A microscope objective is sitting on a spool of solder in a metal tin, in front of a circuit board which has wires running away from it.

Watching Radioactive Decay With A Homemade Spinthariscope

November 17, 2025 by Aaron Beckendorf 2 Comments

Among the many science toys that have fallen out of fashion since we started getting nervous around things like mercury, chlorinated hydrocarbons, and radiation is the spinthariscope, which let people watch the flashes of light on a phosphor screen as a radioactive material decayed behind it. In fact, they hardly expose their viewers to any radiation, which makes [stoppi]’s homemade spinthariscope much safer than it might first seem.

[Stoppi] built the spinthariscope out of the eyepiece of a telescope, a silver-doped zinc sulfide phosphor screen, and the americium-241 capsule from a smoke detector. A bit of epoxy holds the phosphor screen in the lens’s focal plane, and the americium capsule is mounted on a light filter and screwed onto the eyepiece. Since americium is mainly an alpha emitter, almost all of the radiation is contained within the device.

After sitting in a dark room for a few minutes to let one’s eyes adjust, it’s possible to see small flashes of light as alpha particles hit the phosphor screen. The flashes were too faint for a smartphone camera to pick up, so [stoppi] mounted it in a light-tight metal box with a photomultiplier and viewed the signal on an oscilloscope, which revealed many small pulses.

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How Laser Headlights Died In The US

August 18, 2025 by Lewin Day 140 Comments

Automotive headlights started out burning acetylene, before regular electric lightbulbs made them obsolete. In due time, halogen bulbs took over, before the industry began to explore even newer technologies like HID lamps for greater brightness. Laser headlights stood as the next leap forward, promising greater visibility and better light distribution.

Only, the fairytale didn’t last. Just over a decade after laser headlights hit the market, they’re already being abandoned by the manufacturers that brought them to fruition. Laser headlights would end up fighting with one hand behind their back, and ultimately became irrelevant before they ever became the norm.

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White LED Turning Purple: Analyzing A Phosphor Failure

May 30, 2025 by Maya Posch 32 Comments

White LED bulbs are commonplace in households by now, mostly due to their low power usage and high reliability. Crank up the light output enough and you do however get high temperatures and corresponding interesting failure modes. An example is the one demonstrated by the [electronupdate] channel on YouTube with a Philips MR16 LED spot that had developed a distinct purple light output.

The crumbling phosphor coating on top of the now exposed LEDs. (Credit: electronupdate, YouTube)

After popping off the front to expose the PCB with the LED packages, the fault seemed to be due to the phosphor on one of the four LEDs flaking off, exposing the individual 405 nm LEDs underneath. Generally, white LEDs are just UV or 405 nm (‘blue’) LEDs that have a phosphor coating on top that converts the emitted wavelength into broad band visible (white) or another specific wavelength, so this failure mode makes perfect sense.

After putting the PCB under a microscope and having a look at the failed and the other LED packages the crumbled phosphor on not just the one package became obvious, as the remaining three showed clear cracks in the phosphor coating. Whether due to the heat in these high-intensity spot lamps or just age, clearly over time these white LED packages become just bare LEDs without the phosphor coating. Ideally you could dab on some fresh phosphor, but likely the fix is to replace these LED packages every few years until the power supply in the bulb gives up the ghost.

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Double Your Analog Oscilloscope Fun With This Retro Beam Splitter

November 24, 2024 by Dan Maloney 18 Comments

These days, oscilloscope hacking is all about enabling features that the manufacturer baked into the hardware but locked out in the firmware. Those hacks are cool, of course, but back in the days of analog scopes, unlocking new features required a decidedly more hardware-based approach.

For an example of this, take a look at this oscilloscope beam splitter by [Lockdown Electronics]. It’s a simple way to turn a single-channel scope into a dual-channel scope using what amounts to time-division multiplexing. A 555 timer is set up as an astable oscillator generating a 2.5-kHz square wave. That’s fed into the bases of a pair of transistors, one NPN and the other PNP. The collectors of each transistor are connected to the two input signals, each biased to either the positive or negative rail of the power supply. As the 555 swings back and forth it alternately applies each input signal to the output of the beam splitter, which goes to the scope. The result is two independent traces on the analog scope, like magic.

More after the break…

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Fail Of The Week: The Case Of The Curiously Colored Streetlights

October 9, 2024 by Dan Maloney 119 Comments

What color are the street lights in your town? While an unfortunate few still suffer under one of the awful colors offered by vapor discharge lamps, like the pink or orange of sodium or the greenish-white of mercury, most municipalities have moved to energy-saving LED streetlights, with a bright white light that’s generally superior in every way. Unless, of course, things go wrong and the lights start to mysteriously change colors.

