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Renesis

Thu Feb 11 2010, 09:39PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Me and a friend of mine are completely rebuilding an old pinball machine as a school project. We are tearing out all of the old circuitboards and rewiring the playfield to interface with a modern PLC, so it can be used as a fun training model for PLC programming.

This is one of the machines five original 8-digit 7-segment displays. Apparently these are called neon displays, and that is also the only thing hours of googling can tell me about them. Does anyone have any good info on how these work, and how to drive them? Im a complete novice on this old technology, so any info would be appreciated.

No doubt the most logical choice would be to replace them with LED-displays, especially since i have a deadline. But i dont think i need to explain you guys why i would prefer to reuse these badboys.

radiotech

Thu Feb 11 2010, 10:07PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Those displays look like Beckman Planar display panels. We used them in the 70's in Avtron speed indicator systems; They were a real pain to replace--millions of lugs to desolder.

klugesmith

Fri Feb 12 2010, 04:06AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Search for Panaplex.
Or browse the Yahoo group called NEONIXIE-L for work by hobbyists who specialize in these things.

I bet your display is multiplexed (X digits * Y segments). and the board you unsoldered it from might include a set of properly wired digit and segment driver transistors.

You need a DC supply of 180 to 200 volts, with current limiting resistors (of much higher value than those used with LEDs). The anode and cathode driver transistors don't need to be rated more than 60 or 80 volts if the biasing is done right.
To play with it & just light up segments, get yourself a 200 VDC supply and a 100k series resistor,
try various combinations of pins & see what makes things light up.

Renesis

Fri Feb 12 2010, 01:27PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Klugesmith wrote ...

Search for Panaplex.
Or browse the Yahoo group called NEONIXIE-L for work by hobbyists who specialize in these things.

So thats why i couldn't make anything light up, i was fooling around with a 5V supply. Yes, the board attached to the displays is most likely a driverboard of some kind, their connection with the CPU board is done with one 20pin bus cable. But that is still worthless to me unless i can figure out their pinout/protocol.

I'll be searching through that link you posted in the weekend, it might tell me what i need to know.

Are there anything i should be aware of before i start playing around with a HV supply? Are these displays robust or will a short circuit/reversed voltage destroy it?

tobias

Fri Feb 12 2010, 01:42PM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Renesis

About the 20 pin bus cable. The pinout can be just a simple multiplex... each combination of wires light up a single segment.. it's easy to understand if it is multiplex.. you just have to follow some of the pins that connect the cable on the driver board until the display.. if the pins are connected to each display and each segment.. is just a matter of connecting the driver board to your uP! Ground pins are easy to find out too. The trick part should be Vin.

Conundrum

Fri Feb 12 2010, 06:02PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

you may also find that some of the most used segments do not work reliably or do not light at all. i ran into this problem a few years back...
-A

radiotech

Fri Feb 12 2010, 08:00PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

When we used those planar display stations, there were 5 groups of 4 wires. The circuit board decoded BCD. The high voltage driver chips followed the dec oders and the latches. Look on the cards you removed them from, Dc was 160 V with balast. We tested the stations after repair by using a bcd 0-9 thumbwheel switch. They also have a keep-alive, that
is on always in each segnent, We could see it by looking sideways-down.

Renesis

Fri Feb 12 2010, 11:36PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Alright thanks for the help everyone, i think i've got the info i need now.

Nicko

Sat Feb 13 2010, 07:09AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Renesis wrote ...

Alright thanks for the help everyone, i think i've got the info i need now.

Just to add - when cold-cathode neon discharge tubes get old, the cathodes (the bit that glows) can get coated in impurities - this can result in parts of segments (or even the whole segment) not lighting correctly - this is called "cathode poisoning". Further, the whole tube can out-gas, which can also prevent it lighting at all.

Panaplex is a trademark used for this style of display - technically they are he same as nixies, except that they use a segmented display and are flat. Some of them have alpha-numeric capability as they have a "union jack" arrangement of cathodes so they can display words. Your one seems to have some resistors on its carrier board - looking at their value, they might well be for the decimal points, which being very small, require a far lower current than the main segments.

The tube is actually filled with mostly low pressure neon together with a small amount of argon (3% or so) - its called a "Penning mixture" - this allows more predictable behaviour.

The main problem with driving this type of tube is over-current, which drastically reduces the life of the tube. With over-current, the cathodes "sputter" - the inside of the tube can go silvery with deposited metal and the cathodes erode. i.e. its very important to drive the tubes correctly. Find the data sheet for your tube and look at the segment currents - these must not be exceeded. The driver chips have changed since the display you have was made - there were typically something like a DS8880 - these aren't made any more but if you wish to be authentic, you can get them from various suppliers. Nowadays, most people use MPSA42/92 NPN/PNP HV transistors which are very cheap and easily available. If you want to get cute, uP-compatible cathode driver chips are made by Maxim, Supertex, Allegro and others. Note that the driver chip doesn't need to be rated at 180V - as the tube typically drops about 120V when on, 80 - 100 V devices are sometimes used.

Do NOT do the following test if there are any active components on the display board:
The displays are pretty tough. Assuming that the actual display board contains only passive components, if I were you, I'd get a 180VDC supply (isolated from the mains) and a 47K 1/2 resistor connected to the +ve end. Clip this onto each pin of the display and then wipe the -ve end across the other pins - as you identify each anode (there should be one pin for each digit), you will be able to identify which of the remaining pins is a segment cathode. DO NOT DO THIS ON THE DRIVER BOARD - only the display board. Here is a typical circuit for the driver board:

- note that the example here uses cathode rather than anode current limiting resistors.

The Burroughs Panaplex displays like yours are sometimes multiplexed, however yours seems not to be - there are enough pins for each digit to have its own anode and separate cathodes - there were a few standard designs around depending on the pin-ball manufacturer and there are whole web-sites dedicated to pin-ball system restoration. Panaplex displays also normally have a "keep alive" terminal that is used to help maintain predicable striking, especially in multiplexed units like these.

I have one of these pin-ball displays made into a clock - its in a cheap Ikea box with a nice front made in Photoshop:

Renesis

Sun Feb 14 2010, 09:09PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Nicko wrote ...

...a lot of useful information.

Thanks a lot Nicko, this is great information. Although i've decided not to use my displays for this project, the deadline is just too short. I'll use LEDs.

Maybe i could try to build a clock like yours out of them, that would be a fun project. I might give a few away too if anyone is interested, we'll see.

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