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Comment by Craiggybear | Hacker News Reader

If you put a high-value resistor (between 220K-470K) in parallel across the crystal earpiece the audio will be much stronger/clearer.

OR you can use the high impedance earpiece form an old telephone instead of a crystal earpiece which will give a fantastic, clear and loud output.

My grandfather used to put his "cans" from his crystal set in a pudding bowl and it acted like a speaker -- several people gathered around could hear it at the same time.

A 300pf variable capacitor across the coil will also give you quite effective tuning.

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Thanks for the additional advice. I was planning to make a capacitor out of foil and newspaper interleaved. One day when I've finished all the other things I want to play around with...

The other method is using a variometer (inductive), which is easier than making a variable capacitor:

http://www.crystalradio.net/crystalsets/hassell/index.shtml

Newpaper will be 'fun' to work with as its moisture content will vary with environmental conditions, making its dielectric constant not very..erm..constant - unless you impregnate it with wax or a lacquer.

(Made plenty of crystal sets during my teen years in the mid 70s-80s)

Good link to variometer page -- thanks.

There are lots of ways to homebrew variable capacitors. A sheet of glass and two pieces of scrap copper does well.

I have a "crystal" radio (uses a modern diode) all built on a paper towel tube. The coil is wound on the tube like usual but at the other end of the tube there's also a layer of foil and a smaller tube with a layer of foil that slides inside that to make a variable capacitor.

Good VC advice.

Also, I find that inserting a ferrite rod into the coil (or winding the coil around the ferrite rod to begin with) makes tuning much more effective (sharper) and makes the audio signal much stronger. A steel or iron 6" nail also works if you want to dispense with the ferrite rod.

I once found this site with lots of interesting projects: http://www.sparkbangbuzz.com/

I live very close to the BBC regional broadcast transmitters (100KW apiece) and since boyhood I've made loads of crystal sets as well. A few feet of wire often being quite sufficient as an antenna and the radiator pipe as an earth connection. A good earth connection is very important to the effectiveness of a crystal set.

I made a single transistor regenerative set next and because the local BBC signal was so powerful I literally didn't even need an antenna at all!

Another single transistor was all that was required to drive a speaker loud and clear.

Why would the resistor help? It just lowers the impedance and what you'd really want would be a high impedance earpiece. Or is having a useful effect by reducing the damping factor of the circuit?

The resistor provides loading for the detector. The crystal earpiece has such a high internal impedance that it is more a capacitive than a resistive load.

The high value resistor in parallel with the earpiece provides a working load for the circuit. Without it the audio signal is largely being shunted by the large internal capacitance of the crystal earpiece.

The resistor is not required if using a high impedance moving coil or moving iron earpiece such as that from an old telephone.

Trust me, if using a crystal earpiece the resistor makes a big difference.

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