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Worlds Simplest Radio
radio : by Tommy - August 3rd 2006, 12:58AM
radio
I just finished making quite possibly the world's most simple radio. I had read a few HOWTOs for constructing a crystal radio. Some links offered the basic formulas for figuring the specs needed for the coil if you wanted to tune various frequencies. I purchased 3 spools of "Enamel-Coated Magnet Wire" from Radio Shack for ~$5, and picked up a crystal ear piece while at EPO in Houston.

After reading that just about any round object will do for a coil-form, and not having an empty toilet paper tube, I settled on an empty fish food container. It was small, compact, and just the right size - plus with the screw on lid, I can keep the antenna, ground and ear piece inside. I rinsed out the fish food residue, tapped 4 small holes with a drill bit and started wrapping the 22awg copper wire around the bottle. Every 5 turns I would wrap around a toothpick. After wrapping the bottle - which took a while - I ended up using about 30ft. of wire. (FWIW: my coil came out to be about 118uH) The toothpick wraps were then scraped free of the enamel coating to make antenna taps for rough tuning. (how this works) After ensuring there were no shorts along the coil body, I hooked up the 1N34 diode, ear-piece, ground wire and antenna and was greeted with some shortwave radio broadcasts. The first station I heard ID was WWCR out of Nashville, Tennesee, although there are plenty of others I can hear.

The trouble with such a simple receiver (no variable capacitor) is that there is little selectivity and the listener is bombarded by 2 or more stations at once - a mixing of all of the signals within a range of frequencies. To fix it would add complexity, but it's not hard to do.

For my antenna, I just have an 8' wire tacked up to the wall. For my ground, I have an 8' wire precariously plugged into the ground on an electrical outlet (I wouldn't recommend it, but it does work).

In the picture, the toothpick is shown on the coil with the wraps every 5 turns. The green wire is my ground, the black alligator clip goes to my antenna wire (the clip makes it easily movable for tuning), the diode is at the top of the coil, next to the green spool, and then of course the white high-impedence ear piece. Also note that no solder was required, I opted to use springs to connect points, but twisting and taping would have worked just as well.

If you're interested in trying it out, gather the required parts and let me know how yours turns out.

Grendel sez: For my next trick, I hope to swap the diode out for a razor blade and safety pin! The best explanation of how crystal sets work, that I've found so far, is here.

update: Success! After trying for an evening, I got the razor blade and saftey pin combo to act as a detector. Here's a picture of the setup.
I used regular straight blades (found by more "modern" blades in the shaving section of most stores) and an ordinary safety pin. After reading about the "blued steel" of the WWII-era blades, I learned that the blue hue of the blade was due to a heating treatment that is done to the blade during manufacturing. I wasn't having any luck with my blade so I gripped the blade with pliers, held it horizontally in a flame until I saw the blue appear on top of the blade. I wiped away the black residue from the flame on the bottom and tacked down the blade.

Next, I got a small piece of lead from an automatic pencil, connected it to the safety pin via a small dab of solder, tacked down the pin and hooked up the radio. (Blade goes to the ground, pin goes to ear piece.) You have to be patient and fish around the blade for the right point but you'll find it.

It also doesn't help that I'm not very close to a strong station, so it took a while longer. I'm sure anyone near a 50,000W AM station won't have much trouble. The signal is very weak, even compared to the diode, so make sure you are getting a decent signal with your diode before attempting this method. (I hooked up my ham radio antenna and got alot stronger signal than the aforementioned 8' wire.)

Tip: Try in the top-center of the blade around the hole in the blade - that's my sweet spot.
I've also found out that florist's wire works just as well and is alot cheaper (and more easily found in stores).

update: To make it even easier try a Slinky!
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tags: radio kit
+ Tim L.
  Aug 03, 2006 09:06
  
#769
I have long been fascinated by these fox hole radio setups. Great work!

reply

+ Tim L.
  Aug 04, 2006 14:20
  
#772
http://hometown.aol.com/djadamson7/articles/foxhole.html

reply

+ Tommy G.
  Aug 04, 2006 16:06
  
#773
http://journeytoforever.org/edu_xtal/PLANS2.HTM

I think the ribbon cable is a great one too.

reply

+ Tommy G.
  Aug 04, 2006 16:10
  
#774
This will save you some math calculations if you're winding your own coil:
http://www.ourhousenews.com/inductance.php4

reply

+ Tommy G.
  Aug 04, 2006 16:18
  
#775
More improvements on a theme:
http://www.imagenisp.ca/jsm/Crystal.html

reply

+ anonymous
  Aug 07, 2006 12:41
  
#778
I used to make these as a kid. The selenium coating on the old blades is what gave the junction with the steel pin (a diode).

reply

+ anonymous
  Aug 07, 2006 12:45
  
#780
nostalgia. I made one of these over 50 years ago, when I was 12 years old. The plans were in a Superman comic. Another good detector is lead sulfide, take a bit of lead & some sulfur powder & ignite. Then wrap the blob in a bit of bare copper wire.

reply

+ anonymous
  Aug 07, 2006 13:29
  
#781
How do you make the earpiece?

Vik :v)

reply

+ Tommy G.
  Aug 07, 2006 18:46
  
#785
Welcome Diggers

reply

+ anonymous
  Aug 09, 2006 11:43
  
#790
Actually you can connect the earphone across the diode along with an antenna it will work. It's even got the advantage not missing anything good because you were listening to the wrong station. It gets all of them at once! (an even better performing one is use a germanium transistor: base=antenna, collector=earphone, emitter=earphone and ground)

reply

--- Tommy G.
  Aug 09, 2006 12:35
  
#791
Which kind of Germanium transistor?

--- anonymous
  Aug 10, 2006 13:47
  
#792
I used a 2N170 or 2N35 but almost any small-signal Ge transistor would do.
There are actually two different hookups that work well (easy experiment to try).

+ Tommy G.
  Aug 18, 2006 13:43
  
#805
Gollum has some good plans.

reply

+ anonymous
  Sep 04, 2006 19:14
  
#937
Excellent source, thanks!

reply

+ anonymous
  Sep 12, 2006 14:36
  
#939
Wouldn't it be easier to just buy a radio? (just kiddin')

reply

+ anonymous
  Nov 01, 2006 05:01
  
#962
If you enjoyed this, try a reaction set. 2 trs for rf amp, 1 tr for af and you've got good quality speaker sound. Reaction sets are challengingly unstable.

reply

--- Tommy G.
  Dec 06, 2006 01:23
  
#969
...do you mean a regenerative set?

+ anonymous
  Mar 06, 2007 11:44
  
#995
here you have two good lively forums can ask questions on crystalsets.
http://www.midnightscience.com/rapntap/default.asp
http://radioboard.schmarder.com/index.php

here you have online calcuators for coils/cap... variations
http://crystalradio.net/professorcoyle/

reply

+ Tommy G.
  Apr 24, 2007 21:57
  
#1016
Featured in Make

reply

--- Tommy G.
  Nov 24, 2008 11:10
  
#1459
Make also has an Interesting video about LEDs and how they first came about from early radio experiments.

+ anonymous
  Nov 07, 2007 02:21
  
#1271
Great radios!

Try using a mk484 to amplify the signal!

reply

+ jonn
  Apr 23, 2008 15:06
  
#1416
I wish to get a high impedence headphone

reply

--- Tommy G.
  Nov 24, 2008 11:09
  
#1458
For anyone wishing to get parts, you can check out online sources such as Xtal Man.

...personally, I'd like to get my hands on a Silicon Carbine crystal. If you know of a good source, please post a link here.




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