Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!rutgers!mit-eddie!uw-beaver!zephyr.ens.tek.com!tekig5!brianr
From: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
Newsgroups: sci.electronics
Subject: Re: Design a bar graph
Message-ID: <7217@tekig5.PEN.TEK.COM>
Date: 8 Nov 90 00:55:23 GMT
References: <1990Nov3.200047.11252@athena.mit.edu>
Reply-To: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
Distribution: na
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 23

In article <1990Nov3.200047.11252@athena.mit.edu> chuck@mitlns.mit.edu writes:
>
   (Tedious-to-enter circuit diagram deleted)
>
>      Takes three chips 1 74xx138 and 2 quad and gates. How many did
> it take you?
>
>
>					Chuck@mitlns.mit.edu

One chip:  A 7445 open-collector binary-to-decimal decoder/driver.

The seven LEDs can be wired in series, with the cathode end of the
string going to output 7 of the decoder, and successive cathode/anode
node pairs going to outputs 6 through 1.  Wire the anode end of the
string to output 0, and apply a 20mA current source that has enough
voltage compliance to push through all seven LEDs.  Whichever output
of the 7445 gets turned on will shunt the current to ground, preventing it
from flowing through however many LEDs are "downstream" of that output.
All the LEDs up to that output carry the current, and therefore light up.


Brian Rhodefer