How does the FM Transmitter Work?
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.
When your device is powered by the cigarette lighter cable of the Car Mount kit, your ARCHOS will be able to transmit its sound (usually music) to a specific frequency on nearby radios (the radio of your car). You can set the frequency of the device’s transmitter (the same frequency you will tune your car radio to) by going to Menu > Settings > Sound & Display > FM Transmitter. Then, when you are playing music, go to Menu > Settings > Sound > FM Transmitter and turn it on. Make sure the power cable from the car kit is plugged into the ARCHOS.
