Circuit VR: The Wheatstone Bridge Analog Computer

We are always impressed with something so simple can actually be so complex. For example, what would you think goes into an analog computer? Of course, a “real” analog computer has opamps that can do logarithms, square roots, multiply, and divide. But would it surprise you that you can make an analog device like a slide rule using a Wheatstone bridge — essentially two voltage dividers. You don’t even need any active devices at all. It is an old idea and one that used to show up in electronic magazines now and again. I’ll show you how they work and simulate the device so you don’t have to build it unless you just want to.

A voltage divider is one of the easiest circuits in the world to analyze. Consider two resistors Ra and Rb in series. Voltage comes in at the top of Ra and the bottom of Rb is grounded. The node connecting Ra and Rb — let’s call it Z — is what we’ll consider the output.

Let’s say we have a 10 V battery feeding A and a perfect voltmeter that doesn’t load the circuit connected to Z. By Kirchoff’s current law we know the current through Ra and Rb must be the same. After all, there’s nowhere else for it to go. We also know the voltage drop across Ra plus the voltage drop across Rb must equal to 10 V. Kirchoff, conservation of energy, whatever you want to call it.  Let’s call these quantities I, Va, and Vb. Continue reading “Circuit VR: The Wheatstone Bridge Analog Computer”

A complex arrangement of LEGO gears

Analog Computer Made From LEGO Predicts Tides

Although the tides in the ocean are caused by the motion of the Sun and the Moon, both of which are easy to observe, accurately predicting the tide more than a few days in advance turns out to be rather difficult. The math behind the tidal movement is so complex that some of the earliest analog computers were built specifically to perform tide calculations. Sir William Thomson (better known as Lord Kelvin) designed one such “tide-predicting machine”, an impressive arrangement of gears and pulleys, back in the late 19th century.

[Pepijn de Vos] built a modern interpretation of Thomson’s machine out of LEGO parts, and it’s no less impressive than the original. A total of 96 LEGO gears move perfectly in sync to the ocean’s natural rhythms, while a set of pulleys connect four banks of gears together to create the sum of the constituent frequencies. An ultrasonic sensor reads the output value and sends the result back to a PC.

One interesting problem that [Pepijn] ran into, and which he explains in great detail on his blog, is that LEGO gears can only provide a very limited set of gear ratios. In order to match the tide calculations to any kind of precision, he needed to connect many gears in series without creating too much friction and backlash in the mechanism. Optimizing this setup was a non-trivial task that required a significant amount of computing power by itself.

As you can see in the video embedded below, the machine makes beautifully smooth movements, which correspond quite accurately to the actual motion of tides. If you’re interested in the science behind analog tide predictors, we’ve got an in-depth article about just that.

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Forget Digital Computing, You Need An Analog Computer

The analog computer of decades-gone-by is something many of us younger engineers never got the chance to experience first hand. It’s pretty much a case of reading about them on these fine pages or perhaps looking at a piece of one behind glass in one of the more interesting museums out there. But now, there is another option, (THAT) The Analog Thing. Developed by Berlin-based Analog computer-on-chip specialist Anabrid, THAT is an Open Source analog computer you can build yourself (eventually) or buy from them fully assembled. At least, that’s their plan.

From the 1970s onwards, digital computers became powerful enough to replace analog computers in pretty much every area, and with the increased accuracy this brought, the old analog beasts became obsolete overnight. Now, there seems to be a move to shift back a little, with hybridized analog-digital approaches looking good for some applications, especially where precision is not paramount. After all, that pile of fatty grey matter between your ears is essentially a big analog computer, and that’s pretty good at problem solving.

Looking over the project Wiki there are a few application examples and some explanatory notes. Schematics are shown, albeit only images for now. We can’t find the PCB files either, but the assembly instructions show many bodge wires, so we guess they’re re-spinning the PCB to apply fixes before releasing them properly. This is clearly work-in-progress and as they say on the main site, their focus is on chips for hybrid analog-digital computing, with a focus on energy-efficient approximate methods. With that in mind, we can forgive that the community-focused learning tools are still being worked on. All that said, this is still a very interesting project, and definitely would be a Christmas present this scribe would be more than happy to unwrap.

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The Modding, Restoration, And Demise Of A $3M Analog Computer

How do you rapidly record the output from your three million dollar analog computer in the 1940s when the results are only available on analog meters? The team responsible for the Westinghouse 1947 AC Network Calculator at Georgia Tech was faced with just this problem and came up with a nifty solution — hack the control panel and wire in a special-purpose drafting table.

