Bug transmitter help

Discussion in 'The Projects Forum' started by hc359, Apr 8, 2012.

  1. hc359

    Thread Starter New Member

    Apr 8, 2012
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    Trying to build this: http://www.talkingelectronics.com/pr...ckedBug-1.html

    Alterations: slightly smaller pcb, mostly SMD caps and resistors, no audio stage yet (replaced by an old mp3 player), pn2222's instead of bc-whatevers, air core coils, resistor instead of inductor in buffer stage.

    I've built several of these, and the only one I got to work somewhat properly was a 72Mhz NBFM version (24mhz crystal, first oscillator stage 24mhz as well, the second 72). Also tried to build several WBFM versions like in the original schematic (11/33/66, 11/33/99, 11/55/110, 14.318/42.954/85.908 etc etc etc), but none of these could be adjusted to its proper frequency. NO modulation whatsoever on center frequency, poor signals with modulation 1-2 mhz off to either sides... and the "side" frequencies drift, in fact you could (temporarily) adjust these in at least 10mhz range...except on that center frequency of course. Very little audio power is required for this WB modulation (2 on scale of 30 on the audio player, the NB variety needs about 25). Cranking the volume up won't produce audio on center frequency, the signal just becomes intermittent... Any help appreciated..
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The link does not work.
    Please correct it.

    Bertus
     
  3. hc359

    Thread Starter New Member

    Apr 8, 2012
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  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The inductor at the collector of the output transistor is a must.
    When you replace that with a resistor, the RF signal may interfere with the powersupply.

    Read the following PDF for more info:
    RF-proto-1.pdf

    Bertus
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Don't forget to buy a box of "comdons".:D:D

    Seriously, the audio will sound muffled when heard on an FM radio because the circuit is missing pre-emphasis (treble boost) used by all FM radio stations. The de-emphasis (treble and hiss cut) in all FM radios cuts high audio frequencies like your stereo with its treble tone control turned all the way down.
     
  6. Privateman

    New Member

    Mar 7, 2010
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    For better frequency stability, the aerial should be on a one turn tap from the supply rail
    or at the end of a tank filter from the collector of the O/P stage

    Rab
     
  7. hc359

    Thread Starter New Member

    Apr 8, 2012
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    0
    @Bertus

    Ok I'll try to get an inductor in there tomorrow (it's 1.30AM here...). Teh resistor is SMD and the inductor has to be vertical I guess (the only unused 90deg offset...). Otherwise the board looks ok, I don't have a camera at hand, but all leads are rather short, the top side of PCB is split about 1/4 (V sup) to 3/4 (ground) with an obscene amount of scavenged SMD caps inbetween, and a load of vias...

    @Audioguru

    I know how a crappy homemade transmitter sounds, have built a few (simple VFO with a broadband amp/buffer, unusable due to drift). The audio is not muffled, it's NONEXISTANT on crystal frequency. 5 bars of signal on scanner, but no modulation, no WBFM, no NBFM, no AM, nix, nada... Just two "side frequencies" with decent enough audio about 1mhz up and down.
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    It won't oscillate without an inductor.

    The good news is that this inductor can be hand wound with a very small diameter -2mm dia should work find.
     
  9. hc359

    Thread Starter New Member

    Apr 8, 2012
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    err...it does oscillate, just not where it should. Most range I've gotten out of one on these drifting "phantom" frequencies is about 30m (6v, 0.5m untuned antenna).
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    A cheap FM radio gets overloaded by one of these simple transmitters if they are fairly close together and there are some strong local FM stations. Then "phantom" signals are received at many spots on the dial.

    My cheap Sony Walkman FM stereo radio has a "local-distant" switch that attenuates the radio signal and stops the phantom signals when it is set to local. But then its range is very poor.
     
  11. DickCappels

    Moderator

    Aug 21, 2008
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    If its not oscillating where it should, and you are using an inductor (not just the stray inductance in the circuit), then you should be able to adjust the circuit to the desired overtone by stretching or compressing the inductors.

    The instructions on in the pdf file look pretty complete. Give them a chance after faithfully making the circuit shown in the pdf.



    Is your
     
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