crystal radio trouble shooting

Hello I just Finished a crystal radio but can't get it to pull any thing in but the occasional faint static. Ive tripple checked the circuit and it is correct. However the first thing that came to mind was the variable capacitor,I simply pulled mine out of an old am/Fm radio. Is there a certain type I need for this to work or does it matter? I also am using an old telephone handset, will this work or do I need a crystal earpeice. Please look over my attached pics and see if you can find anything wrong. Any help is appreciated. Thanks, Alec

Attachments:

• 20131003_222610 (1)-1 (52 KB)
• 20131003_222604-1 (19 KB)
• 20131003_222554-1 (23 KB)

To thank the Author because you find the post helpful or well done.

Hi Alec

 

I don't see a coil anywhere there. It would be wired  in parallel with the tuning capacitor.  Also you'll need high impedance headphones or a crystal ear piece to listen with.

 

Peter

To thank the Author because you find the post helpful or well done.

It needs:

1. A Capacitor. Usually variable. 15pF to 450pF is good. But any capacitor will work. You might need a parallel fixed capacitor of different values for LW or MW.
2. A coil of about 500uH (0.5mH ) for MW. SW can use 100uH. LW needs about 2.5mH. This is connected across the capacitor. Sometimes you need to connect the two sections of the capacitor in parallel, especially for LW. One end (the earthy side, [1] on diagram) to moving vanes and the other end [2] to fixed vanes of capacitor.  A tapped coil is common if the capacitor range isn't enough. Try about 200 turns of wire (it doesn't need to be enamel wire or Litz, any thin insulated wire) on a 330ml plastic drink bottle. Cardboard is poor. If Litz or enamel space the turns by about 0.25mm. If it's Cat3 phone wire or Cat 5 network cable single core then the insulation is enough space, wind it tight. More turns is a lower frequency band (longer waves) and less turns shorter waves, higher frequency. You need to have about 1/4 or x4 turns for the next band if each band is a 2:1 variation roughly.
3. A Detector Diode. It needs a resistor load, which can be a magnetic earphone. 4000 Ohms is a typical  headset (2K + 2K on each ear) for a crystal radio. A larger resistor is more sensitive.
4. A Decoupling capacitor on the load to smooth the RF. Usually 10nF to 100nF, this can be be capacitance of a "crystal" earpiece, as these are like a capacitor between 25nF and 100nF.
5. A good earth. Either a 1.2m metal pole or pipe hammered into the ground or the metal work of the central heating. It connects to metal work / moving vanes of capacitor, the  [1] side of coil.
6. An aerial wire about 5m long minimum, 20m is much better. A metal cored clothes line will do with ends insulated using plastic rope. Only a longer wire connects to the fixed vane side of capacitor ([2] side of coil. If the wire is only 5m long it's best connected about 1/4 way along the coil from earthy end. Deluxe Crystal sets have the aerial and earth to a 2nd variable coil.

Some detectors are more sensitive with a slight bias via 2 resistors and a battery. Use a Germanium diode and this isn't a problem.

 

The Earphone needs to be 2,000 to 10,000 Ohms, if magnetic. A telephone earpiece is 30 Ohms to 600 Ohms only, it's no use. A ceramic piezo earpiece (long ago actually crystal) is more sensitive but then you need a resistor or there is no current for the diode to work!

The R1 to allow DC diode current can be 470K. But try 100K and 2M Ohms.

(See Reading Schematics if this doesn't make sense)

See:

Part 1 how a radio works

A more complex crystal set Pocket-size crystal radio

To thank the Author because you find the post helpful or well done.

Thanks a ton guys, I had just pulled this design off youtube but it obviously wasn't correct. I'll add these things and let you know if it works. thanks for the tips, Alec

To thank the Author because you find the post helpful or well done.