I have, for some unknown reason, always been interested in ancient technology, mainly electronics and mechanical things and I have always wanted to make a true old fashioned style crystal radio. A crystal radio, or crystal set as it is sometimes called, is a radio that only uses the power of the radio waves picked up by the antenna to generate the sound heard in the head phones. The reason it is called a crystal set is because they use a mineral crystal as a diode for the detector in the circuit.

Quite a few years ago I found a book in the library called "Radios That Work for Free" by K E Edwards and found a set of plans for a radio that I just had to make, unfortunately I did not have the resources to build it at the time so I made a photo copy of the pages from the book for future reference. Well I was poking around on the net the other day and found a site that sells copies of that old book, along with various other electronics books and parts related to crystal radios and decided to order a copy for myself along with two 365 pf variable capacitors needed for the circuit I copied from the book.

The radio that I am going to build is the second one shown in the book but I am going to make some changes to the original set. The three changes I am going to make to the set are that I am going to build the set into a box instead of just having the control panel attached to a base board. I am also going to make an antique style "Cats Whisker" style detector instead of using a Germanium diode and use a matching transformer so I can use a set of modern headphones instead of trying to find an antique high impedance set or a crappy little crystal earplug.

I have written this instructable assuming the reader has basic wood working and electronics skills like making boxes and soldering, explaining the more involved and specific tasks in detail, as in winding the coil.

Step 1: Circuit and Theory

Here is a scan of the circuit from the book. The circuit is actually pretty simple, just two variable capacitors(C1 & C2), a coil(L) with 8 taps and a tap switch(S), a detector(D) and a set of phones(P).

Without getting into the rather intensive math involved, I have taken classes on this stuff, here is in a nutshell how it all works.

Crystal radios mainly work on AM or Amplitude Modulation radio waves, there are some that well work on FM as well. AM radio waves are actually composed of two waves combined together, one is a high frequency wave called the carrier wave and the other is the lower frequency sound waves being transmitted. The carrier wave is on the order of 750 to 1500 kHz. and is used to transmit the sound wave. The way it works is that the two waves are added together, or modulated, producing a signal that is of the same frequency as the carrier wave but its amplitude is determined by the sound wave(see second picture).