If you’ve noticed this trend in your area, relax; [NanoPalomaki] has an in-depth and surprisingly interesting analysis of why LED streetlights are changing colors. After examining a few streetlights removed from service thanks to changing from white to purple, he discovered a simple explanation. White LEDs aren’t emitting white light directly; rather, the white light comes from phosphors coating the underlying LED, which emits a deep blue light. The defunct units all showed signs of phosphor degradation. In some cases, the phosphors seemed discolored, as if they experienced overheating or chemical changes. In other LEDs the phosphor layer was physically separated from the backing, exposing the underlying LEDs completely. The color of these damaged modules was significantly shifted toward the blue end of the spectrum, which was obviously why they were removed from service.

Now, a discolored LED here and there does not exactly constitute a streetlight emergency, but it’s happening to enough cities that people are starting to take notice. The obvious solution would be for municipalities to replace the dodgy units Even in the unlikely event that a city would get some compensation from the manufacturer, this seems like an expensive proposition. Luckily, [NanoPalomaki] tested a solution: he mixed a wideband phosphor into a UV-curable resin and painted it onto the lens of each defective LED in the fixture. Two coats seemed to do the trick.

We have to admit that we have a hard time visualizing a city employee painstakingly painting LEDs when swapping out a fixture would take an electrician a few minutes, but at least it’s an option. And, it’s something for hobbyists and homeowners faced with the problem of wonky white LEDs to keep in mind too.

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Real Radar Scope CRT Shows Flights Using ADS-B

April 15, 2023 by Dan Maloney 9 Comments

Real-time flight data used to be something that was only available to air traffic controllers, hunched over radar scopes in darkened rooms watching the comings and goings of flights as glowing phosphor traces on their screens. But that was then; now, flight tracking is as simple as pulling up a web page. But where’s the fun in that?

To bring some of that old-school feel to his flight tracking, [redacted] has been working on this ADS-B scope that uses a real radar CRT. As you can imagine, this project is pretty complex, starting with driving the 5FP7 CRT, a 5″ round-face tube with a long-persistence P7-type phosphor. The tube needs about 7 kV for the anode, which is delivered via a homebrew power supply complete with a custom flyback transformer. There’s also a lot going on with the X-Y deflection amps and beam intensity control.

The software side has a lot going on as well. ADS-B data comes from an SDR dongle using dump1090 running on a Raspberry Pi 3B. The latitude and longitude of each plane within range — about 5 nautical miles — is translated to vector coordinates, and as the “radar” sweeps past the location, a pip lights up on the scope. And no, you’re not seeing things if you see two colors in the video below; as [TubeTime] helpfully explains, P7 is a cascade phosphor that initially emits a bright-blue light with some UV in it, which then charges up a long-persistence green phosphor.

Even though multicolored icons and satellite imagery may be more useful for flight tracking, we really like the simple retro look [redacted] has managed to pull off here, not to mention the hackery needed to do it.

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A Loving Look Inside Vacuum Fluorescent Displays

February 26, 2023 by Dan Maloney 20 Comments

Everyone knows we’re big fans of displays that differ from the plain old flat-panel LCDs that seem to adorn most devices these days. It’s a bit boring when the front panel of your widget is the same thing you stare at hour after hour while using your phone. Give us the chunky, blocky goodness of a vacuum fluorescent display (VFD) any day of the week for visual interest and retro appeal.

From the video below, it seems like [Posy] certainly is in the VFD fandom too, rolling out as he does example after example of unique and complicated displays, mostly from audio equipment that had its heyday in the 1990s. In some ways, the video is just a love letter to the VFD, and that’s just fine with us. But the teardowns do provide some insights into how VFDs work, as well as suggest ways to tweak the overall look of a VFD.

For example, consider the classy white VFDs that graced a lot of home audio gear back in the day. It turns out, the phosphors used in those displays weren’t white, but closer to the blue-green color that VFDs are often associated with. But put a pink filter between the display and the world, and suddenly those turquoise phosphors look white. [Posy] does a lot of fiddling with the stock filters to change the look of his VFDs, some to good effect, others less so.

As for the internals of VFDs, [Posy]’s look at a damaged display reveals a lot about how they work. With a loose scrap of conductor shorting one of the cathodes inside the tube, the damaged VFD isn’t much to look at, and is beyond reasonable repair, but it’s kind of cool to examine the spring mechanisms that take up slack as the cathodes heat up and expand.

Thanks to [Posy] for this heartfelt look into the VFDs of yesterday. If you need more about how VFDs work, we’ve covered that before, too.

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