We live in a world full of electronics, and yet there's one thing we're all mostly oblivious to. Everything electrical you own, including whatever device you're reading this on, has a magnetic field. Simply put, where there's electricity, there's magnetism, but despite all that, we have no real way of sensing these magnetic fields, until now. With just a few parts, I'll show you how to make a pair of headphones that allow you to listen in on all of the magnetic fields given off by your appliances.
Check the video for a quick demonstration.
Step 1: Acquire Materials
Before you start, you're going to need to collect a few components. Most of these are pretty common, so they shouldn't be hard to find.
- Headphones (1 pair) - Buy or salvage a cheap pair you don't mind never using again, they're going to be, well... altered.
- LM386 ICs (2) - These will function as the amplifiers for this device. They shouldn't be too hard to find at your local Radio Shack, should you be lucky enough to live near one.
- 8 Pin IC Sockets (2)
- 100uf Capacitors (2) - These are technically optional, but they increase the gain of the amplifiers significantly.
- Carriage Bolts (2) - Mine were 5/8" in diameter, but it really depends on the size of your headphones.
- Enamel Coated Magnet Wire - Make sure you have a significant amount, since we'll be using it to wrap coils.
- 9 Volt Battery Clip (1)
- 9 Volt Battery (1) - To power the device...
- SPST Switch (1) - Also optional, but much easier than removing the battery from the clip.
- Extra Bits of Wire - While the magnet wire could be used for the whole thing, I find using regular wire a bit easier.
- Soldering Iron and Solder
- Wire Strippers / Cutters
- Paper Towels
- Electric Drill
- Some Type of Adhesive
Step 2: Preparing the Headphones
To start, we need to get rid of all the excess wires that will get in our way later on. If your headphones were cheaply made like mine, the wires should be easily accessible, and therefore easily removable. Try to remove all the wires you see, including the cord with the plug (though hold on to that, it could be useful someday) and any wires going through the band. Try to remove any plastic that might also get in the way of where you'll be drilling the holes for the bolts.
Step 3: Adding the Coils.
This step is probably the most dangerous, both for you and your headphones, so be sure to use common sense when drilling the hole. Start with a small hole, and continue to enlarge it, either with the drill or a file, until the bolt fits snug inside of it. It should be able to hold itself without needing a nut on the other end. Failure to do so will most likely result in the headphones coming apart as pictured.
When wrapping the coils, try to avoid wrapping backwards over the coil. I can't give a specific number of coils to shoot for, but I'd recommend taking the full advantage of the space on the bolt, since the more coils you wrap, the more sensitive the circuit will be. After you wrap the coils, be sure to sand off the enamel from the ends of the magnet wire.
Step 4: Preparing the Speakers
Plug your soldering iron in, and let it heat while you open up the speaker casings. If your headphones have cushions, they should in most cases come off pretty easily. In my case, after removing the cushions, I had a plastic lid with three tabs that needed to be pushed in to remove it, thankfully, the speaker was part of that lid, and it also was easy to remove.
Once you've gained access to the speaker, you'll find more wires. It's alright to cut all of these out, but make sure to at least remember which terminals on the speaker are different. In my case, there were four tabs, made of two pairs of connected tabs, most likely to make it easier to manufacture.
Cut four lengths of magnet wire, roughly a foot long, and make sure to sand off the ends.
Is your soldering iron ready yet? Good. After heating the terminals of the speaker to remove any remaining wire, solder one of the strands of magnet wire to a terminal, and another strand to the other terminal not connected to the first one. Do this for both of the speakers.
A good way to test your connection (something I highly recommend doing after every solder joint), quickly connect both wires from whatever speaker you're testing to the 9 volt battery. If all is well, you should hear a rustling noise when you first connect it.
After you've made the necessary connections, reassemble the speaker and run the wires out of the casing.
