FM Transmitter

[crivoli]

I have built this simple FM transmitter.

http://www.uoguelph.ca/~antoon/circ/fmt1.htm

I have attached a picture, sorry it isn't the best picture


Is there any other way I can test this without trying to find the transmission on my radio?

Thanks

View attachment 39006

 

[audioguru2]

The coil is spread out way too much. The circuit operates at 100MHz where 5pF is a lot. Your breadboard has about 30pF between its tracks so won't work at such a high frequency.

Use a pcb or Veroboard with very short traces and an FM radio to listen for it.
Mine looks like this and works very well:

View attachment 39007

 

[crivoli]

Hey audioguru. Thanks for the reply.

Can you send me a copy of your schematic. The one I copied off of says not to use an electrolytic caps, but I don;t have any ceramic that go higher than 550pf, so I substituded other for them.

Also, I am not using magnet wire, I am using plain copper wire. I am not sure it will work unless I get the magnet stuff.

If I could get a copy of your schematic that would be great.

Also, if I use pcb or Veroboard do I have to solder the parts before I can even test it? I am not familiar with those types of boards.

Thanks again
newb

 

[audioguru2]

Your coil is supposed to be close wound. Yours is way too open wound.

Somebody had a transmitter like yours that didn't work. I built it and its frequency drifted all over when the battery voltage dropped, when the temperature changed and when something got near it or moved away from it. When it did work it sounded like it didn't have any treble sound.

So I fixed it with a low dropout voltage regulator, an RF output amplifier and treble boost like FM radio stations use.

I used Veroboard which is also called stripboard. It has copper strips running in one direction. I cut them with a drill bit and use the parts and a few jumper wires to connect the circuit together.

If my schematic is not clear because this site reduced its size to fit this page, then download it and open it. 

View attachment 39008

 

[crivoli]

audioguru,

i appreciate your quick response. This schematic looks a bit more complicated than I would like to build. Are there any stable transmitters that are a little less complicated?

Also, Is building a receiver for one of these transmitters very hard?

I would like the transmitter to send the audio from the microphone to a receiver and amplify the sounds through an 8 ohm speaker. I have never seen a schematic for a simple receiver.

Thanks again

 

[audioguru2]

This schematic looks a bit more complicated than I would like to build. Are there any stable transmitters that are a little less complicated?

A stable FM transmitter needs a voltage regulator and an RF output amplifier. For good sound it needs treble boost. A less complicated design won't be stable and won't sound good.

Is building a receiver for one of these transmitters very hard?

It is difficult to find special parts for a radio receiver. Cheap kits were sold but worked very poorly.
You can buy a cheap radio that works poorly, or buy a good one for just a little more money.

 

[crivoli]

So am I understanding this correctly?  You are saying that a radio is pretty much the only thing that should be used as a receiver?

If so, what is a good solution for a wireless intercom, or a baby monitor? Do those run on FM transmitters or some other type of transmitter?

Thanks

 

[audioguru2]

An FM radio, car radio or home stereo receiver can receive small FM transmitters well.

Wireless intercoms and baby monitors have their own receivers that operate at frequencies different from the FM broadcast band. FM modulation is usually used since it avoids interference.

 

[crivoli]

So you still need to build an FM transmitter for a baby monitor device, but the transmitter is not sending a frequency between 88-108 correct?

I am wondering if this is even worth trying!!  :-\

 

[audioguru2]

So you still need to build an FM transmitter for a baby monitor device, but the transmitter is not sending a frequency between 88-108 correct?

Yes, but you would also need to build a receiver with the same frequency.
I don't know what frequency they use. I don't think it is the 88Mhz to 108MHz FM broadcast band because in my area, all frequencies are used by radio stations.

 

[crivoli]

Have you ever seen an example schematic for a transmitter/receiver that run off of something other than 88-108?

I am really wanting to make this thing wireless, and with the limitations of IR I don't think that will be possible to use.

Should I pick up a book on RF and get to know it better?

Thanks again

 

[audioguru2]

It is fairly easy to make a radio transmitter.
It is difficult to make a radio because it has many more parts than a transmitter and some of the parts are available only in huge quantities.

 

[crivoli]

OK, that makes sense. I appreciate all of your great help. I may look into something else to try out with this. I am glad you know what you are talking about

Hey, one more thing..... I have seen FM transmitter and receiver IC's.  Could this do the trick for me? If one IC sends the signal and the other IC receives it? And if so could the signal be amplified if it were audio through the LM386?

I found a IC receiver here, is this anythign close to something that could work?

http://www.semiconductors.philips.com/acrobat_download/datasheets/TDA7000_CNV_2.pdf

Just something to think about

Thanks

 

[audioguru2]

That receiver IC hasn't been made for many years and isn't good. Some stores might have it or a newer one that also doesn't work well.
Try one. It might be OK for your application.

 

[crivoli]

I took apart the remote from my sons remote controlled car ( he is 2, and the remote is very basic ). It is a 27MHz remote and I tested the distance and it works very well throught my house.

Shuold I try building a very basic 27 or 49 MHz remote used in RC cars or walkie talkies? I am sure I can find some simple schematic for that?  Any thoughts?

Thanks

 

[audioguru2]

Did you want to transmit and receive a voice or music, or just an on and off command?

 

[crivoli]

I am wanting to transmit voice from a mic to a receiver that would amplify it out of a speaker.

I assume this can be done with a 27MHz signal, am I wrong?

I found some schematic here, can you tell if they work: http://jap.hu/electronic/rf.html

I can't really read them very well...... can you help explain them a little.

Thanks

 

[audioguru2]

The toy car transmitter and receiver just makes the toy car go and stop. No sound.
The walkie-talkie might work but there isn't any text.

An expert might be able to make these circuits without a detailed parts list. The coils must be made exactly for their function. A pcb must be used with the proper parts arrangement for radio.

 

[crivoli]

Well, seeing as how I am obviously NOT an expert ( yet ), is there any practical solution for sending audio wireless to a receiver that I could either make myself or obtain from some other  reasonable source