28k is not a standard 5% resistor value, but two resistors in parallel will be close.

Try it with 100nF. If the radio doesn't produce enough treble audio frequencies then add a 47nF capacitor in parallel with it.

__________________
Uncle $crooge

 

Do you mean, I can do what I said?
I'm using a 150nF capacitor.

Thanks

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Superman returns..

 

You can't buy a 28k resistor. 27k is too low and 30k is too high.
A 200k and a 33k resistor in parallel makes 28.33k.

__________________
Uncle $crooge

 

Yes you can, it's an E96 (1%) value.

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Hi Folks -

I am also in the process of building an fm transmitter...
( http://tacashi.tripod.com/elctrncs/s...r/smplfmtr.htm )

and I also have a question related to installing the microphone...I bought one of those three-wire electret mic deals from Radio Shack and am wondering how exactly it would be installed in the above circuit..the mic appears to have a red, a white, and a ground wire - my question is which wire would go where in the above schematic? Any advice you could give would be appreciated!

Thanks,
- GOWT

 

Red to +V
Black to 0V
White to the input

After you've built this, I strongly recommend building a better circuit since this one gives very poor quality sound.

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The polarity of the input capacitor is unknown, so use an unpolarized 0.33uF (330nF) film capacitor.
Your schematic shows an optional 3-wire electret microphone. I guess the colours are connected like this:

Attached Images

simple FM transmitter.PNG (7.9 KB, 25 views)

__________________
Uncle $crooge

 

Thanks fellas - the explanation and particularly the revised diagram was a big help!

Take Care,
GOWT

 

Oops!! There is 82k but not 28k, sorry about that. So I'll use 180k for R2 and 33k for R3.

Am I correct with these:
Thanks

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Superman returns..

 

If you use a 9V battery then you need a low dropout regulator because the battery voltage quickly drops too low for an ordinary 5V regulator.
Most low dropout regulators need a 100uF output capacitor so that they don't oscillate. I used 100uF for C8.

A battery needs a capacitor across it to stop its voltage from flucuating. I use 100uF for C11. A power supply needs a capacitor across it to stop hum, use a capacitor bigger than 100uF if there is hum. These capacitors are across the battery or power supply which are at the input of the regulator.

For a 7805 regulator, the datasheet recommends a 0.33uF or more capacitor at the input (use 100uF or more), and a 0.1uF ceramic disc at the output.

The regulator also needs 1000pF ceramic disc capacitors at its input (C10) and output (C9) to bypass radio frequencies.

__________________
Uncle $crooge

 

I'm using a 7805, and there is a 100uF also at the output.
I've finished putting all the parts and cut the tracks, I used 100MHz oscilloscope to measure the antenna and tuned C13 until the amplitude is peak. Then I tried to tune C6, I can see that both frequency and amplitude are changing. But I think what I saw is just the carrier, without signal. I've shouted at high frequency, roar like a gorilla at low frequency and even bark like a dog, but nothing changes at the oscilloscope. I used a auto tuning radio and tune at around 96.xMHz which is not being used, and tune C6, it is affected at some where. When I touch C6 or L1 with hand, I can hear some response from the radio.

I think the mic is not working? I'll replace a mic and use a manual tuning radio to try it soon.

Thanks

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Horray it works! (almost)

Connect a separate antenna to the oscilloscope to tune C13 for its peak. If you directly connect the 'scope to the antenna then the capacitance of the 'scope's cable will be in parallel with C13 and will change the tuned frequency.

I don't think you will see modulation on the carrier with a 'scope because it is very small.

Measure the voltage at the microphone and at the collector of the 1st transistor Q1. It should be about +2.5V. Connect the 'scope there to see audio.

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Uncle $crooge

 

First of all...those are called condensor mikes. It's made with a piezo-electric crystal gluded to a diaphram. The piezo-electric crystal produces electricity from vibration. It is on the order a few micro-volts so you will need to amplify the signal with an audio amplifier. I would use an OpAmp for maximum gain. Then you capacitively couple the signal to the varactor diode.

 

No it is not! Read about it in Google.
It is an electret microphone which is a condensor mic (a diaphram that vibrates and forms a high-voltage divider) with high voltage built-in on the electret material. A condensor mic needs an external 48V power supply. The electret mic is newer and different than a condensor mic. An old fashioned piezo "crystal" mic is also completely different.

The output level of an electret mic is not just micro-volts but it is about 10mV with a normal talking loudness at 5cm. It has a FET transistor impedance converter inside that needs a small DC current. You don't want the max gain of about 200,000 from an opamp.

This FM transmitter doesn't have a varactor diode. The capacitance of the oscillator transistor is changed with modulation.

__________________
Uncle $crooge

 

Oh Lord have mercy...lol! I try to help and what do I get? I'll skip that part...lol. I don't know about your elecret thing but I do know a condensor mike requires no power. So I have to assume an electret has a built in amplifier.

Now for sort of question number two. No varactor huh? Well than what do you propose changes the capacitance? Is there a diode in the circuit? The P/N juction will change in shape with it as well but varactors are hyper-abupt. Dude, before you go getting your feathers all ruffled, let me just explain a few things. I am a licensed ham 34 years. I have built every bit of my equipment I use now. I also made and sold FM micro-broadcasters. I'm just trying to help. Please don't attack me like I'm an idiot. Trust me I am far from an idiot in radio electronics. I'll be glad to help when you understand that the student is not ready to challenge the teacher just yet. Comprendo?