Step 5: Amplifier Circuit Part 1
For this step you're going to be using an IC socket, since ICs aren't too keen on heat, and directly soldering to them could cause damage. When first attempting to make this circuit, I found solder had leaked into the socket, and as a result, I wasn't able to fit the IC legs into their holes. To combat this, the second time, I used a small piece of paper towel that I poked the pins of the socket through, to prevent any further solder leakage.
You'll want to follow the schematic in the pictures. Pay careful attention to the polarity of the capacitor. After the soldering is done, place the IC into the socket, again being careful to orient it correctly. The dot should be on the top, which is the same side as the capacitor.
After building the circuit, a good way to test it is to connect one of the speakers as shown with some sort of temporary connection such as alligator clips, and to power the circuit. If you place your finger over pins two and three, you should hear something other than static over the speaker, indicating success.
If you do not hear anything at all, chances are a connection is bad somewhere. Check to see that the enamel on the speaker wires has been sanded off on the ends, that all the solder joints are good, and that none of the pins on the IC socket are accidentally connected.
Remove the speaker, power supply and IC before continuing.
Step 6: Amplifier Circuit Part 2
Cut the leads on the headphones so that you have anywhere from 1-2 inches to work with, and sand off the enamel coating. Carefully solder them to the amplifier circuit as shown in the picture.
Do the same with the coils, making sure that you pull the wires tight before cutting them.
The amplifiers are now complete, and functional. Put the 386 ICs back in the sockets, and connect the 9 volt battery to the circuit in the same places as the last test. You should hear a light static in the earpiece. While I wouldn't recommend trying the headphones on just yet, due to the circuitry still being rather fragile, placing your cell phone near the coil should create a signal loud enough to be heard even without wearing the headphones.
Step 7: Sticky Situation
My apologies for the pun in this step's title, but I just couldn't resist...
Using hot glue, tape, epoxy, or whatever other method you see fit, attach the amplifier circuits to the headphones, along with the switch near the top of the band. Make sure when you attach the switch, the tabs you're going to solder onto are on the top side, so that they're easier to access.
After letting the glue or epoxy cure, move onto the next step.
Step 8: Final Connections
Solder the positive lead of the battery clip into the switch, and solder the other tab on the switch to the positive connections of both amplifiers. Connect the negative end of the battery directly to both negative ends of the amplifiers.
You can now add a battery into the clip, flip the switch and test the circuit. Make sure you hear static from both speakers before continuing. If not, check your connections, or try isolating just the amplifier at fault.
Step 9: Cleaning Up
I'd recommend taping down all of the loose wires to prevent noise on the circuit (yes the amplifiers can be that sensitive at times), as well as preventing anything from coming loose. Then, just add the battery with a rubber band and you're done!
Step 10: Ready for Testing
If all goes well, and the device works, you can now explore the EMF around you. A consistent hum in the background isn't uncommon, being that most of the appliances you use run on 60 Hz alternating current, however not everything sounds like that, here are a few examples I heard.
- Computer - All sorts of strange static, as well as high pitched whistling near the processor. You can also sometimes hear clicking if you're near the hard drive.
- Fan - A rather loud 60 Hz hum with a pretty large field (at least a 6 foot radius) not that unexpected, considering the motor makes use of rather powerful electromagnets.
- Cell Phone - Loud clicking at about 4 to 10 Hz, probably from it checking the tower, along with sounds similar to the computer's processor when using data. If you get really close, you might also hear sounds from it using the internal storage, and even different tones whether your finger is touching the screen or not.
- Wall Wart AC Adapters - Again, a 60 Hz hum, this time from picking up the field of the transformer.
- Earbuds - This circuit can just barely pick them up, but it still is possible to listen to someone else's music, although they tend to notice when you're less than an inch from their ear.
As you can see, there's a lot going on in all of these devices that we're completely oblivious to, but now you have the chance to witness it yourself. Have fun, and if you find anything interesting with your headphones, make sure to share it in the comments!