I recently built an FM transmitter that uses a combination of coil and capacitor as an oscillator to generate the carrier frequency.

I built this transmitter:

enter image description here

Now my problem is that I find it difficult to wind coils properly. The chance of winding properly is very low. I tried to reconstruct nearly 10 to 15 times and it worked hardly twice or thrice. Also the received signal was very faint and consisted of lot of distortion.

Therefore, Now I would like to know if there is a way to build a transmitter that does not involve any coil. I googled it and found that I could use a crystal oscillator or an IC like 555 or LM385, etc.. How can I do it?

  • 1
    \$\begingroup\$ 555 won't work at Band II frequencies. Not even close. Also, in that circuit, C4 should be across L. In practical terms that reduces stray inductances in your tuned cct, which is probably your real problem. \$\endgroup\$ – Brian Drummond Dec 4 '15 at 10:46

I made this circuit before and it works properly. Here are the issues I met, they might be useful for you:

1- Don't use a battery lower than 9v. You can use a higher voltage.

2- Using a very long antenna will help you a lot. If you don't have a long wire, you can connect an aluminium pot :)

3- The frequency is not stable. so, Each time you turn on the circuit, you should tune your receiver (Change the frequency of your receiver) until you get the best result.

4- You should Not use a Mic as "Audio In". And I think it's better to rise the volume (Volume up) of the input audio.

5- The Capacitor C3 is important.

6- Don't worry about the shape or the number of turns of L1. Just bring a pen and turn a wire around it 5 or 8 times. and If you need to change the transmitter frequency, Change the values of C4 and C5.

7- I actually used a lower value of R3. I used 100 ohm instead of 470 ohm.

8- After all of the previous steps. If it still does not work, Change the value of R2 using a potentiometer.

9- My transistor part number is S9014. I did not use 2N3904 because I don't have it.

If you need another circuit, the following circuit is easier to construct and it works too:

enter image description here


fastest a 555 do is 700khz, so it's not FM. learn how to do it correctly with a coil is the best way, or you can buy a kit like this and start with it. http://www.ebay.com/itm/291568842436?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649

It seems to me that your coil is okay, but you have used a wrong npn transistor, for this kind of FM circuit, you need a high frequency transisor such as the 2N3904, not any npn you got in hand.

  • \$\begingroup\$ the circuit uses a 2N3904 transistor. please have a closer look \$\endgroup\$ – Sâu Dec 4 '15 at 12:40
  • \$\begingroup\$ A reminder that FM is (frequency modulation). Not to be confused with what is commonly called the FM band (88-108 Mhz) (USA). I can certainly make a 555 perform frequency modulation. Not at 100 Mhz of course. However, the OP didn't specify what frequency he wanted his transmitter to be (implied by his coils and capacitors). \$\endgroup\$ – Marla Dec 4 '15 at 14:07
  • \$\begingroup\$ I saw it. for a single radio transmitter like that, you need some kind of feedback from the output to the input, which it doesn't have. click into the link I showed you, and there are 2 circuits there. One of them is also a simple transistor transmitter. \$\endgroup\$ – Atmega 328 Dec 4 '15 at 17:36
  • \$\begingroup\$ in that case, you need a special radio receiver. \$\endgroup\$ – Atmega 328 Dec 4 '15 at 17:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.