need help on deciding on which schematic

Discussion in 'The Projects Forum' started by poolman966554, Feb 4, 2014.

  1. poolman966554

    Thread Starter New Member

    Jan 22, 2014
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    0
    Hey everyone! I have a friend that has requested an fm transmitter from me.. I cant build a design myself, i can only replicate it atm.. the forum videos/worksheets have brought me a long way though!
    Ive made a couple of these, as my first attempts as pcb using thru-hole components. Muriatic acid/ hydrogen peroxide to etch copper board, and MS paint to design traces:
    [​IMG]

    It has worked well for me, but always needs a bit of tinkering with tuning... I would like to omit the variable capacitor, and install a sp4t rotary switch, with predetermined capacitors to make it used friendly.(ex: 5pf = 87.5fm etc) obviously id have to adjust the coil length to get my starting parameter, and go from there.

    question is, is this a good schematic to work on?
    ive also found these:
    [​IMG]

    Ill add i have 20 of the maxim2606 IC already..


    then theres this:
    [​IMG]


    And if your not bored yet, i have built this a while back.. but didnt work properly and gave up figuring it was first project screw up.. ****Question is here, if i replicate this schematic(properly), can i simply replace the mic with a pair of rca jacks/ or 3.5mm? and combine then left & right signal as shown in 2nd and 3rd posted schematic?****
    [​IMG]

    Also, will the 3 transistor schematic posted be more "stable"? as i said, i need this to be user friendly.. So much as you'd let you mum use it with out worry lol!

    OR, is there a better alternative that i haven't found yet that one might recommend? (i wont buy one- price sux, and so does range!)

    Stability is the main worry here. And as she travels from time to time, id like to have preset caps to determine freq output..From what ive learned var caps/ pots have a very short sweeping lifespan, and shouldnt be used for frequent tuning.
    Completely open to suggestions, and any advice one might give.
    Hoping to get started ordering parts! Thanks in advance!!
     
  2. burger2227

    Member

    Feb 3, 2014
    190
    24
    The Maxim schematic looks like the best one as it uses no variable capacitors for tuning. Plus you already have the chip! Let us know how it turns out. Maybe I will buy a few off of ya...

    As to the last one, 3 what transistors?
     
  3. poolman966554

    Thread Starter New Member

    Jan 22, 2014
    29
    0
    The maxim does have 2 pots though.. does the pots produce the same effect as a variable cap?

    The 3 transistor schematic uses 2n3904

    Edit: numbers..
     
    Last edited: Feb 4, 2014
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    be sure of mechanical stability, any loose parts in the rf stage will cause drifr and "twanging" when vibrated.
     
  5. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    The pots, if selected carefully, will be as stable as anything for this.

    And You should select the MAXIM-sollution because it is by far the easiest to get to work stable.
     
  6. poolman966554

    Thread Starter New Member

    Jan 22, 2014
    29
    0
    Thanks for the responses! ill give the maxim circuit a try. Thanks!
     
  7. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    Scrap all of the discrete ones, they are crap, broadcasts all over the place and are very frequency unstable and drifts with temperature.
     
  8. poolman966554

    Thread Starter New Member

    Jan 22, 2014
    29
    0
    ok, so ive gathered all parts needed from digikey for the max2606 fm transmitter.. except i just got confused.

    I decided to search for an "antenna" incase they had something better than a dangling wire flopping around..

    they produced a list of rf antennas.. i guessing that is not what i want? (as most listed are surface mount)

    What antenna could i use that isnt just a dangling piece of wire? maybe telescopic? maybe something salvageable??

    Thanks for all the help!
     
  9. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    For this you dont want anything more than a max 12" long piece of wire and a max output power of 5-50 nanowatts. Anything above that and you need a radio license, atleast in sweden you do.

    In other words the range must be restricted to a few meters for it to be legal to use without a license.
     
  10. poolman966554

    Thread Starter New Member

    Jan 22, 2014
    29
    0
    can one glance at schematic at guesstimate the output power? ive never built this before, nor know anything about it. I live in USA if that matters.. They sell fm transmitters here at best buy, but for like $50.. they can transmit around 200ft- 300ft estimation
     
  11. poolman966554

    Thread Starter New Member

    Jan 22, 2014
    29
    0
  12. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    Well.
    With a sensitive frequency counter it would be possible.
    Will eat of the transmitting power quite a bit.
     
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