Joule Thief blocking an AM radio between 560-600 khz

Discussion in 'Wireless & RF Design' started by Rolland B. Heiss, May 12, 2015.

  1. Rolland B. Heiss

    Thread Starter Member

    Feb 4, 2015
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    I discovered this by accident when I set my Joule Thief down next to a radio while both were turned on. The radio went silent. Can anyone tell me more about how and why it does this? I'm very curious. Obviously it must have something to do with the transistor in the Joule Thief circuit.

     
    Last edited: May 12, 2015
  2. Papabravo

    Expert

    Feb 24, 2006
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  3. Rolland B. Heiss

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    Feb 4, 2015
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    Interesting! Thanks for the link. One thing I can't figure out however is that when I briefly touched and shorted one LED on the Joule Thief the radio signal came back even though the Joule Thief was still running. Any ideas as to why and how? Seems like the desensitization effect should have continued to block the frequency on the radio.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    My guess is that shorting the LED either stopped the oscillation, reduced the amplitude, or changed the frequency.
     
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  5. Rolland B. Heiss

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    Feb 4, 2015
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    Hmmm... once again, interesting! I appreciate your responses Papabravo.
     
  6. radiohead

    Active Member

    May 28, 2009
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    Can you post the schematic for the joule thief? Some of us may be able to determine why it generates an AM signal... What is the range of radio interference?
     
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  7. takao21203

    Distinguished Member

    Apr 28, 2012
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    a joule thief is just a blocking oscillator they existed before joule thief and actually were used in 1960s.
     
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  8. Rolland B. Heiss

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    Feb 4, 2015
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    Here is a link to the schematic I used to make mine, minus the switch I hooked up between the circuit shown and the positive battery connection:

    [​IMG]

    P.S. The range of radio interference isn't very far with this. It cancels out the station signal less than a foot away and from about 5 feet or so away you can hear low to high pitched noises on the station.
     
  9. Rolland B. Heiss

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    Feb 4, 2015
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    Thanks for the info. I'll have to learn more about those old blocking oscillators used back then.
     
  10. Art

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    Sep 10, 2007
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    Does anything happen on the radio if you touch the point where you soldered the inductor to the resistor?
     
  11. Rolland B. Heiss

    Thread Starter Member

    Feb 4, 2015
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    I hadn't tried that before. After you suggested doing so nothing happened when I touched the point where I soldered the inductor to the resistor. But then I touched the other side of the resistor between the connection going to the transistor and suddenly the range of blockage expanded after hearing a certain noise on the radio. However, I then turned the Joule Thief off and back on and haven't as of this moment been able to replicate the results. Quite interesting.
     
  12. neonharp

    New Member

    May 10, 2015
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    Hello ,
    From the looks of your video The " Joule Thief " is messing with your AM radio reception . It appears to be a superheterodyne type AM radio . Not knowing what frequency your circuit is running at , I am going to assume that the frequency , or harmonics of that frequency are giving you what we call " Front end overload " which means that the EMI radiated by your circuit is " drowning out " the normal signal that your radio is expecting to receive . Or it could be that it interferes with the local oscillator in the receiver , or even monkeying with the IF or audio circuits .
    Have you tried this with different radio receivers ? There may be something in particular with that radio that is more sensitive .

    Cheers !
     
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  13. takao21203

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    Apr 28, 2012
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    ive recorded this interference with am radio some weeks ago
     
  14. Rolland B. Heiss

    Thread Starter Member

    Feb 4, 2015
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    Thanks for the input neonharp! I tried it with my plug in radio alarm clock as well as the battery powered radio depicted in the video. Same results in the same frequency range.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Adding a few turns on the inductor would make the frequency lower, reducing or possibly avoiding the interference within the AM broadcast band.
     
  16. neonharp

    New Member

    May 10, 2015
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    Yeah , and there are more things that could be tried . I guess the fun of these little projects is to experiment and learn . Maybe try shielding the toroid with aluminum foil , or put the whole thing in a metal box . For the sake of experimentation , you could just stick it inside a metal coffee can ! Try making your connecting wires between the components as short as possible , Or maybe add a small value capacitor somewhere in the circuit . I may build one of these and tinker with it .
     
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