What Is It?

What is this beast of a computer? Machines of this type were developed during and after World War 2, and strictly speaking, belong in the category of scale models rather than true computers. Although these machines were very flexible, they were primarily designed to simulate power distribution grids. There is a lot of theory under the hood, but basically a real world, multi-phase distribution system would be scaled to single-phase at 400 Hz for modeling.

The engineers would “program” the machine by connecting together the appropriate circuit elements (like capacitors, inductors, transmission lines, generators, etc.) on big patch panels. Thus programmed, a 10 kW motor-generator located in the basement would be started up and the simulation was underway. Continue reading “The Modding, Restoration, And Demise Of A $3M Analog Computer”

Hacker’s Discovery Changes Understanding Of The Antikythera Mechanism

With all the trained academics who have pored over the Antikythera mechanism in the 120 years since it was pulled from the Mediterranean Sea, you’d think all of the features of the ancient analog computer would have been discovered by now. But the mechanism still holds secrets, some of which can only be appreciated by someone in tune with the original maker of the device. At least that what appears to have happened with the recent discovery of a hitherto unknown lunar calendar in the Antikythera mechanism. (Video, embedded below.)

The Antikythera mechanism is fascinating in its own right, but the real treat here is that this discovery comes from one of our own community — [Chris] at Clickspring, maker of amazing clocks and other mechanical works of art. When he undertook a reproduction of the Antikythera mechanism using nothing but period-correct materials and tools four years ago, he had no idea that the effort would take the direction it has. The video below — also on Vimeo — sums up the serendipitous discovery, which is based on the unusual number of divisions etched into one of the rings of the mechanisms. Scholars had dismissed this as a mistake, but having walked a mile in the shoes of the mechanism’s creator, [Chris] knew better.

The craftsmanship and ingenuity evidenced in the original led [Chris] and his collaborators to the conclusion that the calendar ring is actually a 354-day calendar that reflects a lunar cycle rather than a solar cycle. The findings are summarized in a scholarly paper in the Horological Journal. Getting a paper accepted in a peer-reviewed journal is no mean feat, so hats off to the authors for not only finding this long-lost feature of the Antikythera mechanism and figuring out its significance, but also for persisting through the writing and publication process while putting other projects on hold. Clickspring fans have extra reason to rejoice, too — more videos are now on the way!

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Homebrew Slide Rule Gets Back To Mathematical Basics

In the grand scheme of things, it really wasn’t all that long ago that a slide rule was part of an engineer’s every day equipment. Long before electronic calculators came along, a couple of sticks of wood inscribed with accurate scales was all it took to do everything from simple multiplication to logarithms and trig functions.

While finding a slide rule these days isn’t impossible, it’s still not exactly easy, and buying one off the shelf isn’t as fun or as instructive as building one yourself. [JavierL90]’s slide rule build started, ironically enough, on the computer, with a Python program designed to graphically plot the various scales needed for the fixed sections of the slide rules (the “stators”) and the moving bit (the “slide”).  His first throught was to laser-engrave the scales, but the route of printing them onto self-adhesive vinyl stock proved to be easier.

With the scale squared away, work turned to the mechanism itself. He chose walnut for the wood, aluminum for the brackets, and a 3D-printed frame holding a thin acrylic window for the sliding cursor. The woodworking is simple but well-done, as is the metalwork. We especially like the method used to create the cursor line — a simple line scored into the acrylic with a razor, which was then filled with red inks. The assembled slide rule is a thing of beauty, looking for all the world like a commercial model, especially when decked out with its custom faux leather carry case.

We have to admit that the use of a slide rule is a life skill that passed us by, but seeing this puts us in the mood for another try. We might have to start really, really simple and work up from there.

Secrets From A 1969 Analog Computer

Today, most of what we think of as a computer uses digital technology. But that wasn’t always the case. From slide rules to mechanical fire solution computers to electronic analog computers, there have been plenty of computers that don’t work on 1s and 0s, but on analog quantities such as angle or voltage. [Ken Shirriff] is working to restore an analog computer from around 1969 provided by [CuriousMarc]. He’ll probably write a few posts, but this month’s one focuses on the op-amps.

For an electronic analog computer, the op-amp was the main processing element. You could feed multiple voltages in to do addition, and gain works for multiplication. If you add a capacitor, you can do integration. But there’s a problem.

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