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

1. 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.

2.

3. 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.

4. It's my variant crystal receiver. It's "reciver for direct amplifier". Very simple circuit.

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