AM audio transmitter and receiver

Discussion in 'The Projects Forum' started by ecjohnny, Dec 18, 2011.

  1. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
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    Hey.. Firstly thanks for replying to my help on previous thread. It did help me alot.

    Next, i am sorry if the AM transmitter topic comes up again, i did search the forum and google on my own but the results are too many but i went through some which help my project alittle. i guess it would be faster and better to make this topic for help.

    I am building a low power audio AM transmitter and receiver. Because my project require the transmitter and receiver to be as small as possible(mini/micro). It requires as little components as possible. operates on 3V-6V button cell battery. Range-wise: very short >1m. I have found this circuit(transmitter) to suit my project so far.
    [​IMG]
    (Source :http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p024.shtml)
    Help needed:
    1) Tune frequency is at 1Mhz? How can i build the receiver as simple as it can to tune to 1Mhz and is the received audio clear/loud/distorted or i need an amplifier/?
    2) By changing the Crystal Oscillator value, i am changing the tune frequency?
    3) Anyway i am looking for alternative and better circuit idea so i hope you guys can suggest better ways as i describe above.

    Thanks for any reply.
     
    Last edited: Dec 19, 2011
  2. MrChips

    Moderator

    Oct 2, 2009
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    A crystal oscillator is designed to lock the frequency to that of the crystal. You cannot modulate the frequency of the oscillator.
     
  3. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    142
    0
    What i mean is change the component (eg.replacing a differ value of crystal oscillator?)

    To clarify, i need a simple AM tx/rx circuit that i can build and by changing the components value (fixed). i can build different frequencies by changing the components. In fact.. i saw some circuit which mention changing the inductor and capacitor value for frequency and there is a theory formula for it.

    For Tx i think the above is the simplest i can find. For Rx i am still finding a circuit that can tune to 1Mhz(fixed) only to receive from above Tx circuit but most circuit using variable cap and uses 9V. i need a low power RX that operate around 3V and as small as possible. i am still trying to understand that turns inductor/antenna part.

    Since my application is only very short range in meters(about >1m). i am still looking at other RF methods.

    Hope someone in this expertise field can help me out. Thanks.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I'm assuming you are talking about a oscillator module, in which the frequency is what you buy. What you see is what you get.

    Why not buy a commercial AM reciever, since it is likely you are dealing with something in the 1Mhz band. If this is not the case you are going to have to give us more information.
     
  5. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    142
    0
    i am not sure what is an oscillator module. What i want to build can be fixed or tunable but most importantly is it has to be small and operates on around 3V(low power) which i am still looking for a circuit.

    For Tx part: I think the smallest i can find is the one is what i've posted but any other better sugestion are welcom
    For Rx part : Still searching for circuit.

    i found some result like single chip AM receiver but that chip is not available at my area.

    Here is a diagram show what i need. As u can see the important thing is low powered and small/simple to build therefore i dont expect the range to be far.
    http://imageshack.us/photo/my-images/513/audiotxrx.png/
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    I have seen circuits using simple oscillator modules. Not too sure I could recommend them. You would be better using an established design, your transformer idea kind of locks you in a little. If you go for AM frequencies (which is what I initially assumed) that would be around 1Mhz.

    Why AM? FM is quieter.

    You mention you have found a circuit. The schematic is designed around a oscillator module, and is not something you wire up yourself.

    Thing is, wiring one from scratch sounds like a good idea to me. It wouldn't use a crystal though, but a tuned circuit you could adjust.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    What you have shown in the first schematic is an oscillator module.
     
  8. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
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    0
    Thanks so that's an oscillator module. Anyway i have finally found what i want on youtube. Here is it
    Transmitter : http://www.youtube.com/watch?v=nz2HNvfTZ3Q&list=UUBQM0dFPLurCoxsw4qoe5LA&index=3&feature=plcp

    Receiver:http://www.youtube.com/watch?v=zY3R8vT0798

    Now.. i am just trying to understand the circuit. Its operating frequency is 97.6Mhz. Can someone help me with this :
    Is there a way i can modify the circuit to change the operating frequency? of my choice? Not by tuning. By a fixed value component.
    Theoretically, i remember frequency is a function of Inductor and capacitor, so is that the way by changing the value of the inductors and capacitor will give me a different operating frequency?
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The frequency IS NOT 97.6MHz. The transmitter and receiver have trimmer capacitors that adjust the frequency.
    The frequency is also changed by the battery voltage running down and by anything that moves toward or moves away from the circuits.

    The "radio" is so simple that stong local radio stations will overload it and make it not work.
    Toy cars use those circuits at 27MHz where there are no strong local radio stations.
     
    Last edited: Dec 19, 2011
    ecjohnny likes this.
  10. wayneh

    Expert

    Sep 9, 2010
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    Yes, the transmitter is tuned to the nominal crystal oscillator frequency. The audio is fine within range, which is only about 20ft max in my experience. You might increase the range with some experimentation and optimizing things like antenna length.
    In theory, yes, but you'll struggle to find oscillators in the AM band, assuming your goal is to use a standard AM receiver. Most oscillators these days are higher frequency.
    I think you'll need to specify a lot more about what you want to accomplish, to receive better suggestions on how you might do that.
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    In order for people to be able to help you get to your goal, could you please explain what you are trying to do. It may be that there are easier or more practicable ways to get the job done.

    If you do not do this, a lot of effort may be spent on eliminating unsuitable methods, and trying to find a workable solution. This will not be a good use of your time or anybody else's.
     
  12. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
    142
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    I am trying to build the two circuit in post #8.
    Thanks for reply. Correct me if i am : So all i have to do is tune/change the trimmer capacitor to change the frequency which is the tank circuit part? (LC circuit). by tweeting around with the tank circuit part and using the frequency and capacitor and inductor formula i can caculate the range of frequency i want to build?

    And the part where you say the local radio station is strong and overload it. How can i overcome it? Can i change it to 27Mhz which you mention?


    Thanks for any help.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Yes.
    At the very high frequency, the tuning capacitor is about 15pF but stray capacitance of the wiring and the circuit board add to it so it might actually be 20pF to 25pF so the formula will be wrong.

    When the coil has more turns of wire and the value of the trimmer capacitor is increased then the circuit will work at 27MHz.
     
    ecjohnny likes this.
  14. ecjohnny

    Thread Starter Senior Member

    Jul 16, 2005
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    Thanks i guess i will have to try it out. Thanks for the help and clarification.
    Anyway does the resistor value affect the circuit? because i am changing the inductor and capacitance of the LC circuit only. i am leaving the rest of the components value same.
    Also in the video, there is this capacitor 10p which i guess is 10pico but in the video comment it said 1micro so which one is it?
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    There is a 10pF capacitor between the collector and emitter of the transistor which provides positive feedback for oscillation.
    There is a 1uF electrolytic capacitor bypassing the battery which is wrong since an electrolytic capacitor is useless at radio frequencies. I use a ceramic 1000pf capacitor in my FM transmitter and it works perfectly at radio frequencies.

    You do not say where in the world you are so maybe you do not have hundreds of strong local FM stations. Then make the circuits for the FM radio band and try them.
     
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