FM transmitter...help!

[elfvenlord]

Heya guys.I just built a FM transmitter but I didn't get anything through my reciever.
I've never built any radio stuff so I dont really know what to check for.Could anyone suggest some simple tests I can do to find the fault(I only have a multi-meter so I dont think I can check frequency)

Does an electret microphone in series with a resistor provide a voltage divider or does it only alter the frequency? I tested the voltage from the Mic and it remained constant when I spoke,is that normal?

Thanks for your help

 

[panic mode]

It is normal because your multimeter is too slow to react to changes.
You could see the waveforms if you use an oscilloscope. There is one that uses PC sound card. Before you can transmit anything, try to build oscillator. This is essential part of any transmitter. Everything else is optional. If you want to modulate it, that's another option. This is what
you use to transfer voice for example but not all transmitters transfer voice. Check for some of the treads about FM transmitters for more details.

 

[elfvenlord]

Hi Panic
So you think I should try to build an oscillator e.g.using a 555 timer and transmitt the signal and then try to tune into it using my reciever.I'll give it a go.
Thanks

 

[elfvenlord]

:shock: I was looking through some recent threads today and came across a thread that said that building FM radio on breadboards was a no no.
I built my transmitter on a breadboard so do you think that maybe that might of been the problem.
I was also wondering if it was alright to use a strand of insulated wire from a 6 core cable ( ? ) for the arial.Does it need to be a solid wire.not twisted.

 

[Nigel Goodwin]

:shock: I was looking through some recent threads today and came across a thread that said that building FM radio on breadboards was a no no.
I built my transmitter on a breadboard so do you think that maybe that might of been the problem.

Probably, it's a waste of time building RF circuits on breadboard, particularly VHF ones.

I was also wondering if it was alright to use a strand of insulated wire from a 6 core cable ( ? ) for the arial.Does it need to be a solid wire.not twisted.

That's fine, any wire will do. The advantage of solid core is that it can be free standing, if it's thick enough.

 

[Rescue1]

RF Circuits are very finicky,espically when there are few Components that make up the oscillator.A breadboard is a big no,and keep all leads as short a possible with every component.Its funny though,I am no advanced designer but I have tried and designed many different small transmitters and have found that some will work well and some just won't even when they are made the same exact way.Good luck and be patient...

 

[elfvenlord]

Hiya Guys
I'll stay away from the breadboard in future when trying to make any radio circuits and I'll rebuild this one on a PCB.
Thanks for your help.

 

[elfvenlord]

by the way does a trimmer capacitor (with 3 pins) have the same pin configuration and work in the same way as a variable resistor. Do they have to connect in a particular way i.e.such as an electrolyte capacitor?

 

[Nigel Goodwin]

by the way does a trimmer capacitor (with 3 pins) have the same pin configuration and work in the same way as a variable resistor. Do they have to connect in a particular way i.e.such as an electrolyte capacitor?

Capacitors only have two connections, if your trimmer has three, two of them will be the same - check with a meter, or simply your eyes.

They are not polarity sensitive, although you should try to connect the moving plate (where the adjuster screw is situated) to the 'cold' end of the circuit, usually either ground or HT.

 

[panic mode]

that's right...
many large components have more pins than nececary simply to
provide good mechanical stability (elcos, transformers, relays, ...).

 

[elfvenlord]

thanks
I could'nt see inside the trimmer capacitor so I was just checking

 

[elfvenlord]

Hi again

you know the funny thing about electronics is you can understand a circuit and know how it works but when it comes to building the damn thing, its a different matter.
But anyway I got round to building my transmitter but I built it on PCB stripboard (still didn't work :lol: ) but I was told that this is also bad for radio circuits...Is that true?
Thanks for the help

 

[Nigel Goodwin]

Hi again

you know the funny thing about electronics is you can understand a circuit and know how it works but when it comes to building the damn thing, its a different matter.
But anyway I got round to building my transmitter but I built it on PCB stripboard (still didn't work :lol: ) but I was told that this is also bad for radio circuits...Is that true?

Yes it's true, but it can work OK - be careful how you lay it out, and be sure to cut the veroboard strips as short as possible (don't leave full length strips running to the edge of the board). Also, it's probably a good idea to remove all unused strips and partial strips, you do this by soldering a biggish blob of solder on the end of the strip and giving it plenty of heat, after a few seconds the track will lift and you can pull it off with pliers.

 

[elfvenlord]

thanks Nigel i'll give it another go today.

 

[chemelec]

Also Useful, Post a Schematic of the Circuit you are Building.

There may be some Errors or other problems in your circuit that were not aware of.

Gary