Słomkowski's technical musings

Playing with software, hardware and touching the sky with paraglider.

Determining the pinout of the unknown vacuum fluorescent display


Once-ubiquitous vacuum fluorescent displays are easy to reverse-engineer. Mysterious green glow goes well with retro-styled devices. I show how to determine the pinout of unknown display using simple technique.

Vacuum fluorescent displays are cool. They are not as iconic as Nixie, but they do nicely as retro display. They are usually custom-made for specific device and so their datasheet is not publicly available. The good news is that they are quite easy to reverse-engineer.

Where to find them

Aside from the display, it’s good idea to salvage power transformer because it often has dedicated winding for powering the filament.

How do they work

The principle of operation is described in detail on Wikipedia, there’s no need to repeat it here. Single segment has three electrodes:

To light the segment up, you have to supply voltage in range 30 - 50 V to both anode and the grid. Shorting the grid to the cathode shuts the whole digit down. This way the multiplexing is done. VFDs have advantage over nixie tubes that they don’t need separate driver for every digit. So this is basically a direct heated triode. Some folks even made audio amplifiers using them!

Vacuum fluorescent display filament should be powered by alternate current from center tapped transformer with center tap connected to the ground. If you provide both anode and filament voltages from the same power supply, the voltage drop along the length of the filament will cause changing effective anode-cathode voltage for every digit. Let’s say your anode voltage is 30 V and filament 5 V, ground connected to left pin. In this case leftmost digit has anode-cathode voltage of 30 V but rightmost - 25 V. This will contribute to significant brightness change.

Usually every device containing the vacuum fluorescent display has accompanying power transformer so you may salvage both parts.

Examination

For the purpose of this article I have examined two vacuum fluorescent displays:

The first thing to to is check whether the display is airtight. When you look at the corner of the VFD, you’re supposed to notice dark and shiny getter spot. If the getter is white, the vacuum within the display is no more and the VFD is irreversibly broken.

Determining the filament voltage

Majority of them has easily determinable filament pins just by looking through the glass. Double check this using the ohmmeter. The cold filament is supposed to have resistance of several ohms, my displays:

It’s widely assumed that the safe voltage for powering filament is when the filament just barely starts to glow dark red. For testing you should use variable voltage power supply. Connect it to the filament and observe it in the dark room. You should gradually increase the voltage by 0.1V starting from 2V. Wait couple of seconds after each increase. Majority VFD filaments start to glow at the voltage between 3 - 5 V. My displays:

I assumed that both of them can be safely powered from 3V power supply.

Identifying anodes and grids

You’ll need:

Follow the steps:

  1. Power up the filament.
  2. Connect negative of the 30 V power supply to one of the filament pins.
  3. Connect at least two crocodile clips to 30V through 51 kΩ resistors.
  4. connect one of the clips to the first grid you can identify by looking through the glass.
  5. swipe the other crocodile on the remaining pins. Some of the segments under selected grid should glow. This way you can identify the anodes.
  6. Proceed with remaining grids.

This way you can easily determine the pinout of almost any VFD.

Determined pinout

For demonstration purposes I reverse-engineered the two aforementioned VFD’s pinouts. Pins and digits are numbered from left to right. I skipped determining the output of additional segments, I limited myself to just digits.

Futaba 9-BT-22ZK

Filament voltage ~3 V, 100 mA.

Pin number Function
1, 2 filament
6 colon’s upper dot anode
7 colon’s lower dot anode
9 anode segment G
10 anode segment F
11 1st digit grid
12 2nd digit grid
13 colon’s grid
14 3rd digit grid
15 anode segment E
16 4rd digit grid
17 anode segment D
18 anode segment C
20 anode segment B
21 anode segment A
22, 23 filament

NEC FIP4B9F

Filament: ~3 V, 100 mA.

Pin number Function
1, 2 filament
4, 7 1st digit grid
10 2nd digit grid
12 colon’s grid
13 anode segment G
14 anode segment F
15 3rd digit grid
16 anode segment E
17 anode segment D
18 anode segment C
19 4th digit grid
20 anode segment B
21 anode segment A
22 filament