October CTT

[lionelsoni]

Here are some comments about the Weekend Workshop article "Barricades for safety:"

Although the lights in the modified circuit do not flash at the same time, neither do they flash completely independently.  Instead, they work in pairs, with one LED in each pair on when the other is off, and vice-versa.

Throughout the article, capacitors are described in units of "mfd", by which the author means "microfarad", although the metric prefix "m" means "milli" or 1/1000, not "micro" or 1/1000000.  In step 8 he switches to "uf", which is close to the correct symbol, µF.  (The "u" is an allowed workaround for the Greek µ.).

Step 3 says that C6 has a 50-volt rating, while the parts list says 35.  Either is okay; but 20 volts or anything above that should be enough for the 16-volt AC input voltage.  All the other capacitors need only a 5-volt rating.

The units of resistors R5-R8 are given in the parts list and in step 7 as "k-ohm" or "kohm", which is 1000 times too large.  They should be just 220 Ω.  The color codes in parentheses are correct.

Numbers and metric unit symbols should not be run together with no space in-between.

The author has made the frequencies of the two LED pairs different by using 4.7 µF (C1) for the timer that drives one pair and 4.4 µF (C2 + C3) for the other.  This can also be accomplished by varying resistor values, to make better use of the excess components in the packages of 10.  For example, omit C3, let both C1 and C2 be 4.7 µF, and replace 220-kΩ R4 by two 100-kΩ resistors in series.

[Bob Keller]

Errors in the story are on us, not the author. I also failed to send him a copy of the final version, where he no doubt would have caught some of  the problems.