Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!portal!cup.portal.com!mmm
From: mmm@cup.portal.com (Mark Robert Thorson)
Newsgroups: sci.electronics
Subject: Re: Driver for Alphanumeric LED's
Message-ID: <19741@cup.portal.com>
Date: 22 Jun 89 00:11:38 GMT
References: <9571@boulder.Colorado.EDU>
Organization: The Portal System (TM)
Lines: 30

First, you need to encode the 12 switch outputs into a binary number.
Two 74148's will do this.  I suggest you arrange the alphanumeric displays
in a matrix, such that there are separate drive lines for each segment
and each digit.  Some displays of this sort come in banks of 4 or 5 chars
already wired up in this fashion.  This means you will need to scan the
display, lighting up one digit at a time.  An oscillator driving a counter
will do this.

Because you have 12 possible messages (mapped to 4 bits) and 8 character 
positions (3 bits coming from the counter) you have seven bits which 
specify which segments are lit for any given message at any given time.
These seven bits can be translated to the 14 segment drivers by two byte-wide
EPROM's.

Most EPROM's have more than seven address inputs.  You can run additional 
bits from the counter into the EPROM for displaying longer messages, like
16 or 32 characters.  This needn't require extra displays.  For example,
if you wanted to run a message Times-Square-style across a display (horizontal
scrolling), you could hook up another oscillator/counter to some of the other
address inputs.  Each increment of this second oscillator would address
the message as it would appear at each character position of scrolling.
To make sure the message always begins scrolling from its initial position,
the Enable Output lines from the "148's can be used to drive a monostable
to clear the horizontal scrolling counter.  This will cause scrolling to
begin from the initial position every time the switch is turned.

It will probably be easiest if you generate the EPROM code by writing a
little program in Basic or whatever language suits you best for short hacks
like these.  This will also make it easier to change the messages, if you
ever wish to.