A weighing scale with a moving-coil meter as a display

Hackaday Prize 2022: Arduino-Powered Weighing Scale Has A Real Analog Display

Digital displays are useful for quick and accurate readout, but lots of people prefer the physical motion of a needle moving along a dial. For instance, many smartwatch users choose an analog face to show the time, and modern cars with digital dashboards often default to showing an analog speedometer. Following this trend, [Miro Pavleski] built a digital weighing scale with an analog display that not only looks neat, but also serves as a good demonstration of the way that modern scales work.

Inside, the device is built up like a typical electronic scale: the heart of the instrument is a load cell that supports the platform and bends in proportion to the weight applied. This bending motion is sensed by a set of strain gauges wired up in a Wheatstone bridge configuration. An HX711 readout chip measures the resulting voltage and converts it to a digital code that is sent to a microcontroller, in this case an Arduino Nano.

Whereas a typical scale would then simply show the resulting number on an LCD display, [Mirko] decided to use a moving coil meter driven by the Arduino’s analog output. That meter was originally designed to show currents, so [Mirko] printed a new background image using kilograms instead.

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Old-school frequency counter

Edge-Mounted Meters Give This Retro Frequency Counter Six Decades Of Display

With regard to retro test gear, one’s thoughts tend to those Nixie-adorned instruments of yore, or the boat-anchor oscilloscopes that came with their own carts simply because there was no other way to move the things. But there were other looks for test gear back in the day, as this frequency counter with a readout using moving-coil meters shows.

We have to admit to never seeing anything like [Charles Ouweland]’s Van Der Heem 9908 electronic counter before. The Netherlands-based company, which was later acquired by Philips, built this six-digit, 1-MHz counter sometime in the 1950s. The display uses six separate edge-mounted panel meters numbered 0 through 9 to show the frequency of the incoming signal. The video below has a demo of what the instrument can do; we don’t know if it was restored at some point, but it still works and it’s actually pretty accurate. Later in the video, he gives a tour of the insides, which is the real treat — the case opens like a briefcase and contains over 20 separate PCBs with a bunch of germanium transistors, all stitched together with point-to-point wiring.

We appreciate the look inside this unique piece of test equipment history. It almost seems like something that would have been on the bench while this Apollo-era IO tester was being prototyped.

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Make A Set Of Headphones From Scratch

There are a variety of ways to enjoy your audio, of which headphones are one. Making a set of headphones is a straightforward enough project, but [madaeon] has taken the art to a new level by building the headphone drivers from scratch rather than using an off-the-shelf pair.

The result is a set of moving coil drivers with a construction technique involving using the semi-opaque thin window from an envelope as a diaphragm and as a former for the coil. Cyanoacrylate adhesive holds everything in place. The diaphragm is suspended across the mouth of a cardboard tube with the coil positioned above a magnet, resulting in the minimum moving mass necessary for as good a sound reproduction as possible. Judge for yourself, there’s a video that we’ve placed below the break.

The drivers are placed in a set of 3D-printed on-ear holders, and while they probably won’t match an expensive set of commercial headphones, we’d hazard a guess that they won’t have too bad quality. At the very least, it’s an interesting design to base further experimentation on.

Surprisingly few home made speaker or headphone drivers have made it onto these pages, probably because of the ubiquity of the ready-made article. An exception is this flexible PCB speaker, and of course we’ve also talked about home made electrostatic speakers. Continue reading “Make A Set Of Headphones From Scratch”

Lighting Tech Dives Into The Guts Of Laser Galvanometers

There’s something magical about a laser light show. Watching that intense beam of light flit back and forth to make shapes and patterns, some of them even animated, is pretty neat. It leaves those of us with a technical bent wondering just exactly how the beam is manipulated that fast.

Wonder no more as [Zenodilodon], a working concert laser tech with a deep junk bin, dives into the innards of closed-loop galvanometers, which lie at the heart of laser light shows. Galvos are closely related to moving-coil analog meters, which use the magnetic field of a coil to deflect a needle against spring force to measure current. Laser galvos, on the other hand, are optimized to move a lightweight mirror back and forth, by tiny amounts but very rapidly, to achieve the deflection needed to trace out shapes.