The way a crystal radio actually works is pretty ingenious, it uses the coil L and variable tuning capacitor C1 as a low pass filter to only allow frequencies at the desired frequency of the carrier wave to be passed to the detector D. The detector then demodulates the signal by only letting the low frequency part of the signal to pass, ie the sound wave, to the headphones where they are then turned back into sound. There are no batteries, no transistors or amplifiers of any kind, the sound is produced solely by the energy of the filtered radio wave.
<p>I made my first successful crystal radio last Fall and plan to make a much nicer one (like this) as soon as I can. There is just something awesome about listening to one!</p>
Wow, this is really elaborate and dedicated! Personally I am not much of a technical person but I am sure a sucker when it comes to antiques. I always feel that they are so classy and makes me feel that I am able to time travel just my interacting with such antiques. However, I do not think I would be able to accomplish building such timeless radio on my own as it just seems too tough for me. But if I were to make one on my own I would most probably put some antique silver designs on it just to add on that vintage feel.
Outstanding! I will certainly use a lot of the information here when I build mine.
what's the purpose of the coil in the circuit?
You are very capricious. Congratulations on the project.
i made something like this out of a book but icant decipher any of the things i here! is there any way to make it louder?<br />
Radio Shack sells a small 9v amp you plug into the headphone jack. <br>Works Real Good. <br>
VERY WELL DONE ! <br>There are schematics to be had for some of the most complicated crystal sets <br>complete with Wave Traps, Ferite wound coils, 600/44 Litz Wire, and lots of other things ! <br>There is even a DX Contest ! for AM Listening ! <br>I have built several of these sets and the best so far was a Push/Pull wired coil with 2 tunable tank circuits. <br>Check out midnightscience.com plenty info there. <br>Also, Crystal Set Society is another good source for AM DX'ing Contest. <br> <br>Great Ible ! <br>More please !
mabey ill just put the scanner in a nice looking box like that one an pretend like when i was a kid :O)
does the floor jack hold your table up
i wanted to build one but now i think buying might help me live longer lol
Excellent job, but how can you use low-impedance earphones with this set?<br />
carrier wave amplitude modulated draw creates an optical illusion!!!<br />
Dual 1T4 regenerative vacuum tube radio receiver =P<br /> <br /> The first tube is an RF amplifier, while the second serve as the regenerative stage.<br />
This has to be, so far, bar none, absolutely, positively, THE BEST Instructable that I have EVER been to. It is also the most professionally documented. The photographs are crisp and clear and are provided not only at the most important steps, but also many of the intermediate steps. The instructions are easily understood, easy to read, with proper grammar and correct spelling. It, in nautical terminology, “Blows all of those mint box instructables clear out of the water and sends them all to Davy Jones' Locker”. Ohm you certainly have done one “bang-up” job, you are the only instructor that I have subscribe to. This Instructable, in my not so humble opinion, deserves the “Grand prize of Grand Prizes award” if there is one.
Well, thank you very much :) I did get second place in the science fair contest, the winner did a very cool Rubin's tube.
Hey Ohm, what book is that? sounds interesting. :)
i have that book the the boy electrician
I finally got around to building a radio! For years, I knew how to build one, but didn't because I live in the city, and putting up a 100 foot antenna simply wouldn't be possible. But then, an idea came to mind. What if I made the coil big, REALLY BIG and had it serve a double purpose? So I took some pieces of wood about a yard and a half long and nailed them together to make the form. Then, I wound all of the stranded wire I had in my house around the form, and my coil was born. Unfortunately, I didn't have any variable capacitors or high Z (Impedance) earpieces, so quickly soldiered together a five minute transistor amplifier (Not push-pull or anything fancy, just a single transistor with a bunch of resistors), and for the headphones I used an old '60s telephone headset. Instead of the variable capacitor, I used a fixed 470pf ceramic disk one that I found lazing around in my parts box. Excited to put it all together, the whole project took about an hour and a half from idea on paper to working device. I'm pretty proud of my creation. I'll be sure to post some pictures soon...
I just built my second radio! I found an LM386 and cobbled together a short antenna, a loopstick from an old TV, and a rectifier diode for the demodulator (detector) and a small capacitor and connected it to an 8 ohm speaker. It sounds great! Pictures coming soon. I am currently working on a Tube Regen using 12dz6 pentodes. I promise I will write more.
I just finished the regen! Works great! Instructable soon. I also built a two tube IT4 radio. I also put up a 100ft antenna and made a 5 minute crystal set.
If you need to wind a coil quickly, stick the form into a lathe and just guide the wire along (wear gloves). Makes it really quick and (worryingly) quite fun. Great 'ible. AlexHalford
That works but it helps to have a way to count the winds, for that I use a micro switch attached to an old calculator with the switch soldered in place of the + button. I enter a 1 and turn it once by hand and it counts from there. Unfortunately with this particular coil being that it has so many taps you kinda have to wind it by hand.
Love the idea with the calculator. Also, you could time yourself with a stopwatch and compare that to the speed of the lathe (i tend to use 65 RPM for coil winding) to figure out how many you've done. AlexHalford
I used plexiglas for a reproduction radio panel once, but sprayed the back of the panel black. This made the front finish very smooth and shiny black, but scratch-proof - more like bakelite. I used white letters on black labelling tape for the control lettering.
Yeah I realized that I should have done it that way after I had already painted the front of the panel. Oh well I well remember that next time.
The holes in the top of the box looked like an upside down smile!!!
The great thing about crystal sets is that you can make most of the bits yourself (apart from the earphone, haven't managed that yet ;-). I have made my own capacitors (aluminium foil sheets in a book), my own detectors (various sulphide minerals including galena and molybdenite, even a rusty razor blade and pencil lead) and of course my own coils. If you do a search on the internet you will find an amazing variety of designs and approaches. It's a great hobby for those that live away from a horse racing station that swamps everything. ;-)
hi, i was just thinking about how to make an earphone by myself, so that the crystal radio will be entirely homemade. I think we can use the piezoelectric caracteristic of quarz to build that kind of earphone. i might try it someday ;)
