This is the most simple and cheap F.M transmitter you can ever find. This circuit is really cool. This runs at very low voltage, by a CR2025 3V battery, current consumption is also low.And the total size of this F.M transmitter(including battery, excluding antenna) is less than that of a matchbox. The circuit has a central RF oscillator NPN transistor BF494 (substitute: BF194). A coil takes care of the output frequency. The coil has to made by yourself. 36SWG wire 2.5turns only in 5mm diameter ferrite rod. Keep the circuit as small as possible. Try to use no wire in the main functional area(transistor and coil). The input from the audio output of computer/PMP/mobile is given to the biased base of the transistor. The transistor gives a RF humming accordingly to the audio input, and the FM wave is spread by the external antenna(Ariel). By using a standard T.V antenna, the range of this transmitter can go upto 1KM radius, using small(15-20cm) Ariel, it can work upto around 50M range. This circuit is most suitable for miniature FM transmitter for use in computer, mobile etc to send music to home theater system without wires, and in homemade wireless walky-talkies.
For input from low power output devices, such as mobile, computer’s sound card, the value of the unlabeled capacitor is 0.1 or 104. For higher wattage outputs, the value is 0.01 or 103. An optional microphone pre-amp can be also added in this circuit to enable it transmit vioce directly. For even more low cost, use a new bicycle spoke for ariel, get it from cycle store at Rs2.00.
Keep in mind that this type of RF projects are allowed for personal use. Commercial use of Radio frequencies(including FM), is subject to Indian law as crime.
Parts list:
Transistor BF494 x1 3/-
Resistor of various value(all 1/4 watt) x4 1/-
Capacitor of various value(all 10V, small, ceramic) x3 2/-
36 SWG copper wire for coil 1 inch 1/-
Total 7/-
Others:
Input audio jack x1 3/-
Ariel(stick antenna) x1 15/-
Battery CR2025 x1 7/-