on, allowing the radio to work as normal when off. It comes with an On/Off switch.