FM Transmitter




About: An electronics hobbyist

Warning-Check your law before opening such FM channel.

This is a short range FM transmitter module.The working is explained in short in the picture attached.This can be very useful if you are new to RF projects.This needs some patience as because you have the inductor coil yourself and the hardest part to adjust to a frequency which is not used by any local radio station.So be careful about it. Don't let anybody know that XD. This use electret mic to convert the audio signal into voltage levels which is then transmitted by my 10'' hookup wire. You can also use any long antenna salvaged from a old radio. The variable capacitor can have a range of 4-40pf. And to make the 0.15uH inductor, take about 0.6mm diameter of enameled copper wire, turn it 4 times around a 4mm cylindrical piece , just make sure everything is tight. then remove the end points isolation by sandpaper and then you can use it on breadboard of soldering board.After building the circuit ,take a screwdriver and rotate the variable cap to a value and turn on your radio and tune until you find any noise in the radio.Then try to play a song and hold the mic near it. You might need several keep your patience.I would rather suggest to use the components as suggested in the pic. So now..go ahead!!!

Step 1: Parts List

1. condenser mic

2 1megohm 1pc,10k 3pc, 100ohm 1pc,100k 1pc, 1k 1pc resistors

3 4-40 pF variable capacitor

4 2 100nf(104) cap, 1 10nf(103) cap, 1 4.7 pF cap

5 2 2N3904 npn bjt

6 0.6mm enameled copper wire

7 wires

8 breadboard solder kits

9 9 volt battery and clip

Step 2: Building

Connect the components as per the schematic in your breadboard first. You can use any 10-15'' hookup wire for antenna

Step 3: Tuning

Now the real thing. after building, take your radio first. Then take a screw driver as shown if required to rotate the trimmer capcitor. Now change slowly and slowly the Mhz frequency band knob until you hear some disturbing noise, .This will not take place easily as it is difficult to find exactly your transmitting frequency. It will be nice if you have any frequency determining meter from your lab. Or else keep your patience.Maybe you have to rotate the trimpot using the screw driver . Your transmitting frequency must be such that ,that frequency is used by no one else.I have kept a value which is not used by any radio station in my locality. So better keep a song playing near the Mic. and then tune it. After several attempts, You Will be able to hear the noise along with some few disturbances(some AM signals). If you find note the frequency. Then after wards it will be easier.

Step 4: Soldering

Now remove all components from breadboard and start to solder. Keep the schematic beside you.Carefully solder all components. You will hear a better noise on a soldered board rather than your breadboard circuit. Keep the length of components as short as possible.

Step 5: Finally

So you are done. Attach a 9V clip and keep your songs or talks playing.......



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    6 Discussions


    Question 2 months ago on Step 5

    Sir how to set the frequency band for fm transmitter have any formula??


    Question 3 months ago on Step 5

    after making the circuit, how to select the frequency value on receiver of FM

    1 more answer

    Answer 3 months ago

    The capacitance of the variable capacitor will determine the frequency value of transmitted fm signal. Actually this requires some trial and error methods. You have to take a radio and set to a particular frequency such that it is being used by no other channel. Now slowly rotate the variable cap and you'll find a point where the radio sound becomes dead silent or it can make a very high pitched sound nearby the frequency. That point will be that operating frequency. The cap value has a highest value of 40pF


    5 months ago

    Do you have the computations for your circuit? Can I see it. Thanks in advance.


    1 year ago

    Interesting design. However, If you were going to use this as a baby monitor, I would either make in transmit below or above the typical FM band that people use everyday. And that is because then you could have more security if it transmits in a non-typical frequency. But then you have to adjust your receiver to the same frequency out of the typical FM band as well. Just an idea.

    1 reply

    Reply 1 year ago

    Yes that's good, also I used a frequency to test it which was not used by any local radio station. For making it beyond normal typical frequency...normal radios might not have that frequency. In that case a separate receiver must be made...I will try to make that one in a few days.