WBO2 display

[Ludis Langens]

"Robert W. Hughes" wrote:
> 
> The output of the WBO2 is connected to a 12bit A/D (finally found one
> cheap enough). This 12 bit output is connected to the address lines of a
> pair of 2732s. Stored in the 2732s at each address is the equivalent AFR
> in BCD without the leading 1. [...]
> 
> This approach is not particularly low in chip count or elegant in
> programmability but all the parts are cheap and anyone who can program
> the Eprom for their car can duplicate it.

Here's a lower chip count, more elegant approach:

Use a multiplexed 4 digit display.  Use one EPROM four times as big
(27128).  Program the EPROM with the seven segment (and decimal point)
"bitmap" of the required digit.  Use a 4013 dual-D F/F as a two bit
digit counter.  Drive the two extra EPROM address inputs from this
counter.  Use four AND (or NAND) gates to decode the counter into
seperate digit select lines.  Buffer the segment and digit lines to LED
current levels.

This approach will need a clock - but the A/D probably already needed
one.  The segment drivers could be as simple as transistors (and
resistors) - there is no need to find a fancy/expensive BCD to 7 segment
driver chip.  It might be possible to use a "quad driver" type chip to
combine the AND gates and digit drivers into one package.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/


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