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FM Transmitter

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Is there a simple way to remove the electret mic and replace it with my audio jack?
Remove the mic and R1.
Add three resistors to attenuate the stereo line-level inputs and mix them into mono.

I used enamel-insulated wire that is 1mm thick to make my coils. I got the wire from a speaker's crossover coil. I spaced my coils away from the board and away from other parts and wires.
 

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Hi Audioguru,

I like the design of your FM transmitter. I can see the preemphasis cap is C4 and calculate its turnover frequency is ~2,260 Hz, depending on the tolerance of C4.
I use 5% tolerance film capacitors. C4 has one value for Europe and Australia and a different value for North America.

How do you generate the modulation? Does C7 have variable capacitance with voltage? If so, is it ceramic X7R or what type of cap did you use? I don't see any other components that could generate the modulation.
The modulation changes the current in the oscillator transistor Q2 which changes its collector to emitter capacitance. When its capacitance changes then its frequency also changes which is FM. The changing current also creates a little amount of AM that FM radios ignor.
Some simple FM transistors use an ordinary diode or junction of a transistor as a voltage-controlled-capacitor.

Bob[/QUOTE]
 
I was off on the ID of the coils. They are about 1/8". Hmmm... don't know what to use as a former to wind them; pencil is too wide.
 
Just get yourself a proper transmitter from here FM Transmitter Kits and Radio Transmitters - Aareff Systems . I use the 30 watt model with an half wave vertical and get almost 8 miles range lol.

Sounds like a great idea if I want Ofcom to slap me with a massive fine and a criminal record for broadcasting without a license. If you'd have read the thread, you'd have seen that I want to build something, not buy a finished product.


BrownOut - If it's 3.2mm (1/8th inch) it sounds like you wrapped it round the metal shaft of a screwdriver.

Audioguru - I assume it's OK to use a potentiometer between the L and R pins to adjust pan, then have a resistor coming from the pot's slider? I'm not familiar with the output characteristics of standard audio jacks, could you suggest appropriate values for the resistors and explain the reason for those values?
 
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I wrapped my coils around the smooth side of a drill bit.

You can use a balance control (L-R pan) if you need one but usually it is not needed.

This circuit is too simple for this thread to go on and on and on and on and on and on and on and on and on and on and on and on and on and on and on and on for two pages.
Simply make a simple FM transmitter and tell us how well or how poorly it works.
 
The last FM thread was 5 or 6 pages long. The modulation is these simple circuits exploit the depleation capacitance in the transistor's B-C junction, which is essentially a reversed biased diode, not unlike those circuits that use an additional diode for modulation. These reversed biased diodes have a characteristic capacitance, the value of which is modulated by the reverse voltage, which is in turn modulated by the modulating signal. The modulator itself has very little voltage gain due to the fact that the collector's impeadance is very low at the lower modulating frequency. Thus, the signal is nearly all frequency modulated. In the O/P's circuit, the frequency determining components are C5, C6, L1 and the voltage dependent capacitance of Q2's B-E junction. If this network has high enough Q, then it should oscillate. That's why you want to make your own coils. BTW, one of the determanents of the coil's Q is it's resistance. You should use at least 1mm wire. You can go bigger, don't go smaller. Wind the max number of loops. You can trim some off later for tuning. Much messier to try to put them on.

To the O/P: just keep asking your questions and ignor all the noise. That's what this forum is for.

PS: I poked around and found a cleaner design ( see insert ) This one has all the component values specified, and shows a simple way to connect the antenna. Others will say it isn't worth building, but it shoud be fine for a first transmitter. You can get it working and add the other stuff later.
 

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Well I already bought the parts for audioguru's transmitter, so I'll give it a go.

When I get them all I'll post a final design schematic including my supply and any modifications I've made for your review. Don't want it going horribly wrong! :D
 
I thought you were building the original curcuit. I was concerned because not all the components were specified, and the drawing was wrong.
 
The very simple "Miniature FM Transmitter" does not work. I fixed it.
Somebody made it and asked on the forum why it didn't work. I explained all its problems and made it to prove my theories about it then I improved it in 4 stages of modifications until my Mod4 circuit was finished and was working extremely well.

My Mod4 circuit uses the standard value of 30k for R3. It will have distortion if 33k is used.
 
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