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I have been following the tutorial below to build an AM radio, and when powered on I was picking up what id presume to be radio waves (what I'd describe as a mishmash of "whooshing" sporadic oscillations).

http://www.instructables.com/id/Simple-OP-Amp-radio/?ALLSTEPS

This was great, but I wanted to single out a specific frequency and listen to a radio station. I did some research and figured out that I have to add a resonance circuit called an LC circuit to single out specific frequencies. I made one with a variable inductor I made with 47 coils in 10cm with square steel railroad spike of diameter 1.5 cm in the center and a capacitor like below. I experimented with 220 pF capacitors in parallel to make 220, 440, and 660 pF and tried using my variable inductor to tune into a station for each of them but it did not work.

schematic with added tuner (apologize for the poor drawing)

I chose that coil count using the following equation to tune into mid freq radio singles (535-1500 khz), I made sure it covered the lowest frequency and then I could shorten it to get the higher frequencies.:

lc circuit equation

At this point I was stumped, I heard that you had to put a diode in to single out high frequency singles so desperate, I inserted it between pin 3 and my LC circuit but it didn't help. I also made a home made variable capacitor but likewise I coundn't tune into a station. Most importantly I heard you had to ground the circuit so I attached a wire for the tuner circuit to my computer(which is ground) and it changed the tone slightly but still couldn't tune into a station.

I started to wonder whether there were any am stations in my area due to its falling popularity, but a quick test with my mother's car radio yielded handful of them.

I apologize if my troubleshooting may seem stupid but I'm a little new to this circuit building stuff. I Thank you for reading thus far, and I appreciate any assistance you guys may offer.

My current setup ^Current setup pic

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The way you connected your coil - well it is wrong; instead of singling out the desired frequency, it suppresses it. It has to be connected parallel to the the amplifiers input, through a germanium diode (you may potentially use a silicone one, but you'll need to add a DC shift circuit).

This is what you want:

http://www.mikroe.com/old/books/rrbook/chapter3/21a.gif

Ignore the extra output from the the coil, just connect the diode parallel to the coil.

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  • \$\begingroup\$ I apologize but I dont quite get what you mean in parallel to the input. Do you mean have it go parallel to the op amp and have one end on the input and the other on the output, or have it connect to the negative terminal of the batery? and by input do you mean the negative input or positive. I have the germanium diode but not sure where to put it as I don't understand the inductors placement. I also tried leaving the coil and placing a germanium diode in parallel, but don't think thats what you meant. I tried mimicking the picture in a few other ways but I think I'm missing the point. \$\endgroup\$ – 23scurtu Aug 7 '15 at 6:56
  • \$\begingroup\$ Did you read the link? \$\endgroup\$ – ilkhd Aug 7 '15 at 7:35
  • \$\begingroup\$ Yes I tried connecting the capacitor and the inductor the negative then having the prong arm go through a diode. I then tried out having that go straight to the 3 prong, and having it go to another capacitor than connects to negative and then to the 3 prong. I then tried not connecting the inductor and capacitor to the negative, but putting the diode on the prong arm and then to the 3 prong. Lastly I tried just putting the diode parallel to the other two and in series with another capacitor parallel to the other two. \$\endgroup\$ – 23scurtu Aug 7 '15 at 7:43
  • \$\begingroup\$ Did you try exactly like the diagram shows? Because this is the only right way; BTW "3 prong" is called antenna. \$\endgroup\$ – ilkhd Aug 7 '15 at 8:00
  • \$\begingroup\$ I followed the picture exactly. By putting in the variable resistor It made a really loud low frequency square wave noise, so I took it out and was left with the following circuit: i.imgur.com/u3avW7d.png Also I replaced the 100pF capacitor with a 220 one but idk how much that effects it as I dont currently own a 100pF capacitor \$\endgroup\$ – 23scurtu Aug 7 '15 at 18:23
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Try modifying your schematic like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Note that the loop aerial provides a better signal reception and you must make sure it is relatively long(compared to the one in the figure) in order to make your radio work.The schematic is taken from an educational AM experiments kit that worked.

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  • \$\begingroup\$ I tried this out, and all lengths of the coil would emit a mid frequency square wave noise, and when the prong would be taken off the coil it would emit a painfully loud and high frequency screech. I have a large wire antenna attached to a sheet of tin foil. I tried coiling up a large length of wire to make an aerial but that did not help either. \$\endgroup\$ – 23scurtu Aug 7 '15 at 7:24
  • \$\begingroup\$ Check your inductor.The image you posted shows that the turns of wire are too far away from each other.Using the inductor as variable may be the problem.I would recommend you to buy a variable capacitor and a/more fixed value inductor(s) for maximum efficiency. \$\endgroup\$ – Daniel Tork Aug 7 '15 at 10:55
  • \$\begingroup\$ Also,it is hard to make such a device that way with the knowledge you currently have,since you said that you are new to this kind of design.It could be better if you start off with some elaborated tutorials to get you going. \$\endgroup\$ – Daniel Tork Aug 7 '15 at 11:05
  • \$\begingroup\$ I tried finding an elaborated tutorial, but all the ones I've found online don't go too in depth. That's why I finally came to this forum \$\endgroup\$ – 23scurtu Aug 7 '15 at 18:29

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