It’s A Variable Capacitor, But Not As We Know It

September 17, 2025 by No comments

Radio experimenters often need a variable capacitor to tune their circuits, as the saying goes, for maximum smoke. In decades past these were readily available from almost any scrap radio, but the varicap diode and then the PLL have removed the need for them in consumer electronics. There have been various attempts at building variable capacitors, and here’s [radiofun232] with a novel approach.

A traditional tuning capacitor has a set of meshed semicircular plates that have more of their surface facing each other depending on how far their shaft is turned. The capacitor presented in the first video below has two plates joined by a hinge in a similar manner to the covers of a book. It’s made of tinplate, and the plates can be opened or closed by means of a screw.

The result is a capacitor with a range from 50 to 150 picofarads, and in the second video we can see it used with a simple transistor oscillator to make a variable frequency oscillator. This can form the basis of a simple direct conversion receiver.

We like this device, it’s simple and a bit rough and ready, but it’s a very effective. If you’d like to see another unusual take on a variable capacitor, take a look at this one using drinks cans.

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Smooth! Non-Planar 3D Ironing

September 17, 2025 by 14 Comments

Is 2025 finally the year of non-planar 3D printing? Maybe it won’t have to be if [Ten Tech] gets his way!

Ironing is the act of going over the top surface of your print again with the nozzle, re-melting it flat. Usually, this is limited to working on boring horizontal surfaces, but no more! This post-processing script from [Tenger Technologies], coupled with a heated, ball-shaped attachment, lets you iron the top of arbitrary surfaces.

At first, [Ten Tech] tried out non-planar ironing with a normal nozzle. Indeed, we’ve seen exactly this approach taken last year.  But that approach fails at moderate angles because the edge on the nozzle digs in, and the surrounding hot-end parts drag.

[Ten Tech]’s special sauce is taking inspiration from the ball-end mill finishing step in subtractive CNC work: he affixed the round tip of a rivet on the end of a nozzle, and insulating that new tool turned it into an iron that could smooth arbitrary curvy top layers.

One post-processing script later, and the proof of concept is working. Check out the video below to see it in action. As it stands, this requires a toolhead swap and the calibration of a whole bunch of new parameters, but it’s a very promising new idea for the community to iterate on. We love the idea of a dedicated tool and post-processing smoother script working together in concert.

Will 2025 be the year of non-planar 3DP? We’ve seen not one but two superb multi-axis non-planar printer designs so far this year: one from [Joshua Bird] and the other from [Daniel] of [Fractal Robotics]. In both cases, they are not just new machines, but are also supported with novel open-source slicers to make them work. Now [Ten Tech]’s ironer throws its hat in the ring. What will we see next?

Thanks to [Gustav Persson] for the tip!

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Forgotten Internet: The Story Of Email

September 17, 2025 by 7 Comments

It is a common occurrence in old movies: Our hero checks in at a hotel in some exotic locale, and the desk clerk says, “Ah, Mr. Barker, there’s a letter for you.” Or maybe a telegram. Either way, since humans learned to write, they’ve been obsessed with getting their writing in the hands of someone else. Back when we were wondering what people would do if they had a computer in their homes, most of us never guessed it would be: write to each other. Yet that turned out to be the killer app, or, at least, one of them.

What’s interesting about the hotel mail was that you had to plan ahead and know when your recipient would be there. Otherwise, you had to send your note to their home address, and it would have to wait. Telegrams were a little better because they were fast, but you still had to know where to send the message.

Early Days

An ad from the 1970s with a prominent Telex number

In addition to visiting a telegraph office, or post office, to send a note somewhere, commercial users started wanting something better at the early part of the twentieth century. This led to dedicated teletype lines. By 1933, though, a network of Teletype machines — Telex — arose. Before the Internet, it was very common for a company to advertise its Telex number — or TWX number, a competing network from the phone company and, later, Western Union — if they dealt with business accounts.

Fax machines came later, and the hardware was cheap enough that the average person was slightly more likely to have a fax machine or the use of one than a Telex.

