Sensing a Magnet with Local Sourcing

I had a small project going on–never mind exactly what–and I needed to detect a magnet. Normally, that wouldn’t be a big problem. I have a huge hoard of components and gear to the point that it is a running joke among my friends that we can be talking about building something and I will have all the parts we need. However, lately a lot of my stuff is in… let’s say storage (again, never mind exactly why) and I didn’t have anything handy that would do the job.

Options

If I had time, there are plenty of options for detecting a magnet. Even if you ignore exotic things like SQUID (superconducting quantum interference device) there’s plenty of ways to detect a magnet. One of the oldest and the simplest is to use a reed switch. This is just a switch made with a thin piece of ferrous material. When a magnet is nearby, the thin piece of metal moves and makes or breaks the contact.

These used to be common in alarm systems to detect an open or closed door. However, a trip to Radio Shack revealed that they no longer carry things like that as–apparently–it cuts into floorspace for the cell phones.

I started to think about robbing a sensor from an old computer fan or some other consumer item with a magnetic sensor onboard. I also thought about making some graphene and rolling my own Hall effect sensor, but decided that was too much work.

Browsing

I was about to give up on Radio Shack, but decided to skim through the two cabinets of parts they still carry just to get an idea of what I could and could not expect to find in the future. Then something caught my eye. They still carry a wide selection of relays. (Well, perhaps wide is too kind of a word, but they had a fair number.) It hit me that a relay is a magnetic device, it just generates its own electromagnetic field to open and close the contacts.

I picked up a small 5 V reed relay. They don’t show it online, but they do have several similar ones, so you can probably pick up something comparable at your local location. I didn’t want to get a very large relay because I figured it would take more external magnetic field to operate the contacts. You have to wonder why they have so many relays, unless they just bought a lot and are still selling out of some warehouse. Not that relays don’t have their use, but there’s plenty of better alternatives for almost any application you can think of.

The Fridge Test

I got home and pulled a rare earth magnet off the refrigerator and grabbed an ohm meter. Sure enough, I could reliably operate the relay contacts with the strong magnet. My project was in business!

Of course, your mileage may vary. The construction of any particular relay may or may not be conducive to external activation. You may have to experiment with the exact magnet, but those are easy to find in lots of local outlets, including home improvement and hobby stores.

The Mother of Invention

When you are cooking, sometimes it helps to know that you can substitute one thing for another. The same is true of electronic components. Need a bridge rectifier? Make one out of diodes. You can probably substitute an op amp for most comparator applications. LEDs can detect light and speakers can act as microphones although in both cases the results are not as good as  parts that are supposed to do those functions.

I do think it is interesting, though, that Radio Shack has such a blend of odd things. You can get a lot of Arduino shields, for example. You can also get a lot of relays. However, I noticed there was only one MOSFET in the cabinet and it wasn’t a “logic level” FET. Seems like it would be like going into an office supply store and finding PCs and carbon paper, but no LCD screen wipes.

Regardless of Radio Shack’s ability to keep up with the times or not, I was glad they had relays. When you are looking to scrounge something in a hurry, don’t forget to think about auto supply places, home improvement stores, craft stores, and even dollar stores — I’ve seen a one-dollar, open-door detector before, but I knew from a previous attempt they have the sensor integrated with the electronics and are hard to scrounge.

Postscript: After I completed my little project, while looking for something totally unrelated, I ran across this homebrew reed switch. Certainly another option.

 

 

3D Printing Permanent Magnets

Researchers at TU Wien wanted to create magnets with exactly the right magnetic field for a particular application. Their solution? 3D printing of magnets. Previously, it has been difficult to produce permanent magnets with a specific shape of the magnetic field. The resulting magnets will be a boon to magnetic sensor construction.

Previously, after designing a magnet with a specific shape and magnetic field, a researcher would have to create tooling for injection molding. This is expensive and time-consuming and often not worth it for small quantities of magnets.

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Ultra Simple Magnetic Levitator

Want to build a magnetic levitator in under two hours? With a total of 7 parts, including the coil, it just cannot get simpler than what [How-ToDo] shows here! It is not only an extremely simple circuit, it also has the advantage of using only discrete components: a MOSFET, hall effect sensor, diode and two resistors, that’s it.

The circuit works by sensing the position of the levitating magnet, using the hall effect sensor , then turns the coil on and off in response via the MOSFET. The magnet moves upwards when the coil is energized and falls down when it is not. This adjustment is made hundreds of times a second, and the result is that the magnets stays floating in mid air.

