FM Transmitter Help

Discussion in 'The Projects Forum' started by PoutineRocks, Nov 11, 2012.

  1. PoutineRocks

    Thread Starter New Member

    Nov 11, 2012
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    I've recently become interested in electronics, and I figured an FM transmitter would make for a good first project, but I can't seem to get it to work.

    I'm using this http://www.circuitdiagram.org/images/car-mp3-fm-transmitter-circuit.gif as the circuit. I've triple checked all the connections and made sure none of the leads are touching each other, but I can't find the output anywhere on the FM band using an old boombox. I've tried adjusting the trimmer capacitor and the loops of wire with no luck.

    What steps should I take to troubleshoot it, or is the circuit I chose a dud?

    I am aware that there are legal issues with FM transmitters, and this is intended to be practice building circuits.
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Instead of tuning the radio to the circuit, I'd suggest you tune the circuit to the radio. Find a clean, dead band on your radio (so that when it's tuned, it's complete silence--no static or anything), and slowly and carefully adjust the trimmer capacitor on your circuit. Eventually you should be able to hear it on the radio.

    Another question: Do you have this built on a solderless breadboard or a PCB? Breadboards tend to be lousy at high (audio) frequency, so that might create a few issues.
     
  3. PoutineRocks

    Thread Starter New Member

    Nov 11, 2012
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    Sorry, I guess I should have explained my testing methodology better. I would adjust the trimmer capacitor a little bit, then scan through all the radio bands. I would continue to do that until I had the trimmer capacitor back to it's original location, and then I would make slight adjustments to the loop of wire, and repeat the adjustment of the trimmer capacitor.

    The idea was that once I found where on the radio band it was transmitting at, I could adjust it so it would be at a static free place, but I was unable to find it anywhere.

    I am building it on a solderless breadboard, but I've tried both high and low signal inputs, and have the antenna of the circuit right next to the antenna on the radio.
     
  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Yeah, trying to find a signal that way probably won't work. Most radio stations that you here have very powerful transmitters, and your homemade one probably won't be able to broadcast over theirs. Try my suggestion earlier--find a dead (completely silent) station on the FM band, and only make adjustments to the circuit.

    I have no idea what you mean by "both high and low signal inputs". Do you mean you're just putting in no input at all and then trying to tune the radio to where you hear silence, or vice versa? Though that might work, if you're lucky, you'd be better off doing what I mentioned before. Tune the radio before even turning on your circuit to a dead station, and work from there.

    Being built on a solderless breadboard could be part of the problem too. There's a lot of metal in there, and loose connections, that can cause interference or poor conduction. I suggest trying to build it on veroboard or something like that. You'll have a much better chance of it working.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    They copied the preamp in MY FM transmitter circuit but did not test theirs to see that IT DOES NOT WORK when the supply is only 1.5V.

    My circuit was designed to use a 5.0V regulated supply and the transistor does nothing and is always cutoff when its supply is only 1.5V.

    The radio parts are also wrong.
     
  6. PoutineRocks

    Thread Starter New Member

    Nov 11, 2012
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    I meant that I changed the input (output on my mp3 player) to both high and low volumes.
    Would that just make the output of the transmitter a lot fuzzier, or would it completely stop it from working?
    What exactly do you mean by that? The circuit is poorly designed and won't work as a result, or it just has a poor output quality?

    I did some more testing tonight, and found that I had my transistors backwards, but when I flipped them I still had no luck. I also tried testing it at 1.5V and 5V since that's what Audioguru's circuit used, but again, no luck.

    Is it worth continuing to troubleshoot, or is there a better FM Transmitter circuit (but still fairly simple) that I should build?
     
    Last edited: Nov 11, 2012
  7. Audioguru

    New Member

    Dec 20, 2007
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    A solderless breadboard has too much capacitance between the rows of contacts and between the jumper wires. The rows and wires also have too much series inductance. Maybe a 1MHz circuit might work but certainly not a 100MHz FM transmitter for the FM broadcast band.

    There are hundreds of FM transmitter circuits in Google that work.

    Here is my FM transmitter circuit:
     
  8. nerdegutta

    Moderator

    Dec 15, 2009
    2,517
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    Nice!

    Is L1 and L2 10 turns 5mm air core coils?
     
  9. debjit625

    Well-Known Member

    Apr 17, 2010
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    I didn't understood one thing about Audioguru's FM circuit ,in the pre amp section for the mic a by-pass capacitor is been used C4.Why C4 should be 100nF for Europe and Australia and 150nF for North America ?
    Am I missing something...
     
  10. Sensacell

    Well-Known Member

    Jun 19, 2012
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    A solderless breadboard is totally hopeless at RF.

    When building circuits for RF frequencies you have to employ a totally different mentality, you need to consider that even a 1/2" long piece of wire becomes a significant inductance at 100 Mhz. The clips on a solderless breadboard become significant capacitors that are parasitically connected everywhere.

    It's a little daunting at first, but you can learn some good clean RF prototype building practices from looking at how others do it.

    Look at some of the homebrew projects on the ARRL website, you can learn some clever tricks.

    http://www.arrl.org/building-equipment
     
  11. vk6zgo

    Active Member

    Jul 21, 2012
    677
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    Audioguru has included pre-emphasis,which is a requirement of the standards current in all parts of the world.

    The actual standard differs slightly between NA & some other countries

    Pre-emphasis is used because in normal program material the level of higher frequency components is less than lower frequency components.

    With a flat audio response,these higher frequency components create lower deviation,& hence,have a lower immunity to interference.

    A tailored frequency response curve is used,which increases the resultant deviation for a given audio input signal level as the modulating frequency increases.

    The Receiver does the opposite,incorporating de-emphasis to restore the response at the Rx output to a "flat" response.
     
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  12. Audioguru

    New Member

    Dec 20, 2007
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    My coils use 1mm enamelled copper wire and are made like this:
     
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  13. Audioguru

    New Member

    Dec 20, 2007
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    ALL FM radio stations use pre-emphasis which boosts high audio frequencies. ALL FM radios use matching de-emphasis which cuts the boost down to normal and at the same time reduces hiss.
    The pre-emphasis ends at 15kHz and higher frequencies are sharply reduced to avoid beating with the 19khz stereo pilot tone.

    Cheap simple FM transmitter toys do not use pre-emphasis so they sound muffled (like your stereo with its treble tone control turned all the way down) when heard on an FM radio.
    My FM transmitter has pre-emphasis so it sounds crisp and clear when heard on an FM radio.

    Some AM radio stations also use pre-emphasis to boost high audio frequencies so that they do not sound as bad as stations that don't. The narrow bandwidth of AM radios provides de-emphasis.

    Here is the pre-emphasis response curves but they are missing the sharp drop above 15kHz:
     
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  14. debjit625

    Well-Known Member

    Apr 17, 2010
    790
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    I already got that by internet search...
    I never got time to properly study transmitting technology and their principals as I am much into digital electronics on other area, but always wanted to study about them in depth ,I will do it as I get some time...by the way thanks for the pre/de - emphasis stuff … learnt something new
     
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