During World War II, GIs in the field built really amazing simple radios to listen too. These were made with materials that they could get their hands on and were small enough to carry around in a big pocket. You can modify this design if you want to set it up so that itâ€™s tuneable too! Subscribe Link
Earphone Update: A bunch of folks in the comments have mentioned that you need to use special earphones. Raymond sent me an email explaining the earphone controversy.
An earphone converts electrical energy to sound waves by vibrating the air. We detect sound by sensing these movements in the air. The difference between the crystal earphone and regular headsets is that the crystal earphone is very sensitive to voltage change where as the regular headphones require current change to make its diaphragm move. Since a crystal radio is not going to produce large amounts of current the crystal earphones are the best match and will produce noise from the microvolts being produce by the antenna- coil combination.”
37 thoughts on “Make a Foxhole Radio – Weekend Project Podcast”
Is the bluing an oxide formed on the surface of the razor? Does a rusty blade work because the iron oxide contacted with the graphite pencil lead forms a semiconductor?
I’ve read about the earphone being made from iron, wire, and bamboo, a tin can, or something else for a resonator.
Honestly, it’s magic to me, but my research indicates that it’s the oxide coating that comes with bluing steel and rust that make it work! If someone can explain how the detector works in simple language, I’d love to hear it!
great work on this one bre, great project!
Hi Bre — this is my first time visiting the site and I enjoyed the tutorial immensely. Looking forward to getting the PDF.
You can find the blued razor blades online. Or, if you can’t find those you can use a hacksaw blade. They are treated on the cutting end (the little notches) which is where you would want to put your whisker.
Better yet, you can make a crystal radio (which is what this essentially is) but with a much easier to find diode. Germanium diode. Look it up.
next project, disposable battery powered amplifier made from similarly available parts?
Great project and good video to go with it. Thank Bre for explaining why some things didn’t work , trial and error is a big part of learning for the younger Makers out there and I think this was presented well.
Thanks everyone! I’m really pleased with this video and project! It’s magic! The astonishment as I listened to it was like the first time I developed photos in a darkroom.
Make this project and post pictures of your radio up on the Make: flickr pool!
I am working on a very old rusty memory of helping a neighbor with a Boy Scout project maybe 50 years ago and was hoping this presentation would help connect it for me.
This radio appears to get only one station ? So looking at this picture I tried to remember what might be the difference between this and the old Boy Scout one, and could only remember something to this effect.
Put more turns on the coil [hundreds?] and leave one end without a connection to the circuit. At the tack that would have make the other connection, put a long narrow piece of metal, brass or copper as I remember, but it would seem spring steel or even a paper clip properly bent seems logical. Instead of connecting to the other end of the coil have this tack mounted strip of metal extend and touch the windings of the coil under it’s own spring pressure, to the windings of the coil, to make the connection. The piece I remember was made pointed so it would only touch between two wires at once, rather exactly, and bent and pointed directly down on the coil. [The coil, once made, had to have the shellac off of one side of the wires where the contact would touch it. Rubbing the pointed contact spring metal strip back and forth across it might work] Thus, so it could be moved back and forth over the wires of the coil so connection could be varied from including in the circuit, maybe only 50 windings, up to several hundred windings, in so doing, thereby finding many stations. I remember doing this “tuning”, along with the razor blade stuff.
OK, a very old recollection, some electronics engineers out there tell me if this is right???
I’m not selling it, but this project looks similar but uses a diode, mentioned earlier, instead of the razor. Razor ?, hack saw blade ?, or whatever, I don’t remember the diode though.
Really doubting that the blades where heated to turn them blue, because that take the temper out of them. allowing them to dull faster. Remember guns are blued to give them sone corrosion protection I googled bluing and found http://en.wikipedia.org/wiki/Bluing_(steel). Bluing is a controlled corrosion process. My guess in in the heating and cooling of the blade you used a layer of oxide was created.
Bre; if my memory of the story about the foxhole radio is correct the GIs use the foil from cigarette packages to make a variable capacitor to give selective tuning. They may well have made coils with taps or two coils where they could have adjusted coupling between them as well.
Bre, Connect the ground to a cold water pipe. You will have a better signal.
In B4 Cryptonomicon
I really like this. My first crystal radio used a variable capacitor from an old ham radio antenna tuner for a tuner and a germanium diode, but they aren’t that easy to come by. I like this because you should be able to make it with stuff ordinary people have in their house.
Here is my guess at why the razor blade detector works:
You are making a diode here. The diode allows current to flow only one way so when an “AC” signal, like a radio wave comes in, the diode cuts of the bottom half of the sine wave.
I think you are making a Schottky diode, a diode made with only a metal/semiconductor boundary. My *guess* is that the graphite acts as the metal conductor and the oxide formed when heating the razor blade forms the semiconductor. You could probably replace the pencil with he safety pin, if you were careful to not scrape through the oxide layer.
I just don’t know what kind of oxide is formed on the razor blades with heat. Does anyone know?
You should be able to make a tuner with a wiper made of springy metal rubbing against the coil as described by highball if you don’t have a variable capacitor hanging around your garage.
Further wikipeding (it’s not as easy to switch to a verb as googling) revealed that it is a “A non-linear resistance property of the blued steel of razor blades, foreshadowing the same property that would later be discovered in semi-conductor diode junctions…” that provides the diode-like effect.
If I’m not mistaken a variable resistor should be able to be used to change the stations.
How to make a variable resistor!
