In the 60’s and 70’s there were many ways to display numeric data. Nixie tubes, Vacuum Florescent Displays (VFD), micro projection systems, you name it. All of them had advantages and drawbacks. One of the simplest ways to display data was the RCA Numitron. [Alec] at Technology Connections has a bit of a love/hate relationship with these displays.

The Numitron is simply a seven-segment display built from light bulb filaments. The filaments run at 5 V, and by their nature are current limited.  Seven elements versus the usual ten seen in Nixie tubes reduced the number of switching elements (transistors, relays, or tubes) needed to drive them, and the single low-voltage supply was also much simpler than Nixie or even VFD systems.

Sounds perfect, right? Well, [Alec] has a bone to pick with this technology. The displays were quite dim, poorly assembled, and not very pleasing to look at. RCA didn’t bother tilting the “8” to fit the decimal point in! Even the display background was gray, causing the numbers to wash out in ambient light. Black would have been much better. In [Alec]’s words, the best way to describe the display would be “Janky,” yet he still enjoys them. In fact, he built a fancy retro-industrial-themed clock with them.

The Numitron was not a failure, though — we know variants of this display ended up in everything from gas pumps to aircraft cockpit gauges. You can even build an LED-based replica clock — no glowing filaments necessary.

[Gilles Messier] at the Our Own Devices YouTube channel recently took a look at an interesting device — an electric lantern powered by a candle. At first glance, this sounds completely absurd. Why use a candle to power LEDs when you can use the light from the candle itself? This gadget has a trick up its sleeve, though. It lets candle light out and uses the heat from the candle flame to generate power for the LEDs.

The small Peltier “solid-state heat pump” module in the lantern acts as a thermoelectric generator, converting heat from the candle into electricity for the LEDs. The genius of the device is how it handles the candle “exhaust”.  A bimetallic disk in the chimney of the lantern closes when the air inside the device is hot. The Peltier device converts the heat differential to electricity, causing the air inside the lantern to cool. Meanwhile, the candle is beginning to starve for oxygen.  Once the air cools down a bit, the disk bends, allowing stale smoke out, and fresh air in, allowing the candle to burn brightly again. Then the cycle repeats.

[Gilles] does a deep dive into the efficiency of the lantern, which is worth the price of admission alone. These lanterns are pretty expensive — but Peltier modules are well-known by hackers. We’re sure it won’t be too hard to knock together a cheap version at home.