Hi all
A new guy checking in here. Looks like a nice forum and nice people - think I'll stick around for a while
I have a little headache regarding how I should solve this in a tidy manner so I'm asking you if you have any good advice.
I have an old radio with a VFD-display which is worn out. There is no way I can find a replacement display so I decided to build my own display with 7 (+1)-segment LED displays. There are 12 digits.
The logic behind this is a bit obscure (not hard, but why the H.. did they do it like this hehe).
The controlling logic is:
uPD8279C-5 for segment multiplexing
MC14515B for display multiplexing
(Output buffer for VFD consists of 3x uPA80C, -75V lines)
I'm connecting at the 5V side of the output drivers.
The case is:
1: The display select is a "low" output, the 11 others are "high" (thought it was the other way around before i made a test circuit).
2: Segment output is "high" when active.
3: I use common cathode displays.
The first experiment was using a NPN transistor for sinking the cathode when the display select was high. Bad result as I saw on a scope that it was the other way around
In the next attempt I used one more NPN transistor to make an inverter for the display select signal. It works okay, at least it seems to, but the display output is somewhat low, so I decided to buffer the segment output too, but that gives me a headache as the base voltage is too low when I have to pass it through a series resistor, LED and cathode sink transistor. It just won't light up properly. And on top of that, it requires too many transistors to be nice, 12+12+8 = 32 transistors :s (BTW: I'm using BC547C as they're cheap and reliable with a fairly high hFE).
I will post a picture of the existing circuit when I get home.
Summary of the circuit and signals:
Matrix of 12 x 8 segment (7+ DP) LED common cathode
8 active high signals for segments
12 active low signals for displays
None of the circuits can sink or source a LED, I need buffers.
I hope this wasn't too confusing, so just ask if you're wondering about something.
Would appreciate any help available
PS: Getting parts here in Norway is a pain as we only have a couple of shops for electronics and they're via mail order (1-3 weeks delivery).