WTF is Degaussing?

The modern office has become a sea of LCD monitors. It’s hard to believe that only a few years ago we were sitting behind Cathode Ray Tubes (CRTs). People have already forgotten the heat, the dust, and the lovely high frequency squeal from their flyback transformers.

Image by Søren Peo Pedersen via wikipedia
Image by Søren Peo Pedersen via wikipedia

There was one feature of those old monitors which seems to be poorly understood. The lowly degauss button. On some monitors it was a physical button. On others, it was a magnet icon on the On Screen Display (OSD). Pressing it rewarded the user with around 5 seconds of a wavy display accompanied by a loud hum.

But what exactly did this button do? It seems that many never knew the purpose of that silly little button, beyond the light-and-sound show. The truth is that degaussing is rather important. Not only to CRTs, but in many other electronic and industrial applications.


Of Shadow Masks and Aperture Grilles

Close up of a shadow mask by Rauenstein via Wikipedia
Close up of a shadow mask by Rauenstein via Wikipedia

A CRT has quite a few components. There are three electron guns as well as steering and convergence coils at the rear (yoke) of the tube. The front of the tube has a phosphor-coated glass plate which forms the screen. Just behind that glass is a metal grid called the shadow mask. If you had enough money for a Sony screen, the shadow mask was replaced by the famous Trinitron aperture grille, a fine mesh of wires which performed a similar function. The shadow mask or aperture grille’s  job is to ensure that the right beams of electrons hit the red, green, or blue phosphor coatings on the front of the screen.

This all required a very precise alignment. Any stray magnetic fields imprinted on the mask would cause the electron beams to bend as they flew through the tube. Too strong a magnetic field, and your TV or monitor would start showing rainbows like something out of a 1960’s acid trip movie. Even the Earth’s own magnetic field could become imprinted on the shadow mask. Simply turning a TV from North to East could cause problems. The official term for it was “Color Purity”.

magnet-trickThese issues were well known from the early days of color TV sets. To combat this, manufacturers added a degaussing coil to their sets. A coil of wire wrapped around the front of the tube, just behind the bezel of the set. When the set was powered on, the coil would be fed with mains voltage. This is the well-known ‘fwoomp and buzz’ those old TV sets and monitors would make when you first turned them on. The 50 Hz or 60 Hz AC would create a strong moving magnetic field. This field would effectively erase the imprinted magnetic fields on the shadow mask or aperture grille.

Running high current through the thin degaussing coil would quickly lead to a fire. Sets avoided this by using a Positive Temperature Coefficient (PTC) thermistor in-line with the coil. The current itself (or a small heating coil) would heat up the PTC, causing resistance to increase, and current through the coil to drop. After about 5 seconds, the coil was completely shut down, and the screen was (hopefully) degaussed.

As time went on monitors became embedded systems. The PTC devices were replaced by transistors controlled by the monitor’s main microcontroller. Monitor manufacturers knew that their sets were higher resolution than the average TV set, and thus even more sensitive to magnetic fields. Users are also more likely to move a monitor while using it. This lead the manufacturers to add a degauss button to the front of their sets. A push of the button would energize the coil for a few seconds under software control. Some monitors would also limit the number of times a user could push the button, ensuring the coil didn’t get too hot.

Holding a magnet near the front of a black and white (or a monochrome ‘green screen’) CRT created visible distortion, but no lasting damage. Mid-century hackers who tried the same trick with their first color TV quickly learned that the rainbow effect stayed long after the magnet was moved away. In extreme cases like these, the internal degaussing coil wouldn’t be strong enough to clear the shadow mask.

Commercial degaussing coil
Commercial degaussing coil

When all else failed, a handheld degaussing coil or wand could be used. Literally waving the magic wand in front of the screen would usually clear things up. It was of course possible to permanently damage the shadow mask. Back in 2007, I was working for a radar company which had been slow to switch to LCD monitors. Being a radar shop, we had a few strong magnetron magnets lying around. One of these magnets was passed around among the engineers. Leaving the magnet under your monitor overnight would guarantee rainbows in the morning, and a shiny new LCD within a few days.

Queen Mary, showing her degaussing coil

CRTs aren’t the only devices which use degaussing coils. The term was originally coined in 1945 by Charles F. Goodeve of the Royal Canadian Naval Volunteer Reserve (RCNVR). German mines were capable of detecting the magnetic fields in a naval ship’s steel hull. Coils were used to mask this field. The Queen Mary is one of the more famous ships fitted with a degaussing coil to avoid the deadly mines.

Even mechanical wristwatches can benefit from a bit of degaussing. A watch which has been magnetized will typically run fast. Typically this is due to the steel balance spring becoming a weak magnet. The coils of the spring stick together as the balance wheel winds and unwinds each second. A degaussing coil (or in this case, more properly a demagnetizer) can quickly eliminate the problem.

A story on degaussing wouldn’t be complete without mentioning magnetic media. Handheld or tabletop degaussing coils can be used to bulk erase floppy disks, magnetic tape, even hard disks. One has to wonder if the degaussing coils in monitors were responsible for floppy disks becoming corrupted back in the old days.

So there you have it. The magic degaussing button demystified!

The Booths Of Hamvention

Hamvention was last weekend in Dayton, Ohio. Last weekend was also the Bay Area Maker Faire, and if you want tens of thousands of people who actually make stuff there’s really only one place to be. Bonus: you can also check out the US Air Force Museum at Wright-Patterson AFB. The ‘Space’ hangar was closed, so that’ll be another trip next year.