As [Zeno] explains in his teardown of some galvos that have seen better days, this means using a very low-mass permanent magnet armature surrounded by coils. The armature is connected to the mirror on one end, and a sensor on the other to provide positional feedback. We found this part fascinating; it hadn’t occurred to us that laser galvos would benefit from closed-loop control. And the fact that a tiny wiggling vane can modulate light from an IR LED enough to generate a control signal is pretty cool too.

The video below may be a bit long, but it’s an interesting glimpse into the day-to-day life of a lighting tech. It puts a little perspective on some of the laser projection projects we’ve seen, like this giant Asteroids game.

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Follow The Bouncing Needles Of This Analog Meter Clock

Our community never seems to tire of clock builds. There are seemingly infinite ways to mark the passage of time, and finding unique ways to display it is endlessly fascinating.

There’s something about this analog voltmeter clock that really seems to have caught on with the Redditors who commented on the r/DIY thread where we first spotted this. [ElegantAlchemist]’s design is very simple – just a trio of moving coil meters with nice industrial-looking bezels. The meters were wired for 300 volts AC, so the rectifier and smoothing cap were removed and the series resistance was substituted for one more appropriate for the 0-5VDC range needed for the project. New dial faces showing hours, minutes and seconds were whipped up in Corel Draw, and everything was put into a sturdy and colorful aluminum “stomp box” normally used for effects pedals. An Arduino Nano and an RTC drive the meters with a nice, bouncy action. Simple, cheap to build, and a real crowd pleaser.

The observant reader will note a similarity to a clock we covered a while back. That one chose 3D-printed cases for an airplane instrument cluster look. We like the spare case design in [ElegantAlchemist]’s build, but wonder how this clock would look in a fine wood case.

DIY Ribbon Element Upgrades A Studio Microphone

For those with some experience with pro audio, the term “ribbon microphone” tends to conjure up an image of one of those big, chunky mics from the Golden Age of radio, the kind adorned with the station’s callsign and crooned into by the latest heartthrob dreamboat singer. This DIY ribbon mic is none of those things, but it’s still really cool.

Of course the ribbon mic isn’t always huge, and the technology behind it is far from obsolete. [Frank Olsen]’s ribbon mic starts out with gutting a run-of-the-mill studio mic of its element, leaving only the body and connector behind. The element that he constructs, mostly from small scraps of aluminum and blocks of acrylic, looks very much like the ribbon element in commercial mics: a pair of magnets with a thin, corrugated strip of foil suspended between them. The foil was corrugated by passing it through a jig that [Frank] built, which is a neat tool, but he says that a paper crimper used for crafting would work too. There’s some pretty fussy work behind the cartridge build, but everything went together and fit nicely in the old mic body. The video below was narrated using the mic, so we know it works.

Fun fact: the ribbon microphone was invented by Walter Schottky. That Walter Schottky. Need more on how these mics work? Our colleague [Al Williams] has you covered with this article on the basics.

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Old Wattmeter Uses Magnetics To Do The Math

Measuring power transfer through a circuit seems a simple task. Measure the current and voltage, do a little math courtesy of [Joule] and [Ohm], and you’ve got your answer. But what if you want to design an instrument that does the math automatically? And what if you had to do this strictly electromechanically?

That’s the question [Shahriar] tackles in his teardown of an old lab-grade wattmeter. The video is somewhat of a departure for him, honestly; we’re used to seeing instruments come across his bench that would punch a seven-figure hole in one’s wallet if acquired new. These wattmeters are from Weston Instruments and are beautiful examples of sturdy, mid-century industrial design, and seem to have been in service until at least 2013. The heavy bakelite cases and sturdy binding posts for current and voltage inputs make it seem like the meters could laugh off a tumble to the floor.

But as [Shahriar] discovers upon teardown of a sacrificial meter, the electromechanical movement behind the instrument is quite delicate. The wattmeter uses a moving coil meter much like any other panel meter, but replaces the permanent magnet stator with a pair of coils. The voltage binding posts are connected to the fine wire of the moving coil through a series resistance, while the current is passed through the heavier windings of the stator coils. The two magnetic fields act together, multiplying the voltage by the current, and deflect a needle against a spring preload to indicate the power. It’s quite clever, and the inner workings are a joy to behold.

We just love looking inside old electronics, and moving coil meters especially. They’re great gadgets, and fun to repurpose, too.

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