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Thread: Variable capacitor and Crystal radio (Newbie to wiring)

  1. #1 Variable capacitor and Crystal radio (Newbie to wiring) 
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    Hello, I admit that I don't have much understanding of anything but the simplest circuits, but could you help me here?

    I am trying to build an AM radio from scratch. I guess it's a good way to learn more about electronics. Here is my plan:



    I know I'm missing something. This should let me select a station by adjusting capacitance in an LC circuit. But, the signal is still modulate it. To demodulate it, I apparently need a second capacitor there somewhere. And I have no idea how should that one part work.

    1) Where should I place a capacitor?
    2) How should I pick it's capacitance?
    3) Is there anything horribly wrong with this schematic?

    Also, I've had some trouble finding a variable capacitor, so I decided to make one myself, with aluminium foil! ... Is that even possible?..

    What I need is, a capacitor that can vary from 100 micro farads to 0.
    All I have is some aluminium foil, enough wires, and 3 normal capacitors: 1000, 47, and 10 micro farads. Perhaps there is a circuit to make a normal one into adjustable?

    I'd try to do it with aluminium, but I can't come up with calculations. From what I see, if it's flat plate conductor, even a whole square meter of aluminum would have to be placed at a fraction of a micrometer away from each other. That just can't be right.


    So guys, please answer my questions and don't be harsh on me, I'm just beginning to study electronics.


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  3. #2  
    Forum Ph.D. Leszek Luchowski's Avatar
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    1) and 2) : you don't really need one. The diode plus the inertia of your headphones will do your demodulation for you.

    3) : yes, there is one serious error: it doesn't make sense to have a circuit consisting of two parts that are only connected at one point. And your circuit is just that, the single connection being the diode. I suggest connecting the emitter of your transistor to the ground point of the oscillator.

    You will also most probably need a resistor between the "+" of your battery and the base of the transistor.

    I am sure you can find a lot of ready schematics on the Web.

    Good luck - Leszek.


    Leszek. Pronounced [LEH-sheck]. The wondering Slav.
    History teaches us that we don't learn from history.
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  4. #3  
    New Member Peter's Avatar
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    It's my variant crystal receiver. It's "reciver for direct amplifier". Very simple circuit.
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