The biggest draw for Hamvention is the swap meet. Every year, thousands of cars pull up, set up a few tables and tents, and hock their wares. Everything from radios from the 1920s to computers from the 1980s can be found at the swap meet. This post is not about the swap meet; I still have several hundred pictures to go through, organize, label, and upload. Instead, this post is about the booths of Hamvention. Everything imaginable could be found at Hamvention, from the usual ARRL folks, to the preppers selling expired MREs, and even a few heros of Open Hardware.

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The SEC Has a Thing for Crowdfunding

Kickstarter is not a store. Indiegogo is not a store. No matter what crowdfunding platform you’re on, you’re not in a store. This is an undeniable truth, and no matter how angry you are about not being able to bring a cooler with a blender to the beach this summer, you did not buy this cool cooler, you were merely giving someone money to develop this cooler.

This reality may seem strange for the most vocal Internet commenters out there, leading them to the conclusion their pledge for a crowdfunding campaign was an investment. Surely there must be some guarantee in a single pledge, and if it’s not exchanging money for some consumer goods, it is exchanging money for a stake in a company. If that were true, backers of the Oculus Rift would have received several thousand dollars each, instead of a $600 VR headset.

Crowdfunding is not a store, and according to Kickstarter and Indiegogo, it is not an investment, either. Last week, the Securities and Exchange Commission’s rules for “crowdfunded investing”, “Regulation Crowdfunding”, or “Title III Crowdfunding” kicked into gear. Is this the beginning of slack-jawed gawkers throwing their life savings into a pit of despair filled with idiotic consumer products that violate the laws of physics?

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Meter All The Phases: Three Phase Energy Meter With OpenWrt

Keeping track your overall electricity usage is a good thing, and it’s even better if you know where all the kilowatt-hours are going. [Anurag Chugh’s] house has the three phases coming from the electrical distribution box tidily organized: One for the lighting and fans, one for household appliances, and one for the hot water supply. To monitor and analyze the electrical fingerprint of his house, [Anurag] installed a 3 phase energy meter and hooked it up to the internet.

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Black Line Follower: A Modern Bristlebot

It’s been a while since we’ve seen much action on the bristlebot front, which is too bad. So we’re happy to see [Extreme Electronics]’s take on the classic introductory “robot”: the Black Line Follower. The beauty of these things is their simplicity, so we’ll just point you to his build instructions and leave the rest to you.

The original bristlebot is a fantastic introduction to electronics, because it’s simple enough that you can cobble one together in no time. A battery, a pager motor, and a toothbrush head are all you need. But it goes where it wants, rather than where you want it to go.

Adding steering is as simple as tying two bristlebots together and firing one motor at a time to execute a turn. The Black Line Follower is of this style.

Of course, any good idea can be taken to extremes, as in this giant weight-shifting bristlebot, or this super-tiny IR-controlled bristlebot.

But that was more than five years ago now. What happened to the mighty engines of bristlebot creativity? Has the b-bot seen its finest hour? Or are we just waiting for the next generation to wiggle up to the plate?

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Hackaday Prize Entry: Worldwide Educational Infrastructure

The future of education is STEM, and for the next generation to be fitter, happier, and more productive, classrooms around the world must start teaching programming, computer engineering, science, maths, and electronics to grade school students. In industrialized countries, this isn’t a problem: they have enough money for iPads, Chromebooks, and a fast Internet connection. For developing economies? That problem is a little harder to solve. Children in these countries go to school, but there are no racks of iPads, no computers, and even electricity isn’t a given. To solve this problem, [Eric] has created a portable classroom for his entry into this year’s Hackaday Prize.

Classrooms don’t need much, but the best education will invariably need computers and the Internet. Simply by the virtue of Wikipedia, a connection to the Internet multiplies the efforts of any teacher, and is perhaps the best investment anyone can make in the education of a child. This was the idea behind the One Laptop Per Child project a decade ago, but since then, ARM boards running Linux have become incredibly cheap, and we’re getting to a point where cheap Internet everywhere is a real possibility.

To build this portable classroom, [Eric] is relying on the Raspberry Pi. Yes, there are cheaper options, but the Pi is good enough. A connection to online resources is required, and for that [Eric] is turning to the Outernet. It’s a system that will broadcast educational material down from orbit, using ground stations made from cheap and portable KU band satellite dishes and cheap receivers.

When it comes to educational resources for very rural communities, the options are limited. With [Eric]’s project, the possibilities for educating students on the basics of living in the modern world become much easier, and makes for a great entry into this year’s Hackaday Prize.

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Sweet 3D Printer

Dylan’s Candy Bar is an upscale sweet shop in Manhattan. In a stunning proof that 3D printing has become buzzword-worthy, they’ve announced a deal with Katjes Magic Candy Factory to bring four 3D gummy printers to the US (specifically, to New York, Chicago, Los Angeles, and Miami).

The device looks a bit like a classic 3D printer, but it extrudes eight different gummis in a variety of flavors. The store offers twenty designs but you can also print text or your own drawings (including, apparently, your face).

Each creation costs about $20. Time will tell if this is just a stunt, or if we are going to see food printers cropping up at a mall near you. You can see a video they posted to Twitter below along with a video from the product roll out of the printer in question.

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