This is the kind of project that can make a kid get interested in science: it combines easy construction with visually amazing behavior, and can teach you basic concepts (electromagnetism and basic electronics in this case). Excellent for a school project.

For the more advanced enthusiast, more sophisticated levitator design based on an Atmega8 micro-controller will be of interest.

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Magnets for a Machinist

We’re not sure if [Stefan Gotteswinter] ever makes anything but tools to make more tools in his shop. This nice set of toolmaker’s magnets are no exception to the trend.

We can gather that [Stefan] is a professional machinist by trade. Like all professionals who do the same thing for work and play, he was spoiled by the nicer tools at work. One tool in particular, a toolmaker’s magnet, always came in handy. These are strong magnets that have been ground flat, square, and parallel.

He really only needed one magnet, so he started to build a 20 x 20 x 100 mm one. It would be made out of alternating mild steel and brass plates. The steel plates would have a hole drilled through them and he’d place a correctly oriented magnet in the middle. It would all be clamped and glued together.

The build was going pretty well when he decided that he couldn’t really trust the glue alone. He had just begun grinding, but decided to switch to a quick drilling operation. Two brass rods through the whole assembly would be enough to hold it together. He started drilling, and then, suddenly, he had two magnets.

The assembly had broken in half. He decided that, all things considered, two 20 x 20 x 50 mm magnets were also handy. So he completed the drilling, and ground the new set of magnets to be a perfect match to each other.  In the end he had a tool that looks just as expensive as the commercial option. There is also a video series on the magnets, part 1 and part 2, viewable after the break.

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Electric Train Demonstrator

If you ever want to pique a kid’s interest in technology, it is best to bring out something simple, yet cool. There was a time that showing a kid how a crystal radio could pull in a radio station from all the way across town fit the bill. Now, that’s a yawner as the kid probably carries a high-tech cell phone with a formidable radio already. Your latest FPGA project is probably too complicated to grasp, and your Arduino capacitance meter is–no offense–too boring to meet the cool factor criterion.

There’s an old school project usually called an “electromagnetic train” that works well (Ohio State has a good write up about it as a PDF file). You coil some bare copper wire around a tubular form to make a tunnel. Then a AAA battery with some magnets make the train. When you put the train in the tunnel, the magnetic forces propel the train through the tunnel. Well, either that or it shoots it out. If that happens, turn the train around and try again. There’s a few of these in Internet videos and you can see one of them (from [BeardedScienceGuy]) below.

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Homemade EDM Can Cut Through Difficult Materials Like Magnets With Ease

Many years ago [ScorchWorks] built an electrical-discharge machining tool (EDM) and recently decided to write about it. And there’s a video embedded after the break.

The build is based on the designs described in the book “Build an EDM” by Robert Langolois. An EDM works by creating lots of little electrical discharges between an electrode in the desired shape and a material underneath a dielectric solvent bath. This dissolves the material exactly where the operator would like it dissolved. It is one of the most precise and gentle machining operations possible.

His EDM is built mostly out of found parts. The power supply is a microwave oven transformer rewired with 18 gauge wire to drop the voltage to sixty volts instead of the oven’s original boost to 1.5kV.  The power resistor comes from a dryer element robbed from a unit sitting beside the road. The control board was etched using a hand traced schematic on the copper with a Sharpie.

The linear motion element are two square brass tubes, one sliding inside the other. A stepper motor slowly drives the electrode into the part. Coolant is pumped through the electrode which is held by a little 3D printed part.

The EDM works well, and he has a few example parts showing its ability to perform difficult cuts. Things such as a hole through a razor blade., a small hole through a very small piece of thick steel, and even a hole through a magnet.

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Real World Race Track is Real Hack

[Rulof] never ceases to impress us with what he comes up with and how he hacks it together. Seriously, how did he even know that the obscure umbrella part he used in this project existed, let alone thought of it when the time came to make a magnet mount? His hack this time is a real world, tabletop race track made for his little brother, and by his account, his brother is going crazy for it.

His race track is on a rotating table and consists of the following collection of parts: a motor, bicycle wheel, casters from a travel bag, rubber bands (where did he get such large ones?), toy car and steering wheel from his brother, skateboard wheels, the aforementioned umbrella part and hard drive magnets. In the video below we like how he paints the track surface by holding his paint brush fixed in place and letting the track rotate under it.

From the video you can see the race track has got [Rulof] hooked. Hopefully he lets his brother have ample turns too, but we’re not too sure. Some additions we can imagine would be robotics for the obstacles, lighting, sounds and a few simulated explosion effects (puffs of flour?).

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