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need help in fm transmitter


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i got a very simple fm transmitter yet very stable, low power and have reasonable range (the author claims)

i'm having a problem to get it working.. :(

I don't know if the 28pF capacitor which is connected to the collector has something to do with the parallel resonant circuit.

Also, what's the use of 2pF to connect the antenna?

Hopefully someone here have good knowledge bout transmitters..
Thanks a lot!



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Hi Keile,
Of course the 28p cap affects the parallel resonant circuit, but may have a value that is much too high. What is the total range of values of the trimmer cap "Tr" that is in parallel with it?

Have you seen the many FM transmitters in the "RF Circuits" of our Projects Section? Click on Projects at the top of the screen. Most of our transmitters use the same oscillator as yours, except for the following items:
1) No 28p cap, but just a trimmer cap. Try that and compare the range of yours.
2) No 62p cap at the emitter. Try removing yours.
3) 9V battery, not just 1.5V like yours has. Try 9V.
4) A microphone preamp. Scream into your mic or add a preamp.
5) Some have the antenna connected directly to the collector, others have a small cap like yours. Some have the antenna connected to a tap on the coil. Try shorting your 2P cap.
6) Most have more than only 3 or 4 turns of the coil. Try 6 or 8 turns, tightly spaced but not touching.

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thanks a lot for explanation audioguru!

actually for the mic i wanted to replace it with a 555 timer to generate tone so an amp might not be needed?

also, the trimmer capacitor has no value.. i don't know.. maybe it is just optional..

btw, i got the schematic at this site: http://tacashi.tripod.com/elctrncs/smplfmtr/smplfmtr.htm

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Hi Keile,
Didn't you install a trimmer cap? It is very important and the author's PCB shows a value range from 5pF to 40pF. It is used to tune the transmitter's FM frequency.

Did you jump here from another post about remote control? This simple circuit's FM frequency changes as the battery runs down and the author says, "slowly", so you will lose control unless you re-tune it (how?) or voltage-regulate it.

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to audioguru:

uhh sorry.. actually i just want this thread to focus on transmitter thing.. and to ask all about its variations.. i have researched lots of transmitters already and always wondered why it differed in many ways..

wow.. i didn't know that the capacitor has a value of 5 to 40pF.. sorry i don't know how to distinguish different types of capacitor.

to MP:

yes.. when i'd seen the circuit i think that it is what my project should be.. though there are some components i didn't understand like the varactor diode and the JFETs... so u know, i can't use it in my project until I understand what those components do.

btw, thanks for the help.. i really appreciate it :)

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  • 4 weeks later...

:) go to talking electronics web site you may find what you seek, in transmitters car trackers etc and other things, the circuit looks a bit similiar or other to the beeper bug circuit that sends a tone to a nearby radio, this transmitter can be used to track stolen objects via the radio

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