We know that building of Fm transmitter for first time is quite hard and we get confused about the components, parts, design, pcb, transmission frequency, inductor value and making of inductor , polarity of the capacitor and many things.
Here i am going to explain you a building of simple fm transmitter using a two transistor . I will try to explain as possible as i can. lets starts with the components required.
C1 & C2 =4.7uF(electrolytic)
C3,C4 &C7 = 100nF( ceramic 104)
C6 = 4.7 pF (ceramic )
C5= 10-30 pF (Variable capacitor or also known as TRIMMER )
R4 & R5 =10k
R6= 100 ohm
L1= 8 turns of 22 Gauge copper wire around a simple ball pen.( I will upload a photo of it for more idea)
Battery (Power Supply) = 12-9 V DC (Always use battery and avoid adepter so i will
reduce noise in transmission )
HOW FM CIRCUIT WORK ? :
The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz. Audio from audio jack is fed into the audio amplifier stage built around the first transistor. Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit (the 8 turn coil and the trimcap) by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is connected in a Colpitts oscillator circuit. It works on the principal of COLPITTS Oscillator.
ANTENNA IS MUST IN THE CIRCUIT:
A impudence matching ANTENNA is must required in the circuit,without Antenna circuit will never works .Generally we uses a telescopic antenna . which through the waves in the atmosphere .
A SHORT NOTE ON INDUCTOR :
It is easy to make take an 22 Gauge enameled copper wire and wound it on the ball-pen and make 8 turns on it tightly. i shown in photo below.
FM CIRCUIT CALIBRATION ( IMPORTANT PART :- PLZ KEEP PATIENCE MAY BE OF CIRCUIT ) YOU NOT GET SUCCESS IN
FIRST TRY OF
Place the transmitter about 2-10 feet from a FM radio. Set the radio to somewhere about 89 - 90 MHz. Walk back to the Fm Transmitter and turn it on. Spread the winding of the coil apart by approximately 1mm from each other. No coil winding should be touching another winding. Use a small screw driver to tune the trim cap. Remove the screwdriver from the trim screw after every adjustment so the LC circuit is not affected by stray capacitance. Or use a plastic screwdriver. If you have difficulty finding the transmitting frequency then have a second person tune up and down the FM dial after every adjustment. One full turn of the trim cap will cover its full range of capacitance from 10pF to 30pF. The normal FM band tunes in over about one tenth of the full range of the tuning cap. So it is best to adjust it in steps of 5 to 10 degrees at each turn. So tuning takes a little patience but is not difficult. The reason that there must be at least 2-10 ft. separation between the radio and the TRANSMITTER is that the Transmitter emits harmonics; it does not only emit on one frequency but on several different frequencies close to each other.
Don't leave a circuit by just trying sometime try and try until you get some distortion in the fm receiver. once you get some distortion in it. that means your fm circuit is working. and if not then try to check all components and the soldering of your circuits and power supply .if you getting confusing about the power supply put one 2 Volt LED in series with 1K resistor in parallel with battery so you can easily get the on-off you fm transmitter.
ABOUT THE RANGE AND QUALITY (STRENGTH) :
I personally maid this project with great success i got a 1 km perfect range with proper quality of the signal after that up to 1.5 km small distortion starts and after that quality factor go down and then lost it. you should have to maintain the circuit in the metal case to protect it form the electrical isolation .
The range and quality are inversely personal to each other. Both can be increased by increasing the input voltage upto certain limit.
I JUST UPLOADING THE PICS FOR YOUR REFERENCE THAT I HAD MAID IT LIKE THAT. YOU CAN TRY IT BY YOUR OWEN WAY BY MAKING PCB OR ON SIMPLE HOLED PCB.
|I used the simple copper plate and glue the other copper parts to place it on it|
|A telescopic antennae of proper height.|
|A inductor of the copper|
A SIDE PHOTO OF THE CIRCUIT
If you have any problem and any question about it then you are always welcome.