At 03:10 PM 12/03/97 +0200, you wrote:
>The drive is now referenced to the 'ground' potential and consists of similar
>segment and digit drivers.  They are made to work at 0 .. 12V positive these
>days while in the past the voltages were higher.

I hope it is a 12V display. Anything higher would be a pain to use.

>
>You need to have some form of intensity balancing in long displays if you have
>a DC fillament voltage as the drive voltage varies from 12V at one end to

It may be worth driving the filament through some kind of an H bridge.

>The drivers on a Futaba type LT display I have are NE594 and have 8 High side
>drivers with TTL inputs.  With your 5 digit display you would need 5 + 8
>drivers if you want to use the decimal points and the colon, just the
>numbers would need 4 + 7 so you will need 2 ICs 8 or 7 drivers per
>IC would be enough.  You could suffer the high side drivers with 2 transistors
>each but a lot more work.

With 5 + 8 drivers, I would be using up all the PIC 16C84 I/O lines. A
1-of-n decoder for the digit drivers will save 2 lines.

How much current does the grid and anode take? An alternative to high side
drivers could be using open collector TTL gates with pull-up resistors. A
7405, depending on the max voltage it can take, can be used to drive 6 lines.
That would mean 3 of these chips. Other possibilities are an LM339(4 lines,
4 chips) or CMOS gates operating off a 12V supply with a 0-5V signal on
their inputs. The decoder could be an extra chip unless I find an open
collector decoder like the 7441 or a CMOS decoder. I wonder what the input
characteristics of CMOS gates operating on a 12V supply are. If it is
suitable, I could use 2 hex inverters and one decoder(3 chips). I might even
be able to cobble together some kind of an H bridge for the filament with the
spare gates.

Regards

Prashant
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