From the Mailbag: Understanding the Homopolar Motor

Wes wrote in to say:

I am an Electrical Engineer (graduated May ’72, Texas Tech U), but I never saw or even heard of a homopolar motor until last week, when I saw an electric motor made from four parts on National Geographic’s program, “None of the Above“. When I first saw it, I figured it must be a hoax. A DC motor had to have a commutator and two magnets.

Only when I was browsing around in Wikipedia did I find an article on the motor. I happened to have everything I needed, so I built one, not really expecting it to work. To my great surprise, it spun up to a few thousand RPMs in seconds. I read Wikipedia’s theory of operation, but it didn’t make sense. Today, I came across your wonderfully clear and simple explanation, and now I understand the motor perfectly.

I simply cannot thank you enough for your drawing and explanation.

Thanks for writing in— we’re glad to hear you enjoyed learning something new! The instructions for making the motor and the discussion of how it works are in our articles:

Simple Relay Shield v 2.0

relay shield

A minor bump for one of our little open source Arduino add-ons. The Simple Relay Shield is an easy to use single-relay board that does one thing, and does it well: It adds a beefy little mechanical relay to an Arduino, which you control through pin Digital 4.

relay shield

Version 2.0 adds the ability — by popular request — to control it from a pin other than D4. Solder the jumper in the normal way (in location JP), and it works on pin D4. Hook it up to any other digital pin, say to D7, and now you have a relay on that pin. The Simple Relay Shield is available as a complete soldering kit or as a bare PCB, and you can find documentation on our wiki.

A Vintage Bliley Crystal

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They don’t make — or package — them like they used to. This is a vintage radio crystal from the Bliley Electric Company.  Bliley is still around, making modern oscillators and even space flight hardware, but this vintage unit is a beauty.

Introduced in the 1930′s, the Bliley LD2 was a popular frequency standard for amateur radio operators. A 1935 advertisement in QST magazine claimed efficiency and extremely low drift (<8 ppm/°C), guaranteed operation, an improved holder, and a cost of only $4.80, or $82.79 in 2014 dollars.  This particular unit is calibrated at 3.9895 MHz (“3989.5 KC”), for a radio wavelength of about 75.2 m.
Bliley Crystal 9

 

Unlike most modern crystals, this type comes apart easily. Inside, two rectangular steel plates sandwich a thin slice of quartz crystal, all held pressed together with a simple spring.

If you’re interested to learn more, there’s a wealth of additional information about vintage crystals and the Bliley company available online, here.

Interactive art at Helix

Romy Randev of Looma is installing his latest piece, Penumbra at the Helix museum in Los Altos.

Penumbra is an interactive installation that responds to movement in its environment. Without any human interaction, Penumbra is disguised as a decorative glass wall. However, each colored glass tile illuminates individually as sensors that respond to movements control LEDs behind the glass.

Penumbra makes use of our Octolively modules, and we’ll be at Helix with Romy on Saturday March 29th, starting at 2:00 PM to talk about the art and tech behind Penumbra. Event information is available from Helix.

Three Fives in IEEE Spectrum

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Our Three Fives Kit was featured in this month’s IEEE Spectrum. From the article:

Just as DNA models, star maps, and periodic tables serve as reminders of fundamentals that can get obscured by day-to-day minutiae, so too the Three Fives kit is a reminder that even the most complex digital processor is still at its heart just a collection of very simple components.

You can read the full article and see pictures of it in use in a sample circuit over at IEEE Spectrum.

Basics: AVR Target Boards and Arduino Compatibility

Diavolino
Gary writes:

I have fallen in love with your Diavolinos – thank you!
My question: does the “Simple target board” allow for the 6-pin FTDI Friend hookup to upload sketches? This is quick and easy with the Diavolino. I’m new to reading circuits and stuff, and I cannot tell looking at the target board. It says to use in-system programmer, but I prefer to not buy another interface. Thanks!

Excellent question! It is certainly possible, but not as quick and easy.  Both the Diavolino and our ATmegaXX8 target boards boards use the same chip, usually the ATmega328P.  But, one might say that our ATmegaXX8 board is a simple AVR target board optimized for use with an AVR ISP programmer (like the USBtinyISP), whereas the Diavolino is a simple target board optimized for use with the FTDI interface.

XX8 Target Board

Versus a “bare” target board (with just the chip and power), there are four things that you would normally add, in order to use the FTDI interface to upload a sketch from within the Arduino environment:

Continue reading

Cool Wearable Electronics

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Mascot costumes are often hot and sweaty, and mascots are expected to enthusiastically energize their teams. At FIRST robotics competitions, there are also traditions of dance parties during delays caused by technical difficulties, leading to even more activity in a warm, heavy costume than would normally be expected.

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Cardinalbotics, an FRC team from San Francisco, made a mascot costume using an application of wearable electronics I had not considered before: cooling fans. The cardinal head was made of fabric sewn over shaped foam, with fans on the sides to keep everything cool inside.

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The fans were wired up to an on/off switch and adjustment pot which were hidden in a velcro compartment in the back of the head.

Larson Scanner modding

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Anton wrote in to ask:

I want to purchase your Larson Scanner kit but need to be able to run it from household current … is there a converter that you would recommend?

Since the Larson Scanner normally runs from 3 V DC, a regulated 3 V power supply can be hooked up in place of the 2xAA battery box. However, 5 V power supplies (like this one) are much more common, and the circuit can be run from 5 V with only minor changes. if you replace the nine 16 ohm resistors in the Larson Scanner kit with 120 – 150 ohm resistors, you can power it from 5 V directly.

Another related question we occasionally get is how to run the Larson Scanner with green LEDs. (Note: by green, we mean “pure” green LEDs which have a forward voltage of about 3 V. Older style yellow-green LEDs with a forward voltage of ~2 V can be used as drop-in replacements for the red ones.) 

If running off of battery will work for you, this is an even simpler change: merely replace the 3 V battery holder with a 4.5 V one, such as a 3xAA. It is a happy coincidence that the circuit can run with red LEDs at 3 V or green LEDs at 4.5 V using the same 16 ohm resistor value. So how about running a green Larson Scanner from a 5 V power supply? Replace the nine 16 ohm resistors with 39 ohm resistors and you should be good to go.

You can find the documentation for the Larson Scanner and more stories about modding it on our wiki. We’d love to hear about any mods you do to the Larson Scanner in the comments or see pictures in the flickr pool.