Blinking light report

[sweetser at TheWorld.com (Doug Sweetser)]

Hello Folks:

Thanks for all the suggestions.  Boston Public Library is a great
resource that remains free.  Saturday afternoon I was at the shelf
containing TK9965, but none of the three books by Forrest Mimms was
there despite the claims of the web site.  There was a book on the 556
timer, like the 555 but with two timing circuits.  There appeared to
be an appropriate circuit: a cyclic 4-stage sequential timer.  Off to
Radio Shack I went.

The Shacks of today often do not have people who understand digital
circuits.  There is too much turnover, the staff is young, often
foreign born.  One store had no one who wanted to fish through their
standardized bins for parts.  Another did have a guy who knew his
bins. Forty-seven dollars latter, I had a breadboard, 4 555 timers, 4
50 uF capacitors, LEDs, and a hundred resistors.

There was a digital circuit class at Harvard Extension School that
looked interesting to me.  I bought the tool box filled with EE tools,
but never took the class.  That lack of real experience showed on
Saturday night.  I was unable to construct a working version of the
timing circuit.  I made all kinds of errors, like only orienting one
of the four chips correctly.  Sunday morning I just wanted to build
one blinking light every 5 seconds.  Using two 555 chips, one as the
cyclic signal source, the other to turn on only briefly, I was able to
get one of the lights going.  

What I can do once, I could repeat.  Two identical circuits where
sitting on the same board.  When I supplied the power, one always
blinked first, followed shortly by the next.  Perfect.  Actually it
turned out to be near perfect, since as someone pointed out, there is
drift.  I used standard 5% tolerance parts.  If I bought more
expensive lower tolerance parts, I would think the drift would go
down, but don't know by how much.

I hope to shoot the video on Thursday. Over the weekend I'll be taking
a video class at Cambridge Center for Adult Ed.  If/when I get a still
from that, I'll capture a few stills and forward a URL to the group.

doug