Inside A Selective Voltmeter

[Martin Lorton] has a vintage Harmon 4200B selective voltmeter that needed repair. He picked it up on eBay, and he knew it wasn’t working, but it was in good condition, especially for the price. He’s posted four videos about what’s inside and how he’s fixing it. You can see the first installment below.

The 4200B is an RMS voltmeter and is selective because it has a tuned circuit to adjust to a particular frequency. The unit uses discrete components and has an analog meter along with an LCD counter.

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A Single Chip Computer For The 8051 Generation

The Intel 8051 series of 8-bit microcontrollers is long-discontinued by its original manufacturer, but lives on as a core included in all manner of more recent chips. It’s easy to understand and program, so it remains a fixture despite much faster replacements appearing.

If you can’t find an original 40-pin DIP don’t worry, because [mit41301] has produced a board in a compatible 40-pin format. It’s called the single chip computer not because such a thing is a novelty in 2025, but because it has no need for the support chips which would have come with the original.

The modern 8051 clone in use is a CH558 or CH559, both chips with far more onboard than the original. The pins are brought out to one side only of the board, because on the original the other side would interface with an external RAM chip. It speaks serial, and can be used through either a USB-to-serial or Bluetooth-to-serial chip. There’s MCS-BASIC for it, so programming should be straightforward.

We can see the attraction of this board even though we reach for much more accomplished modern CPUs by choice. Several decades ago the original 8051 on Intel dev boards was our university teaching microcontoller, so there remains here a soft spot for it. We certainly see other 8051 designs, as for example this Arduino clone.

Supercon 2024: An Immersive Motion Rehabilitation Device

When you’ve had some kind of injury, rehabilitation can be challenging. You often need to be careful about how you’re using the affected parts of your body, as well as pursue careful exercises for repair and restoration of function. It can be tedious and tiring work, for patients and treating practitioners alike.

Juan Diego Zambrano, Abdelrahman Farag, and Ivan Hernandez have been working on new technology to aid those going through this challenging process. Their talk at the 2024 Hackaday Supercon covers an innovative motion monitoring device intended to aid rehabilitation goals in a medical context.

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Hackaday Podcast Episode 320: A Lot Of Cool 3D Printing, DIY Penicillin, And An Optical Twofer

This week, Hackaday’s Elliot Williams and Kristina Panos met up across the universe to bring you the latest news, mystery sound, and of course, a big bunch of hacks from the previous week.

In Hackaday news, the 2025 Pet Hacks Contest rolls on. You have until June 10th to show us what you’ve got, so head over to Hackaday.IO and get started today!

On What’s That Sound, Kristina actually got it this time, although she couldn’t quite muster the correct name for it, however at Hackaday we’ll be calling it the “glassophone” from now on. Congratulations to [disaster_recovered] who fared better and wins a limited edition Hackaday Podcast t-shirt!

After that, it’s on to the hacks and such, beginning with a complete and completely-documented wireless USB autopsy. We take a look at a lovely 3D-printed downspout, some DIY penicillin, and a jellybean iMac that’s hiding a modern PC. Finally, we explore a really cool 3D printing technology, and ask what happened to typing ‘www.’.

Check out the links below if you want to follow along, and as always, tell us what you think about this episode in the comments!

Download in DRM-free MP3 and savor at your leisure.

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Oscilloscope Digital Storage, 1990s Style

You’re designing an oscilloscope with modest storage — only 15,000 samples per channel. However, the sample rate is at 5 Gs/s, and you have to store all four channels at that speed and depth. While there is a bit of a challenge implied, this is quite doable using today’s technology. But what about in the 1990s when the Tektronix TDS 684B appeared on the market? [Tom Verbure] wondered how it was able to do such a thing. He found out, and since he wrote it up, now you can find out, too.

Inside the scope, there are two PCBs. There’s a CPU board, of course. But there’s not enough memory there to account for the scope’s capability. That much high-speed memory would have been tough in those days, anyway. The memory is actually on the analog board along with the inputs and digitizers. That should be a clue.

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This Week In Security: Encrypted Messaging, NSO’s Judgement, And AI CVE DDoS

Cryptographic messaging has been in the news a lot recently. Like the formal audit of WhatsApp (the actual PDF). And the results are good. There are some minor potential problems that the audit highlights, but they are of questionable real-world impact. The most consequential is how easy it is to add additional members to a group chat. Or to put it another way, there are no cryptographic guarantees associated with adding a new user to a group.

The good news is that WhatsApp groups don’t allow new members to read previous messages. So a user getting added to a group doesn’t reveal historic messages. But a user added without being noticed can snoop on future messages. There’s an obvious question, as to how this is a weakness. Isn’t it redundant, since anyone with the permission to add someone to a group, can already read the messages from that group?

That’s where the lack of cryptography comes in. To put it simply, the WhatsApp servers could add users to groups, even if none of the existing users actually requested the addition. It’s not a vulnerability per se, but definitely a design choice to keep in mind. Keep an eye on the members in your groups, just in case. Continue reading “This Week In Security: Encrypted Messaging, NSO’s Judgement, And AI CVE DDoS”

Triggering Lightning And Safely Guiding It Using A Drone

Every year lightning strikes cause a lot of damage — with the high-voltage discharges being a major risk to buildings, infrastructure, and the continued existence of squishy bags of mostly salty water. While some ways exist to reduce their impact such as lightning rods, these passive systems can only be deployed in select locations and cannot prevent the build-up of the charge that leads up to the plasma discharge event. But the drone-based system recently tested by Japan’s NTT, the world’s fourth largest telecommunications company, could provide a more proactive solution.

The idea is pretty simple: fly a drone that is protected by a specially designed metal cage close to a thundercloud with a conductive tether leading back to the ground. By providing a very short path to ground, the built-up charge in said cloud will readily discharge into this cage and from there back to the ground.

To test this idea, NTT researchers took commercial drones fitted with such a protective cage and exposed them to artificial lightning. The drones turned out to be fine up to 150 kA which is five times more than natural lightning. Afterwards the full system was tested with a real thunderstorm, during which the drone took a hit and kept flying, although the protective cage partially melted.

Expanding on this experiment, NTT imagines that a system like this could protect cities and sensitive areas, and possibly even use and store the thus captured energy rather than just leading it to ground. While this latter idea would need some seriously effective charging technologies, the idea of proactively discharging thunderclouds is perhaps not so crazy. We would need to see someone run the numbers on the potential effectiveness, of course, but we are all in favor of (safe) lightning experiments like this.

If you’re wondering why channeling lightning away from critical infrastructure is such a big deal, you may want to read up on Apollo 12.