an even more precise timer.
Response: Joe, I have used this
method in past columns. The 558
was used in this instance to mix
things up and show the reader that
there are alternatives to a digital
world. — TJ
Reading through the November
2003 issue, I ran across a request from
Mr. James Tadlock concerning the
4N25 optoisolator. Having used this
type of device in many of my previous
designs, I have to say that you are correct in telling Mr. Tadlock not to ground
pin 6 — otherwise known as the base.
To ground the base would guarantee
that the circuit would just sit there —
dumb and happy, doing nothing.
However, the best way to handle
the base in the 4N25 is to place a
100K resistor from pin 6 to ground.
Adding this resistor will cause the Icbo
current to be bled off through the
100K resistor and will guarantee that
the base is not pre-charged.
Norman A. Howard
In your May 2004 column, the
Precision 15 Minute Timer (Figure 5
in the May issue) shows a 558 IC with
the same pin numbers being utilized
on all four timers. This can't be right,
Response: Oops! This is what
happens when you do a copy and
paste of identical modules in a
schematic and forget to follow-up
with a proper pinout. Here's the
corrected schematic (Figure 8). — TJ
In the May 2004 issue, you give
sources for downloadable data
sheets. There is another one I like to
use — www.questlink.com It is
similar to ChipDocs, but free.
Build a White-Box TiVo
For under $200.00, you can transform a PC
into a full-featured personal video recorder.
Free clip art and document templates for
Microsoft Word 97 on up. Templates include
a simple will, power of attorney, birthday
cards, Christmas newsletters, and more.
http: // office.microsoft.com/clipart/
NUTS & VOLTS
Circle #138 on the Reader Service Card.