Crystal Set Design

Discussion in 'Wireless & RF Design' started by jkele, Dec 1, 2010.

  1. jkele

    Thread Starter New Member

    Oct 17, 2010
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    I have been looking up schematics of the Crystal Set, from what i know You have an variable inductor, capacitor, germanium diode, and earphones. I know that you connect the inductor to the capaictor via a tap for tuning, and an antenna. From all the schematics that i have seen it says you must connect the diode, and the earphones to the inductor via a tap, why cant we connect it to the variable capacitor?
     
  2. hgmjr

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  3. JDT

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    Feb 12, 2009
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    I recommend the link.

    Very important to have long wire antenna. 50 - 100m and good earth. Copper rod in the ground.

    This was my first electronics project almost 50 years ago! Me and my dad went on a long trip to London just to buy a germanium diode!

    It worked first time and look where it's got me.

    Are there still any AM stations about?
     
  4. bertus

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    Apr 5, 2008
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    AC0AC likes this.
  5. BlackCow

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    May 11, 2009
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    Ah, my first radio project. I just put my output into a guitar amp though. It kind of defeats the purpose of using no power, but it works if you can't get sensitive headphones.

    I used a variable capacitor though, not sure what a variable inductor looks like :confused:
     
  6. beenthere

    Retired Moderator

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  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    One form of variable inductor used cardboard oatmeal box with a single layor winding. After securing winding with shelac, or nail polish, a strip was cleand with sandpapper. A brass rod was supported over the exposed windings with a metel bead riding on the rod.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    In the case of a crystal set, the inductor taps are "Coarse Tuning", and the variable capacitor can be thought of as "Fine tuning", if that helps you understand the placement of parts.

    It could be the other way around, technically, but it is mechanically simpler in the traditional setup.
     
  9. beenthere

    Retired Moderator

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    That was a class act. Most homemade designs used an initial set of 15 turns of wire, followed by a loop made by twisting the wire before laying it on the oatmeal bow. Thereafter, a loop was added every 7 turns (and I can't recall the total number of sections right now - the info comes from an old magazine). The loops were cleaned of the enamel and coarse tuning was done by a clip to one of the loops.
     
  10. jkele

    Thread Starter New Member

    Oct 17, 2010
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    So if i wanted to prove to my self that it still works the other way around i could have the connections to the diode going from the variable capacitor?
     
  11. thatoneguy

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    Using a slug variable inductor and variable cap will make it difficult to find the same station twice, but it should work if the inductor has the correct value range.
     
  12. durhambats

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    Oct 13, 2010
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    my design proved the importance of maintaining a high Q.

    I used a MASSIVE air cored coil - about 6" long and over 2 in diameter - with a big moving plate capacitor to form the tuned circuit: and a germanuim transistor asthe detector.
    I also tried a simple traditional ge diode detector with a FET buffer.
    Both designs provd very sensitive and precise.
     
  13. beenthere

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    That is actually part of the fun. The output is quite tiny, so listening is best done in the dark while lying in bed. The purpose is not to catch the news.

    There are all those atmospheric noises, plus ones from sources that can't be imagined. It's too bad the Cold War is over, so you can't hear the propaganda broadcasts, but there are still stations with interestingly strange music and unrecognizable languages.
     
  14. AC0AC

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    May 5, 2010
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    Thank you very much for this link - 73, Alan AC0AC
     
  15. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The reason for "tapping down" on the inductor with the diode is to reduce the loading of the tank coil. There is a finite amount of power drawn by the headset and the diode. This will lower the Q (and selectivity) of the coil. For this reason you want the most sensitive and high impedance headset you can get.

    Eric
     
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