FM Adaptor for Car Stereo

Nagaraj Kulkarni

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If your car has an FM radio with stereo output but no inbuilt cassette player, this circuit will come handy for listening to your favourite collection of music from your personal audio player through the FM-stereo car radio.

For the circuit to work, your personal audio player should have a provision for stereo output socket.

Hooked to the output of the personal audio player, this FM adaptor can modulate audio over the FM band of 88 to 108 MHz. The modulated frequency can be conveniently received by the car stereo’s FM receiver.

Fig. 1: FM rebroadcast circuit
Fig. 1: FM rebroadcast circuit

The FM adaptor is basically a low-power, single-transistor transmitter with a range of 15 to 20 metres. It can also be used for rebroadcasting TV audio or your favourite music from an audio system to a pocket or headphone FM receiver without disturbing others in the room or outdoors.

You can easily rig this FM modulator using any general-purpose npn transistor configured as a voltage-controlled oscillator (refer Fig. 1). Frequency modulation is achieved by modulating the base-collector voltage, thereby varying (modulating) the depth of the depletion layer of the reverse-biased base-collector junction, which results in a change in capacitance at the collector and, in turn, a change in the resonant frequency of the collector circuit.

Inductor coil L1 can be made by winding eight turns of 20SWG enamelled copper wire around a 5mm dia.

Solder the complete circuit inside a 1.5V, AA-size single-cell battery holder, which can be retrieved from a toy. Make sure that the trimmer capacitor (VC1) is soldered firmly and accessible easily (using a non-metallic screwdriver) for varying the capacitance.

For antenna, use an 8cm long straight 28SWG copper wire or a telescopic antenna (which can be retrieved from a pocket radio).

Fig. 2: AA-size single-battery holder andassembled FM modulator
Fig. 2: AA-size single-battery holder andassembled FM modulator

Solder the stereo pin directly onto the battery holder (see Fig. 2). The power switch is not essential as the current drawn is only a few milliam-peres and power consumed is around 125 mW. A typical AA-size alkaline battery can be removed easily when the gadget is not in use.

Make the circuit as compact as possible to fit on the battery holder. Check the assembled circuit for connections. If alright, solder it firmly onto the battery holder and insert the battery.

For testing, use a battery-powered FM receiver, and not an AC powered system, and proceed as follows:
1. Tune the FM radio to dead space, i.e., frequencies within the FM radio band that are silent or only have some hiss. Frequencies near 108 MHz are typically dead space.
2. Plug in the FM modulator to the audio output of the personal audio player. Extend its antenna and keep the transmitter approximately a metre away from the FM radio. Play some music in the personal audio player and slowly adjust the variable capacitor (trimmer) using a non-metallic screwdriver until you hear music from the FM radio. This process requires careful capacitor tuning.
3. Increase the transmitter-to-radio distance and fine-tune the trimmer for its optimal audio level.
4. When you hear music clearly on the FM radio, apply some hot wax on the trimmer capacitor to fix its position.

Now the FM modulator is ready for operation. Plug it into your personal audio player for rebroadcasting the audio being received by your in-car stereo or any FM receiver over a distance of 10 to 20 metres.


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