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American Science And Surplus Ends Online Sales

September 17, 2025 by 20 Comments

For nearly 90 years, American Science and Surplus has been shipping out weird and wonderful stuff to customers far and wide. In the pre-Internet days, getting their latest catalog in the mail — notable for its hand-drawn illustrations and whimsical style — was always exciting. From Romanian gas masks to odd-ball components, there was no telling what new wonders each issue would bring. In time, the printed catalog gave way to a website, but the eclectic offerings and hand-drawn images remained.

Unfortunately, those days are officially no more. Earlier this week, American Science and Surplus had to make the difficult decision to shutter their entire mail order division. It’s no secret that the company as a whole had been struggling over the last few years. Like many small businesses they were hit hard during the COVID-19 years, and while they made it through that particular storm, they faced skyrocketing operational costs.

Earlier this year, the company turned to crowd funding to help stay afloat. That they were able to raise almost $200,000 speaks to how much support they had from their community of customers, but while it put the company in a better position, the writing was on the wall. The warehouse space required to support their mail order operations was simply too expensive to remain viable.

But it’s not all bad news. At least two of the company’s physical storefronts, located in Milwaukee, Wisconsin and Geneva, Illinois will remain open and operate under the ownership of the employees themselves. The fate of the third store in Park Ridge, Illinois is less clear. They currently don’t have a buyer, but it sounds like they haven’t given up hope of selling it yet.

Anyone in the Illinois area feel like getting some buddies together and buying a turn-key surplus business?

Naturally Radioactive Food And Safe Food Radiation Levels

September 17, 2025 by 19 Comments

There was a recent recall of so-called ‘radioactive shrimp’ that were potentially contaminated with cesium-137 (Cs-137). But contamination isn’t an all-or-nothing affair, so you might wonder exactly how hot the shrimp were. As it turns out, the FDA’s report makes clear that the contamination was far below the legal threshold for Cs-137. In addition, not all of the recalled shrimp was definitely contaminated, as disappointing as all of this must be to those who had hoped to gain radioactive Super Shrimp powers.

After US customs detected elevated radiation levels in the shrimp that was imported from Indonesia, entry for it was denied, yet even for these known to be contaminated batches the measured level was below 68 Bq/kg. The FDA limit here is 1,200 Bq/kg, and the radiation level from the potassium-40 in bananas is around the same level as these ‘radioactive shrimp’, which explains why bananas can trigger radiation detectors when they pass through customs.

But this event raised many questions about how sensible these radiation checks are when even similar or higher levels of all-natural radioactive isotopes in foods pass without issues. Are we overreacting? How hot is too hot?

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Oil-Based Sprengel Pump Really Sucks

September 17, 2025 by 15 Comments

Have you heard of the Sprengel pump? It’s how they drew hard vacuum back before mechanical pumps were perfected — the first light bulbs had their vacuums drawn with Sprengel pumps, for example. It worked by using droplets of a particular liquid to catch air particles, and push them out a narrow tube, thereby slowly evacuating a chamber. The catch is that that liquid used to be mercury, which isn’t something many of us have on hand in kilogram quantities anymore. [Gabriel Wolffe] had the brainwave that one might substitute modern vacuum pump oil for mercury, and built a pump to test that idea.

Even better, unlike the last (mercury-based) Sprengel pump we saw, [Gabriel] set up his build so that no glassblowing is required. Yes, yes, scientific glassblowing used to be an essential skill taught in every technical college in the world. Nowadays, we’re glad to have a design that lets us solder brass fittings together. Technically you still have to cut an eyedropper, but that’s as complex as the glasswork gets. Being able to circulate oil with a plastic tube and peristaltic pump is great, too.

If you try it, you need to spring for vacuum pump oil. This type of pump is limited in the vacuum it can draw by the vapor pressure of the fluid in use, and just any oil won’t do. Most have vapor pressures far in excess of anything useful. In the old days, only mercury would cut it, but modern chemistry has come up with very stable oils that will do nearly as well.

How well? [Gabriel] isn’t sure; he bottomed out his gauge at 30 inches of Mercury (102 kPa). It may not be any lower than that, but it’s fair to say the pump draws a healthy vacuum without any unhealthy liquid metals. Enough to brew up some tubes, perhaps.

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