Build an antique style crystal radio

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.

Comments

[kynite]

what's the purpose of the coil in the circuit?

[clovisfiesta]

You are very capricious. Congratulations on the project.

[the "wow shammy"]

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?

[Dr.Bill]

Radio Shack sells a small 9v amp you plug into the headphone jack.
Works Real Good.

[the "wow shammy"]

thanks!

[Dr.Bill]

VERY WELL DONE !
There are schematics to be had for some of the most complicated crystal sets
complete with Wave Traps, Ferite wound coils, 600/44 Litz Wire, and lots of other things !
There is even a DX Contest ! for AM Listening !
I have built several of these sets and the best so far was a Push/Pull wired coil with 2 tunable tank circuits.
Check out midnightscience.com plenty info there.
Also, Crystal Set Society is another good source for AM DX'ing Contest.

Great Ible !
More please !

[riverreaper]

mabey ill just put the scanner in a nice looking box like that one an pretend like when i was a kid :O)

[riverreaper]

does the floor jack hold your table up

[riverreaper]

i wanted to build one but now i think buying might help me live longer lol

[ElectricUmbrella]

Excellent job, but how can you use low-impedance earphones with this set?

[uptight]

carrier wave amplitude modulated draw creates an optical illusion!!!

[Xellers]

Dual 1T4 regenerative vacuum tube radio receiver =P

The first tube is an RF amplifier, while the second serve as the regenerative stage.

[Normjr]

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.

[Ohm]

Well, thank you very much :) I did get second place in the science fair contest, the winner did a very cool Rubin's tube.

[zoltzerino]

Ive been looking into this and i can get all the parts instantly except ceramic headphones (are these necessarily?) because if they are not ill just use a normal pair. ZZZZ

[Ohm]

No for a crystal radio you have to use high impedance headphones or as it mentions in the instructable, you have to use an impedance matching transformer to run normal low impedance headphones. You could also use a crystal ear phone which is high impedance as well.

[zoltzerino]

Well to give me an idea on how to gauge it is 8 ohms high or low impedance? ZZZZ

[Dencohe]

Not sure if anyone has responded to you, but 8 ohms is very low impedance, high impedance runs 1k+ and if you get into serious xtal set building a 1k to 8 ohm transformers does not match a high Q tank circuit load very well. You need more of a 40K + ohm transformer to match that to 8 ohms but for this circuit using magnet wire and an air core 1k to 8 ohm that radio shack sells should be perfect. That is what I used.

[StopPre]

Hey Ohm, what book is that? sounds interesting. :)

[Xellers]

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...

[Xellers]

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.

[zoltzerino]

LM386, isn't that an operational amplifier DIL IC? ZZZZ

[Xellers]

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.

[mattccc]

i have that book the the boy electrician

[alexhalford]

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

[Ohm]

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.

[alexhalford]

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

[scooter750]

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.

[Ohm]

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.

[Dencohe]

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[Ohm]

It shouldn't make too much of a difference, I can't find my book right now with the calculations for the coil but it should be OK. Also since the coil is tapped your breaking up the coil into smaller coils with the taps and strong stations tend to be tune in on several of the taps.

[Dencohe]

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[Ohm]

You well need to post some photos when it is done, I can't wait :D Are you building it into a box like I did or are you just mounting the panel to a base board?

[Dencohe]

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[Ohm]

Why did you order a rotary switch from Bangkok? There are so many suppliers that would be more local, even if you had to order it over the internet? I can get them locally for $2. Should look cool mounted breadboard style like that and the original set that was shown in the book was just a panel mounted to a base board.

[Dencohe]

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[Dencohe]

Hey Ohm here it is, All done and everything working like you said, its the best set ive ever made. It has a kinda modern future theme. Hope you like it. Not so good at soldering but was able to do everything

[thewoodcarver]

Very nice work from both of you !! I have made several dozen sets none as fancy as these .....started back in the mid 60's when AM was still king and you couldn't touch the dial and not hit another station ......now I get 3 stations a local Christen station ESPN and a Spanish station ......I have tried changing antennas and different crystals Galina fools gold store bought and still 3 .....I may take a crack at making this one ....keep up the fine work

[Ohm]

This one is pretty good because it has good range so I get about 7 or so stations, 2 Spanish, 1 Christian, 2 sports and about 2 news. The unfortunate thing about the set is that it is not very selective so the one local station that is only a few miles away covers a lot of other stations, including two music stations. I might try one of the other sets that is in the same book that is more of a local set and see if I can get better selectivity with it.

[Ohm]

Looks good, what kind of frequency range did you end up with as your coil was slightly different? The soldering looks pretty good from what I can see and remember it's an acquired art, you learn new tricks each time you heat up the iron.