I just found this one the web, <a rel="nofollow" href="http://josepino.com/other_projects/index.php?making-high-impedance-headphones.jpc">Making High Impedance Headphones</a> It shows how to make a set of high impedance phones out of an old set of modern headphones and two broken digital watches. Might be a good Instructable on it own.<br/>
thanks for the link... this is a really good idea to make such earphones ;) first of all i need to make a 1kHz sine oscillator to define the nominal impedance of this piezoelectric device.
...... i just calculated that to make a earphone of about 1kohm impedance at 1kHz with a coil and a magnet. You need a coil of about 160 mH... And i also calculated that using AWG48 wire you need to make a Multi-Layer coil of about 4cm diameter, 1cm height and 2cm wire-layers width... and that makes about 1600 turns of wire. 1600!! i think i am going to make the piezoelectric earphone like on your link, it is easier ;)
I have actually seen plans on how to make a foxhole speaker out of a tin can a nail and magnet wire. Here is the text from the site "Richard Lucas, who was a POW in Vietnam, built a radio in camp and was also able to improvise an earphone. He writes: Four nails were bound together with cloth from our clothes. Wire was obtained from wire used around the camp which I might add wasn't coated with varnish. It was bare wire, so we wound a layer and, using a candle, we dripped wax over the turns, which were spaced as closed as possible without shorting out (not touching). We repeated this process over and over again until we had about 10 layers of wire, which were insulated from each other layer by a strip of cloth and wax. Then we put this in a piece of bamboo and adjusted it so it was about a 1/32 of an inch from the end. A tin can lid was positioned over the coil of wire and nails. Then connecting it to our "foxhole radio" (basic design as yours) we could here about three radio stations. Our antenna was the barbwire around the camp and the ground was wire laid along the ground to make up the ground. Best listening was at night and it had to be pretty quiet because the earphone was pretty weak. If we had a magnet to set up a bias on the coil, the volume would have been a lot louder."
Excellent - now I can make the whole radio myself. Now, if I could make the wire..... ;-)
That is one problem I am having is there is one station that is real close to where I live and it is drowning out three other stations around it so I think I well have to make a wave trap for it as well which should work pretty good I think. I got an outside antenna installed and that has helped a bit as well, I did not do an instructable on it though, didn't feel like taking photos while on the roof in the 90 degree heat. I am also going to make a two tube amplifier for the set as well, I have all the bits just laying around, is that a good thing or bad :) I might do an instructable on the wave trap and the amp though just for fun.
at the antenna connection make a series resonant circuit to ground at the frequency of the offending station. it goes tearing to ground and the other stations are not affected. use a coil and mica variable cap(series) adjusted to the stations freq. good luck.
The is exactly what a wave trap does except that it used a variable capacitor allowing you to tune it to the desired frequency, I have also seen one that has a coil with a slider and variable cap making it even more tuneable.
i didn't mean to insult u. hope i didn't. ur already all over it. i said mica variable to save the 365 air variable for the main tuning. they're not getting any cheaper. the variable coil arrangement makes it very useful where a lot of am stations can be heard. wouldn't u know, u always want the waeker one. a fixed coil worked ok fr me. i only needed to get rid of 5000w ststion 600 yards away. good luck.
ooooooo I wish I had a 5000W station close by. think of the power I could harness from it using only a AM antenna, a bridge rectifier, and a capacitor!
i need to try a voltage doubler on the wave trap. i think i can drive a darlington pair audio amp. maybe more. i can see a battery charger in the works. here i go againnnnnnnnnnnnnnnnnnnnn.
A few things<br/>The higher the better for antennas (outside definitely) and a good ground (or even two or three seperate ones).<br/>The best coil I have tried so far was the mystery set coil: <a rel="nofollow" href="http://www.clarion.org.au/crystalset/mystery.html">http://www.clarion.org.au/crystalset/mystery.html</a><br/>which managed to keep the loud station within a small band of the tuning and then allowed me to hear some of the others (2 sometimes 3). So a bit of experimenting with the coil might give you better results for your area.<br/>TRIVIA: I have heard that some people living REALLY close to a transmitter have been able to use a crystal set to charge batteries (don't know how long that takes though).<br/>I'd like to see the wave trap so feel free to post that instructable when done. ;-)<br/>Also I forgot to mention that I used silver wire with the molybdenite. There's a site somewhere where someone has compiled a list of the mineral combinations that work. I'll see if I can find it again.<br/>As I mentioned there is just so much scope to this concept - it's great!<br/>
the wave trap is really simple. a coil and variable cap that runs between the ant connection and ground. this is a series resonant circuit. only 2 parts : a tank coil and cap the same as the main tuning. tune the trap to the offending station. it may take some tweeking but what doesn't?
That would actually be a tank circuit, and the premise is easily had on a search. Basically, it's a wide bandpass filter. Kudos to you, Eromanga. You are definitely headed down the old school path ;)
There is a variation worth exploring, one I did not see during cursory exploration of the various linked sites, a project I made as a boy, especially if you have a strong station nearby. You can add a second crystal receiver in the same box, tuned to the strong station, fullwave rectify its output to charge a polarized capacitor, use the capacitor to power a one-transistor audio amplifier. Resulting sound will fill a room. U.S.
my word thats brilliant.
Sam you are making it more complicated than it actually is. This radio should drive an audio amp as is. Using a ceramic capacitor to couple the circuits.
Looks Great ! However using fahnestock clips, for the antenna and ground connections, could add to the old timey looks
Well I got the amplifier done, here are a few photos that I took of it while I was testing it. Mind the mess, I have way to many projects/ hobbies. The first photo is of the whole thing being tested, had to remove the panel and connect the amplifier with jumpers as I have not put connectors on top yet. I am also using my power supply for the heaters, I still need to scrounge around for a battery holder to run them on. It works really well, definitely gets the job done, I can here a few more stations that I could not before and of course the local station is even more domineering over everything else, too bad the music station is hiding just behind it.

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