This Tiny FM Station can transmit the Songs from an MP3 Player though out the home. Its range is around 100 Meters and operates in the 87.5 MHz frequency. The songs can be heard in a Mobile Phone, FM Radio, Music system etc placed in any room. The unique feature of the FM Transmitter is that it has no Trimmer capacitor . Instead a fixed capacitor is added with the coil to form the Tank circuit. So there is no need for tuning the FM Transmitter. The FM radio will pick its frequency during scanning.
You need the following components
1. 47 K Resistor – 1
2. 100 Ohms Resistor -1
3. 0.1 uF ( 104 ) Disc Capacitor -1
4. 0.001 uF ( 102 ) Disc Capacitor -1
5. 33 pF Disc Capacitor -1
6. 10 pF Disc Capacitor -1
7. 0.22 uF ( 224 ) Disc Capacitor -1
8. Transistor 2N 3904 -1
9. Coil – ( 22 SWG – 5 Turn – 3 mm Inner Dia )- 1
10. 9V Battery Snap -1
11. 9V Battery -1
12. Mono Jack pin – 1
How it works?
The Tank circuit comprising the Coil (L1) and the 33 pF Capacitor (C3) along with the transistor T1 forms the FM Modulator for the audio transmission. The audio signals from the MP3 Player are coupled to the base of T1 through the capacitor C1. These Audio signals will be modulated by T1 to the FM frequency 87.5 MHz where no FM station is usually present. Modulated FM signals will be send to the air through the antenna connected. Resistor R1 biases the base of T1 and the capacitor C2 removes noise. C6 is a power supply Decoupling capacitor.
The Air core coil is made up of 22 SWG (Standard Wire Gauge) enameled Copper wire. It has 5 turns with an inner diameter of 3 mm. You can wind the coil around a refill tube of a dot pen. Keep the windings of the coil close. Do not separate them.
Assemble the circuit on a small piece of Perf board with a size equal to the 9V battery so that you can fix it on the side of the battery with double sided tape or glue. Solder the components as close as possible to get maximum performance. Lead length of the capacitors should be minimum. Solder the Jack pin to the input as shown in the diagram with correct polarity. Connect Battery snap to the circuit with correct polarity.
Antenna can be a 30 cm plastic wire or enameled copper wire made in the form of spring. You can also use a Telescopic antenna used in Pocket radio.
9V Battery is used for the circuit. You can also use 6V battery. It even works in 4 volts.
Since there is no Trimmer capacitor in the circuit, no tuning is needed. Power the circuit and connect the MP3 Player in the playing mode. Then Scan the radio. Radio will automatically align with the FM Transmitter and you can hear the song in the radio. Orient the FM transmitter in a position to get good reception in the Radio. Do not keep in hand to avoid noise picking. If properly aligned, its range can be up to 100 meters.
You can modify the circuit into an FM transmitter by connecting a Mic and a 47K biasing resistor in the circuit. So that your speech can be heard in the FM Radio. See image below to make the FM Transmitter.