Simplest AM radio receiver

Other Hardware General Electronics
21

just got all the parts and made a Am radio. it worked! i used a 100pf cap and a inductor like you told me and tuned it using the rod method!

22

Well done, I bet you are well happy.
Can you pick up many stations?

23

i am as very very happy! thank you again! i thought id never understand how a LC circuit works but i do now! i can easily pick up 3 stations, but i used an op amp to amplify the signal, also the signal is also sometimes very noisy, how can i reduce this? capacitor?

24

i next have to learn how fm circuits works. For another time i guess :smiley:

25

The thing I like about a crystal set is that it uses no power, so using an op amp is cheating a bit. :slight_smile:

also the signal is also sometimes very noisy, how can i reduce this? capacitor?

The way to reduce this is to make the LC circuit have a higher Q. This Q is a quality factor and is related to how sharply the frequency response is defined. That means it picks up less of the other signals and more of the wanted signal.
Some capacitors will give a higher Q tuned circuit than others. Here ceramic capacitors are not so good, mica and polystyrene are much better. But the thing that affects the Q the most is the inductor and the grade of ferrite. For an explanation see:- Inductor - Wikipedia

26

Well yes, kinda cheating but i did get the crystal earpiece :slight_smile: it did work but the volume was too low. I used an op amp only then i could hear the other two stations. hmm i see, it depends on the quality of components used. this method of transmission and reception can work with any frequency right? also i was trying to buy a 200pf trimmer cap for this but it seems there aren't any.

27

also i was trying to buy a 200pf trimmer cap for this but it seems there aren't any.

Not as a compression trimmer but you can get a 500pF variable capacitor, but new they are not cheap.
There is this one:-
http://compare.ebay.co.uk/like/120824146657?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar&cbt=y

But again there is this one:-
http://www.radioworld.co.uk/catalog/second_hand_520pf_medium_spaced_variable_capacitor-p-8916.html

28

thanks again! yeah i saw that too. Also remember you said that the germanium diode is not necessary, and it could be used with a silicon rectifier, i tried the running circuit with a general purpose rectifier and it didn't work at all.

29

A germanium diode needs 0.2V across it before it conducts where as a silicon diode need 0.7V. Therefore you need a much larger signal to make it work. Also a rectifier has a larger junction surface area which acts as a capacitor and suppresses the Q of the tuning circuit.

30

thanks again! all queries have been solved :slight_smile: may i ask how do u have so much knowledge of almost everything related to electronics, i always see you answer on everything :slight_smile: is there anything known as variable inductor? also can you guide me on how to make inductors of a specific value?

31

s there anything known as variable inductor

Yes you made one by sliding the rod part way out of the coil.

how do u have so much knowledge

Well I started at an early age and have now reached a much older age. My first job in electronics was in 1966.

32

wow! i plan to do that exactly :astonished: im in my gcse's and ive covered pretty much in electronics. also is it possible to make a LC circuit in the mhz range that would pick up fm in the same way? i understand the demodulation would be different.

33

You can't just jump to FM like that.
Well, at least add regeneration, get some selectivity.

34

oh yes i was thinking about that, im being too erratic :confused: i agree i will first understand Am fully. What is regeneration?

35

I dug out my Regenerative hybrid.
It has a varactor diode (MVAM109) for tuning, I used a voltmeter as a tuning dial.
It uses an NPN transistor for the regeneration.
I'll have to hunt around for the circuit.
There's a LM386 power amp, too, just for kicks.

      • I found that schematic. I tucked it inside one of my books. It's with the other pics in the 'photobucket' link above. * * *
36

Well in theory you could demodulates FM with a crystal set, by using slope detection. This is simply off tuning the signal so it sits on the slope of the selectivity curve. In practice you just tune for maximum volume.

37

Thanks again everyone. @runaway thanks for the photos nd circuit. i also dont get one thing in some basic crystal recivers there are 2 caps. one is in the lc circuit while one is connected to the diodes cathode to ground. what does this do?