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FM transmitter (mod4)

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

  1. Hero999

    Hero999 Banned

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    That will work, I don't know why you've got two capacitors in series, C12 and C14 which will make one 16.6pF capacitor. My guess is that you just connected the two circuits together without thinking about it.
     
  2. chingyg

    chingyg New Member

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    No, its that I doubt there is a 16.6 pF capacitor. you get 10pf 12pf 18pf etc, just that I thought it is an odd number.

    and I dont think It would make much of a difference, would it? I'd use a 15p instead.
     
    Last edited: Aug 15, 2009
  3. mvs sarma

    mvs sarma Well-Known Member

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    After all the air-cored inductor could also be tweaked by little stretch or compress. I feel it is not that serious a matter. one can keep nominal capacitor and manage the inductor value to be within the required value. Final adjustment can always be done by trimmer.
     
  4. dave

    Dave New Member

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  5. chingyg

    chingyg New Member

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    Hey, I found this RF signal strength meter on the net, You think it would work on mod4 like this:
    fm.GIF
    But I cant understand why the LED in this circuit is connect the way it is, isnt the cathode suppose to be connected to the ground, not to the emitter of BC549?
     
    Last edited: Aug 15, 2009
  6. Hero999

    Hero999 Banned

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    Look up capacitors in series in Wikipedia and you'll see what I mean: C12 and C14 form a 16.6pF capacitor.

    Capacitor - Wikipedia, the free encyclopedia
     
  7. chingyg

    chingyg New Member

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    I know what you meant, 1/C t = 1/Ca + 1/Cb ..........
    Its 16.62222pF
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I did not measure my air-coils. I simply copied how they are made from another circuit that works. The coils use 1mm enamel-insulated wire with the turns closely spaced and they were never tweaked. maybe they are 0.1uH or maybe 0.23uH but who cares, mine work perfectly. I spaced the coils away from any other part and spaced away from the circuit board.

    The trimmer capacitor that is set to minimum at 5pf might actually measure 9pF to 12pF in the circuit with stray capacitance added. Then the max frequency is much lower than the calculator shows.
     
  9. transistor495

    transistor495 Member Forum Supporter

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    Hi everyone,

    You know 2N3866 is capable for 5W maximum power dissipation and I assume it needs at least 500mW of input for that. If the mod4 design outputs around 200mW of power(am I correct?), a final 2N3866 stage as Q4 might give a blasting 1W or more of power. I would be rather interested in such a simple add-on modification because in your circuit 2n3866 won't be efficient(ie, input is only below 50mW). However it should work.

    '2N3386' in your circuit is a JFET :)

    I think 2N5109 has less maximum power dissipation than 2N3866. So the design is not good.

    Another good modification I guess mode4(200mW)+2N4427(1W)+2N3866 = 5W plenty of power enough to be get traced by the police :D
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Somebody measured the output of my FM tx mod4 circuit at only 20mW. They souped it up and got 40mW.
     
  11. chingyg

    chingyg New Member

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    So One should rather leave the 2N3904's

    It'd be 2N3904 -> 2N3904 -> 2N5109

    I'd leave the 2N3866 alone then, because if the output of the mode4 not very high, it would be a waste to use 2N3866.

    because regarding to power, 300-400mW is the best I'd hope for. I am not opening a radio station after all.

    Talking about traced by the police, hahaha. I'd rather keep the power nice and low, because I couldn't even find a heat sink for TO39 and I hate to get traced by the cops (happened before)

    and Sorry for the 3386, just a typing error.
     
    Last edited: Aug 15, 2009
  12. chingyg

    chingyg New Member

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    Hey, I built a prototype today. out of the confusion it worked apparently on 96.2MHz, then after about 1 minute, the signal is suddenly cut and sound from a radio station started to appear on my receiver. then after hours of tuning I still couldn't get it working again. another strangle thing is that the voltage are abnormal, the output of the 7805 and the 7809 show 7.45V regardless. I have no Idea why it does that.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If you connect the pins of a 7805 backwards then its output will not be 5V.
    If you overload a 7809 then its output voltage will be too low.
     
  14. chingyg

    chingyg New Member

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    but they are always Vin Gnd and Vout right? is there a way to test them?
     
  15. mvs sarma

    mvs sarma Well-Known Member

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    As you keep the regulator facing you and leads downwards, the order for 7805 and 7809 is In, Gnd, Out
    while for 78L05 and 78L09 the input and output are reversed.
     
  16. Hero999

    Hero999 Banned

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    The pin out is on the datasheet which can be found using Google. I don't see why you're having a problem.
     
  17. chingyg

    chingyg New Member

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    No, I think I found out whats wrong, my meter is messed. it measures everything as 7V.
     
  18. chingyg

    chingyg New Member

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    I found out that there is a huge transmitter about 100m from my house, their signal is all over the radio, So I could tune the circuit property, and the trimmer capacitor I ordered online hasn't arrive yet. I had to live with a set of 10 discrete pairs of capacitors, haha.
     
  19. chingyg

    chingyg New Member

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    Do you guys know ant site, where you can find substitutes for semiconductors.
    I can't get the 2N5109 for the last amplifier stage, I tried to order 2N2219A, but it is out of stock now, would the 2N2222A work.

    ant for the ultra fast diode, FD700, what other substitute can I use for that?
     
  20. whiz115

    whiz115 Member

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    i believe 2N2222A and 2N2219A are very small to take the place of 2N5109.

    looks like we are doing the same project, so let's share my topic and work together on that since we got common aim. :)

    http://www.electro-tech-online.com/threads/rf-signal-amplification.96153/
     
    Last edited: Aug 19, 2009
  21. chingyg

    chingyg New Member

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    BFY51? would that work?
     

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