Building a clock from salvaged Vacuum Fluorescent Displays (Maurycy's blog)

Building a clock from salvaged Vacuum Fluorescent Displays

2026-05-14 (Electronics)

When was disassembling an old calculator (as you do) I found these cool VFD tubes:

Normally, a whole display is built into a single flat package, which doesn't look particularly interesting.<br>These ones have the individual digits inside a standard vacuum-tube housing, which gives them a cool nixie-esque look.

Because vacuum tubes are rather archaic components, they deserve some introduction.<br>The simplest tube has two electrodes: A filament and anode plate.

Similar to an old-school lightbulb, the filament is a tungsten wire that heats up when a current passes through it.<br>On a microscopic scale, heat is vibration of atoms —<br>which knocks some (negatively charged) electrons out of the wire and into the surrounding space.

If the plate has a positive voltage, it attracts some free electrons which allows a current to flow.<br>If the plate is negative, the electrons are repelled and the tube doesn't conduct.

This diode configuration is useful for power supplies,<br>but most tubes added a wire mesh or "grid" between the other electrodes.

The grid shields the electric field of the plate, so the electrons near the filament only feel the grid's voltage.<br>However, because the grid has a very small area, most of the electrons it attracts fly through one of the holes to arrive at the plate.

As a result, a triode tube conducts plate to filament when both the grid and plate have a positive voltage:<br>similar to an n-channel MOSFET.

A Vacuum Fluorescent Display is just a triode but with the<br>anode plate coated in rare-earth oxide (similar to what's found in CRTs) which glows when stuck by electrons in the tube.

In my displays, each tube contains a single grid, but the anode is broken up into seven isolated segments which can be used to form digits.

The grid allows a clever trick:<br>segment lines can be shared between digits as long as only a one tube is on at a time.<br>A single tube is supplied with a positive grid voltage, allowing it to light up,<br>while all the other tubes have a negative grid voltage forcing them to stay dark regardless of the segment voltage.

By quickly cycling between the digits,<br>the control electronics can create the illusion that the whole display is on...<br>but saving a lot of wires:<br>a six digit display with a total of 42 segments only needs 13 signal lines.

I couldn't find a datasheet for my displays, so I had to determine the pinout experimentally:

Find the heater. It should be a pair of pins with a few ohms of resistance

Run 25 - 50 mA through it. The filament should start glowing very dimly . If it's bright, there's to much current.

Apply +12 volts (relative to the heater) to pairs of the remaining pins until a segment lights up: One of those pins is the grid

Moving the one of the +12 wires to a different pin, while keeping the other one connected as is.

If the display goes out, the pin you disconnected was the grid.

If a different segment is on: The pin that stayed connected is the grid.

Now, connect twelve volts to the grid and use apply +12 to the remaining pins to map out the segments.

If you try this, be very careful to not connect 12 volts across the filament.<br>Doing this will likely destroy the tube!

Also, on step #4, only the second outcome is guaranteed accurate if some pins are<br>not connected internally. It may be a good idea to try again but moving the other wire<br>to be sure.

AAA<br>B F<br>B F<br>GGG (Four)<br>C E<br>C E<br>DDD (Dot)

Seg. ASeg. G<br>Seg. FSeg. B<br>Seg. FourSeg. C<br>HeaterSeg. D<br>GridSeg. E<br>Seg. DotHeater

--><br>With that done, I moved on to finding good operating conditions for the display:<br>Increasing the filament current or/and the plate voltage will make the tube brighter...<br>but don't get the current to where the filament is visibly glowing:<br>That will make it hard to read and reduce the tube's life.

Borrowing some math from light bulbs, the expected filament lifespan is (roughly) inversely proportional<br>to current to the 6th power, so it's not a good idea to push it.

You'll often hear stories about that light bulb that's been running for 100 years, but it's no secret how they did it:<br>the bulb is running at 6% of it's rated power!

(Ok, a lot of that loss is from aging, but the original filament resistance was a lot higher than a modern bulb.<br>That made it run less current, at the cost of being even more inefficient and having a very red cast)

The plate voltage can be adjusted more freely, and is mostly a matter of personal preference.<br>However, too much will fade the phosphors faster, and again, there's a non-linear relation:<br>a higher voltage will cause more electrons to flow, but those electrons will also have a higher energy and do more damage.

Mine were happy at 25 mA of heater and a voltage between 10 - 24 volts.<br>I settled on 12 volts, which worked well for a desk clock while not being annoying at night.<br>(keeping in mind that...