It wasn’t long after the development of the LED that LED watches became available. They were prized for their clear light output and low power draw. Neon bulbs, on the other hand, are thirsty for current and often warm or even hot in operation. And yet, [Lucas] found a way to build them into a sweet watch that actually does the job. Nice, right?
The design uses a simple trick to avoid killing the batteries with excessive power draw. The neon lamps are only activated when the user waves a hand above the watch, at which point the lamps light to display the time. Reading the time is a little fiddly, but understandable with the aid of this PDF diagram. Basically, the two electrodes of each neon lamp are driven independently. This gives each of the four lamps three possible states: with the first electrode lit, the second electrode lit, or both lit. Four lamps multiplied by three states equals 12—so the watch can display the hour quite easily. As for minutes, a similar scheme is used with some modifications for clarity. Setting the time is via a light sensor on the watch which picks up flashes from a computer screen.
It reminds us of a time when we once thought nixie tubes were too power hungry for a wristwatch build… until the hackers of the world proved us wrong. Video after the break.
[Thanks to Jan for the tip!]
“Neon bulbs, on the other hand, are thirsty for current …”
What? A LED typically might require 10 mA. A neon bulb is happy on 1 mA.
Now, you get a lot more light out of that LED’s 30 mW vs the neon’s 60 mW, and don’t need a power-hungry boost converter either.
1mA at the voltage that neons operate at isn’t a trivial amount of wattage.
It is fairly low honestly. If you are being a bit crude and assume the difference between Firing and Sustain voltage is dissipated passively a tiny neon bulb like those used here consume like 100mW. A couple times higher than a led. With some careful design though you can get the supply voltage closer to the sustain once the neon fired up. Letting you reach up 60mA as Paul suggested. This is ignoring the Boost Converter (in)efficiency though.
Plus the article does specifically mention Current, not Power/Wattage. “High current” is not something one would generally use with Tube stuff.
Oh. The document shared actually gives the current draw.
These little fellas are of the <1mA type with the watch consuming 30mA from the li-ion while actively displaying. That is more efficient then most Led variety watches ive seen honestly.
This watch is hungry for power. About 100 mW of it even, sometimes. One Hundred Milli-Watts!
It’s a neat little build, but I laughed at this part: “Displaying the time takes about 16 seconds.” That’s kind of an Achilles heel!
It is. It could be made shorter, but then one might need multiple attempts at parsing what the watch says. (Generations younger then me can’t even read an analogue clock anymore :))
I was under the impression that cold cathode lights were very efficient, its the higher voltage coming from an inverter that makes for higher power use. I remember the neon R/C blinkenlights that ran off zinc carbon B+ batteries for years.
When LED watches came out, they were also thirsty for current. I was lucky to get a month on one battery.
Neons get ” often warm or even hot in operation” only if you over drive them. normally those small pilot neons should be drawing around 1/4watts or less..
I think someone is confusing neon lamps with incandescent. Neon lamps do not require a lot of current, nor do they get hot in operation.
In case someone want to build one for their own use: https://www.eluke.nl/2024/02/14/neon-watch-design-information/