Simplest AM radio receiver

I'm trying to understand how am radios work. But i dont understand the most basic and fundamental parts required for reception of am radio. can some one help me out by telling me how to make the simplest AM receiver and please explain LC circuits.

Thanks!

http://en.wikipedia.org/wiki/Crystal_radio

i just saw the circuit shown in wiki, can i feed the audio output to a op amp instead of using crystal earphones?

yes

So basically i need an antenna, a LC circuit that resonates at the required frequency, a diode (can i use any rectifier diode?), a opamp and earphones?

Yes that is all you need. If you wind the coil on a ferrite rod, you can tune the radio by sliding the rod in and out of the coil.

Okay, so how can i calculate the number of turns i need to wind round the core?

so in the lc circuit i can either keep a variable cap or a variable inductor?

One over two Pi root LC gives you the resonant frequency. (sing it)

The number of turns depends on a lot of things like the diameter. If you just want to try for medium wave wind 40 turns. Make the C about 200pF.

can i used a fixed 200uh inductor and just vary the cap? of does it have to be the inductor.

Thanks again

adilmalik: so in the lc circuit i can either keep a variable cap or a variable inductor?

Yes! Most car radios used to use a variable inductor, less subject to drift vs. temperature compared to a variable capacitor.

Okay i think i understand now. So the resonant frequency of my LC circuit should cover the AM range?

This shows you step by step from just household items. http://scitoys.com/scitoys/scitoys/radio/homemade_radio.html

(can i use any rectifier diode?

You can but will be able to pick up more stations if you use a germanium diode in place of a rectifying silicon one.

So the resonant frequency of my LC circuit should cover the AM range?

Yes

thank you again, i finally understand this. 1 more question please, now i know that LC circuits are used during reception of Am but are they also used to transmit RF? example: if i transmit a blank carrier wave and connect it to a LC circuit?

but are they also used to transmit RF

They can be if they are part of the oscillator that generates the carrier. If you have something like a crystal oscillator then you don't need them.

What makes me think GM is in his element with this old technology ?

No offence Mike, I'm one of your biggest fans really.

Crystal sets were old technology when I was a lad. :)

I used to sell crystal sets to people when I was at school. That is where I came up with the ferrite rod tuning method, it was a cost down on the variable capacitor. I made the boxes out of cardboard and used to sell then for three and six ( 3/6 ) and it cost me about two shillings to make. Not sure what that is in new money.

17.5p and 10p. Buys b*gger all these days. I'll bet there's an amateur radio ticket in your past somewhere......

Grumpy_Mike: One over two Pi root LC gives you the resonant frequency. (sing it)

When nobody else is listening preferably... my landlady just gave me a O___o look as though I'm an escaped mental patient ;)

I'll bet there's an amateur radio ticket in your past somewhere.

G8HBR

my landlady just gave me a O___o look

:)