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FM transmitter (mod4)

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, Oct 26, 2006.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The 100uF supply bypass capacitor is good with a battery, but it might not be enough to stop hum from your power supply.

    Most low dropout regulators must have a pretty big output capacitor or they oscillate. The one I used needs 100uF. A 7805 or 78L05 is fine with a smaller output capacitor, a 0.1uF ceramic disc is recommended on its datasheet.

    I mark where the tracks need cutting with a marker then cut them before mounting and soldering the parts.
     
  2. bananasiong

    bananasiong New Member

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    A bigger one is needed for power supply? At the input or at the output?
    The recommended is only 0.1uF, is anything affected if I use bigger one?
    I planed to do so but luckily I didn't, the size of the coil and the trimmer cap is bigger then I thought. Now I'm thinking how to cut the tracks with the components on the board :(

    The difference between 78L05 and 78LS05 is only the size right?

    Thanks
     
  3. Hero999

    Hero999 Banned

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    audioguru has already said, think about it, on the power suppply obviously means before the regulator.

    No it shouldn't be affected, just don't go too overboard, stick to ceramic capacitors (normally under 1:mu:F) because the electrolytics have poor high frequency charicteristics.

    That's never botherd me, if a componant is in the way then temporarly unsolder it.

    Yes, the LM7805 is only rated to 100mA which makes it cheaper.

    EDIT:
    It's output impedance is a bit higher so it's regulation isn't as good as the LM7805 but for small circuits it's more compact and cost effective.
     
    Last edited: Nov 11, 2006
  4. dave

    Dave New Member

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  5. bananasiong

    bananasiong New Member

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    Oh! I mixed up the 7805 with the low drop out voltage regulator...

    For a low drop out voltage regulator, a 100uF is needed if battery is used; bigger capacitor is needed if power supply is used, right?

    For a 7805, just stick on ceramic capacitor, for both the battery and power supply?

    Thanks
     
  6. bananasiong

    bananasiong New Member

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    How about this:
    150k for R2 and 28k for R3? This is very close to your original design.

    Is it critical or I can use either 100nF or 150nF?

    Thanks
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    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.
     
  8. bananasiong

    bananasiong New Member

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

    Thanks
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    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.
     
  10. Hero999

    Hero999 Banned

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

    GivingOutWolfTickets New Member

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    A similar question concerning electret Mics...

    Hi Folks -

    I am also in the process of building an fm transmitter...
    ( http://tacashi.tripod.com/elctrncs/smplfmtr/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
     
  12. Hero999

    Hero999 Banned

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    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.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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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:
     

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  14. GivingOutWolfTickets

    GivingOutWolfTickets New Member

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    Thanks!

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

    Take Care,
    GOWT
     
  15. bananasiong

    bananasiong New Member

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    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
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    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.
     
  17. bananasiong

    bananasiong New Member

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    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 :)
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Horray it works! (almost):D

    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.
     
  19. mr.gone

    mr.gone New Member

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    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.
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    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.
     
  21. mr.gone

    mr.gone New Member

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    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?
     

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