2 Nov
2010
2 Nov
'10
6:03 a.m.
On Mon, Nov 1, 2010 at 4:02 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
On 2010 Nov 1, at 10:23 AM, Dan Roganti wrote:
oh yes, I was preoccupied with the diode matrix, I neglected good ol' ohm's
law on the 4066. There is another digital switch around - the 4066 was an
ol' favorite - with enough power that would replace this , I've seen them in
the mouser catalog.
On Mon, Nov 1, 2010 at 3:10 PM, Brent Hilpert
<hilpert at cs.ubc.ca> wrote:
We must have different specs at hand (different manufacturers perhaps):
Fairchild'76 says 270 ohms typ, 1000 max at 5V/25C, Harris'92 says 470-1050.
Neat suggestion to modify the existing A-F displays, but once you have
the
'138 and a bunch of diodes you could just revert to the technique of
generating 8-F, and get by with 2 chips (247,138), 1 inverter (to disable
the 247 for >=8), and diodes.
The use of the 4066 does present a problem for direct drive of the LEDs:
ON
resistance of the transmission gates is on the order of hundreds of ohms
(with considerable variability over devices and temp).
oh yea, that would make it only 3chips - we have a winner !
Although the utliz.of
the inverter chip is very low.
You could always just use a transistor.
I just didn't want to leave the '247 abandoned ;)
The on resistance of the 4066 (~80ohm) is way less than the 4016(~200ohm)
-
which is why I chose that one - but marginally higher when operating a the
lower 5volts. But you could still compensate by lowering the LED resister
to
keep it just as bright.
Even if you account for the R in the LED drop R, part
of the problem is the
variability between devices, even though intra-device variability is low.
Based on the specs I have power dissipation may also be an issue once you're
pushing current through that internal R (100mW max per switch).
But I like the last solution even more, 2 chips, 74247, 74138, transistor
inverter, a few diodes, viola !
Rube Goldberg would be proud ;)
You can't rule out discretes !
=Dan
--http://www.vintagecomputer.net/ragooman/