This circuit can be used to play your favorite songs from your personal audio player through the FM-stereo vehicle radio if your automobile has an FM radio with stereo output but no built-in cassette player. Your individual music player needs to have a provision for a stereo output socket in order for the circuit to function properly. When this FM adaptor is connected to the output of a personal audio player, it can

modulate audio over the 88-108 MHz FM band. The FM receiver in the car stereo can easily pick up the frequency that has been changed. Fig. 1: FM rebroadcast circuit The FM adapter is essentially a low-power transmitter consisting of a single transistor that has a range of between 15 and 20 meters. It is also possible to use it to rebroadcast TV audio or your preferred music from an audio system to a pocket or headphone FM receiver without bothering other people in the room or outside. Using any general-purpose npn transistor and configuring it as a voltage- controlled oscillator is all that is required to rig up this FM modulator (refer to Fig. 1). Frequency modulation is done by changing the base-collector voltage. This changes the depth of the depletion layer at the reverse-biased base-collector junction, which changes the capacitance at the collector and, in turn, the frequency at which the collector circuit resonates.

Fig. 2: AA-size single-battery holder and assembled FM modulator To make inductor coil L1, wrap eight turns of 20SWG enameled copper wire around a 5mm dia. Solder the whole circuit inside a 1.5V, AA-size, single-cell battery holder that you can get from a toy. Make sure the trimmer capacitor (VC1) is securely soldered and easy to reach (with a non- metallic screwdriver) so that the capacitance can be changed. Use a straight 8cm-long 28SWG copper wire or a telescopic antenna as an antenna (which can be retrieved from a pocket radio). The stereo pin should be soldered directly to the battery holder (see Fig. 2). The power switch is not necessary because the current drawn is only a few milliamperes and the power used is about 125 mW. When the device is not being used, it is easy to take out a typical AA-size alkaline battery. To fit on the battery holder, make the circuit as small as possible. Check the connections in the circuit you just put together. If everything looks good, solder it to the battery holder and put the battery in. Use a battery-powered FM receiver for testing, not a system that runs on AC power, and do the following: Tune the FM radio to "dead space," which is a term for frequencies in the FM radio band that are either silent or only make a hissing sound. Most of the time, there are no signals near 108 MHz. Connect the FM modulator to the personal audio player's audio output. Extend its antenna and keep the transmitter about a meter away from the

FM radio. Play some music on the MP3 player and slowly move the variable capacitor (trimmer) with a screwdriver that isn't metal until you hear music from the FM radio. This process needs careful tuning of the capacitors. Increase the distance between the transmitter and the radio and adjust the trimmer to get the best sound level. When the music on the FM radio is clear, you can fix its position by putting some hot wax on the trimmer capacitor. The FM modulator is now ready to be used. Plug it into your personal audio player and you can send the sound from your car stereo or any other FM receiver up to 10 to 20 meters away.