Take a sheet of paper with a graphite pencil draw a very dark long rectangle. Take 2 paperclips and connect them to the paper, and make sure they’re touching the rectangle. Move the second paperclip around to change the station. The longer the distance between the 2 paperclips the higher the resistance through the graphite semiconductor.
The “detector”, or the coil, is less a detector and more an energy gatherer. Am radio stations are actually electro-magnetic signals, you can’t see them, but they’re flying through the air all the time. The coil captures the electromagnetic radiation, and converts it into a current. This current flows though the wire, is forced in one direction by the diode, and the current pushes the speaker. This signal is sent out fast enough that the speaker vibrates with a sound.
Here’s some pictures of what the variable resistor should look like, approximately. (The black rectangle should be much much darker though. I took these with my phone, so bear with the quality.)
I’m no EE, I’m just a kid, so if I’m wrong don’t give me too much flack :). It should work though.
I won’t give you much flack, but yes you are wrong. A variable resistor will not help with tuning in stations.
The way these radios work is by forming an LC circuit that resonates at the frequency of the station you are tuning. So to tune to a differen station you have to change L or C. In this case you didn’t really put in a physical C.
This was mention above, but to change L you will need to make the coil longer. Then, take fine sandpaper and sand off the varnish along one strip of the coil. Disconnect the connection at the end opposite the antena. Instead, make the connection sweep along the bare strip of the coil. This will give you a variable inductor allowing you to change L.
Ahhh nice :). Yeah a variable resistor would just change the volume. Sorry bout that.
I have built several foxhole radios, and I have never used a blued razor. I use any thin any thin piece of steel that I can find. For example the last foxhole radio I built used a canning jar lid .The real trick in getting a foxhole radio to work is oxidizing the piece of steel (in this case the bluing of a razor blade) a easier way to oxidize the steel is to let it rust . Now, all you need to is touch the pencil head to a thick area of rust and you will pick up sum form of a radio signal. It is also important to know that regular head phones will not let you hear a signal very well, so you will need a low voltage headphone like the one in the video, or you can use a old pair of headphones plugged into the microphone jack on your computer and record the sound.
Totally off topic here – would it be possible to release the podcast in a format that is compatable with the ipod/iphone? I would love to be able to sync my iphone with the Make podcast on my PC without converting each video myself. GREAT weekend project this time, Bre!
p914 – it is. The links under the video are there for that reason.
I think I might have to make one of these… I have some diodes here…
This is an awesome project. A while ago I searched long and hard for a project like this to no avail, so I’m glad to see that this works and is pretty simple to do. One question – where did GI’s get piezoelectric earphones? Presumably they could have used speakers, but those aren’t very available either and neither are magnets to make them. I have also heard of the variable capacitor idea, but I’ve heard of using two pieces of foil in between pages of a book. Anyway, again, great project.
Would it be possible to change the number of turns of wire around the toilet paper roll to change the frequency you are tuning to. I know little about radios and how they work, but I was wondering if would be possible to tune it to the civilian aviation band VHF 108 to 136 MHz. I have know idea, but i would be cool.
Haven’t tried this yet:
My husband and I keep fighting about this… he says it is no accident that you used *that* earphone because it is one with a crystal in it, and just any old (or rather newer) earphone won’t do.
I say that’s a pretty important piece to leave out of the project– especially if people who don’t know better are going to go hacking apart headsets that aren’t the right kind.
he’s the expert, but thoughts?
I have that earphone in my kit from my old school 200 in 1. I didn’t try it out with other earphones, but it looks like you need something like that earphone. I just found a link with some instructions on how to make your own earphone… now that’s diy 1337 right there.
Read to the end – http://bizarrelabs.com/foxhole.htm.
For those of you interested in tuning, you can change the amount of winds around the tube or make a slider to do that too! I can’t wait to see pictures of all these radios people are making… I hope you all upload them to the make flickr pool or send me an email with a pic.
Oh, I just had the coolest idea… case mods! What could you put this radio inside?
On a related note, there’s an episode of the wonderful Secret Life of Machine about radio. It begins with a similar radio and then explains the inner workings of the radiosets and the history of the radios. This is a fascinating watch.
The secret life of machines is required watching for everyone!!! Thanks bf5man!
What is the formula to determine the number of winds for a specific frequency? Is there any other information about this radio type?
Oups, you can watch the radio episode of the Secret Life of Machine here:
built one these 48 years ago with a blue blade. Although I haven’t researched it I rather imagine the foxhole radio is a misnomer. When you were in a foxhole you were generally too busy trying not to get shot and shooting back to have time to make a radio, not to mention that these radios being powered by the radio signal itself needed quite a long antenna. Again, not something you would get out of the foxhole to hang from a tree.
Most likely these were made in the pow camps. There you would have material, opportunity and motivation. The motivation coming from the fact that prisoners were not allowed radios or information about anything outside the prison camps.
I, too, remember making similar radios– I believe Boy’s Life Magazine had an article called “Razorblade Radio”
As mentioned above, blueing is a controlled corrosion of steel. Heating the blade annealed it, thereby making the steel soft, but also opening it to not-so-controlled corrosion.
I wonder whether a piece of blued strapping steel would work? Many items can be used as detectors. Might be fun to post some of the odd ones!
BTW: a reference to this design being used by GI’s in Italy during WWII: http://members.aol.com/djadamson7/articles/foxhole.html
Great site on xtal radios:
Would 20 gauge wire from a craft store work for the wrapping wire. It is all i have
… the power comes from the air would it be possible to collect enough of it to power an led?
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