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I'm totally confused. I have built the following circuit with absolutely no result, not even a click. Even after reading through several other similar questions on the board I haven't debugged it.

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I am using about 15 feet of wire for an antenna and the metal case of a circuit breaker for ground. I've tried using a 100uH coil from Radio Shack and a 15uH choke for the inductor. I've tried a variety of capacitors as well as an air variable 1.4 - 9.7 pF capacitor. From the LC calculators I've checked out I should have been able to get AM stations with the varicap/15uH choke combination. I have a glass type "switching" diode but I'm not absolutely sure it's germanium. I've used two different piezoelectric ceramic disks in place of the headphones. Right now I'm suspecting the piezo disks. Are they not high enough impedance for this to work, or is one of my other parts incorrect?

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"I'm not absolutely sure it's germanium". This will be an issue. A crystal radio requires a germanium diode. It will not work with a Silicon or Schottky diode.

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  • \$\begingroup\$ I wonder if it would be helpful to place a cap in series with the diode, and use a battery and resistor to try to keep the cap biased enough to hold the cap near the diode's conduction voltage? I wonder how much current would be required to keep the cap biased? \$\endgroup\$
    – supercat
    Commented Apr 9, 2013 at 14:59
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Some classmates and I built a working crystal radio using the same basic circuit last semester. Some things we learned with experimentation that may help you:

  1. A better ground increased our amplitude. We used a short wire to a bare screw on a door jamb and got a tiny signal. we changed to a longer wire, and got a better volume. we changed to a prong stuck in a ground at an outlet (outside) and got a huge spike in the volume of the signal.
  2. While we had a long wire for our antenna, we had an increase in both number of stations we picked up, and the clarity of the signals when we attached it to a metal light pole that stuck about 10 feet above us.
  3. We had taken our germanium diode from a crystal radio kit, to ensure it was germanium. After our radio began working, we backtracked to the other diodes we had attempted to use, both glass type of various types, and a Schottky diode; none of them would work.
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Thanks everybody for your answers. I finally ordered a high-z earphone and that solved the "no response whatsoever" problem. Fiddling with the ground improved it as well. I got some new germanium diodes and there was no discernible difference so I suspect the original one I used was in fact germanium. It now works, but a local Spanish AM station is totally drowning out everything else. Oh well, at least I know it was working from the beginning, I just couldn't hear it due to the use of an incorrect sound output device.

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  • \$\begingroup\$ If you want to take this further as a hobby, I suggest you research "notch filters". Filtering out the local AM station will help when looking for other, weaker signals. \$\endgroup\$ Commented Nov 16, 2014 at 2:45
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Sounds like useful answers above. The inductor - you don't give details. An all-important feature of this inductor is that its self-resonant frequency is significantly higher than the stations you are trying to receive. If the inductor is a small axial or radial inductor you might need to rethink it and wind your own. I'm not sure what frequency you are trying to receive but 50 to 200 turns on a 1" diameter broom handle is what i used when i was trying it many years ago. This was intended for reception about 1MHz.

You can of course use the ferrite rod and coil from a commercial radio but if you are into making it all your self then maybe this picture might help: -

enter image description here

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Tuning capacitors for the AM band are usually in the 365 uF range and are variable in capacitance. After the diode insert a very small capacitor in parallel (.oo1 uf). This shunts the residual RF component to ground and pass the audio. Hopefully I remembered this correctly....it been decades since I dabbled with crystal radio

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  • \$\begingroup\$ 365 uF is wrong. \$\endgroup\$
    – Uwe
    Commented May 30, 2023 at 6:56
  • \$\begingroup\$ I did say it was a long time over 4 and a half decades. maybe 365 pf whatever but the small cap is correct in parallel is shunt out the rf and leave the detected audio and a cap in series to pass the audio to a high impedance earphone. The inductor was a ferrite core. The winding I do not remember what the inductance value was. \$\endgroup\$
    – Old_Fossil
    Commented Jun 11, 2023 at 22:38
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Have you tried a 6volt by 6volt transformer? (Phillips t-121) I use this simple doorbell transformer with a set of 32ohm magnetic headphones and it works great. I also use a very long 100' green plastic coated brass clothes line cable for an antenna and a 3' aluminum rod for a ground. In the daytime I pick up a local station. It is quite loud. At night signals are weaker but can listen to Cuba and China with just the phones. (No amp needed.) Connect audio out into the two black wires. One for positive and the other for negative. Then connect the three transformer output wires to a mini 1/8 stereo jack. One red wire for left the other red wire for right and the green wire for your common ground. For some reason the little plastic jack works best for me.

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