MAILBAG
Dear Russell: Re: Thermostat for
Attic Exhaust Fans, November ‘ 10,
page 24. Thanks so much for your
response and for publishing my
question in the November issue of
Nuts & Volts.
It occurs to me that a “heating
only” thermostat is designed to turn
on a furnace when the ambient
temperature drops below a certain
point. In my case, I want the device
to turn on the fans when the
temperature goes above a certain
point in the attic, and turn them off
when the temperature drops down
to the set point.
I am thinking that I should use a
thermostat that is designed for
heating and air conditioning
systems and set it to “cool” instead
of the “heating only” thermostat you
show in your schematic. Does this
make sense?
— Guy Fischetti
Response: Yes, you are right.
I guess I wasn’t thinking.
Dear Russell: Re: Inexpensive
Drafting Software, April ’ 10, page 26.
In the November ‘ 10 issue, the
first article in the mailbag discusses
drafting software. Being an avid
Solid Works user and instructor, I
have become familiar with a 2D
solution which has been available
for only a short time. It’s called
DraftSight and works very similarly
to its big name competitor, and not
the LT version. The major difference
is that it is free to download. Go to
www.3ds.com/products/draftsight
for more information and a
download link.
— David Urasky
Response: Thanks for the info,
David. If anyone uses this software,
I would be interested in more
feedback.
Dear Russell: Re: PC Board
Current Capacity, January ‘ 10,
page 25.
The Military has documents
defining the design requirements for
printed wiring boards.
Look at MIL-STD-275 Printed
Wiring for Electronic Equipment
( www.dscc.dla.mil/Downloads/Mil
Spec/Docs/MIL-STD-275/std275.pdf).
For Type 1 single-sided board, see
Figure 4a, page 34. Type 2 Interior
layers, see Figure 4b, page 35.
— Glenn G.
Response: I am sure many of
our readers who are dealing with a
high current layout will appreciate
this data. Thank you.
FLICKERING CANDLE
QI belong to a synagogue that has an eternal light fixture containing a low wattage bulb which needs moving on a regular basis. As a consequence of moving, it is occasionally
dropped, usually necessitating the replacement of the
lamp. Years ago, we developed a flickering circuit which
would give the appearance of a flickering flame when in
position. I think the time has come to move the circuitry
into a more reliable design using LEDs. Could you help me
with a circuit that would cause four to six orange-yellow
LEDs to appear to flicker randomly like a candle and
provide a means of battery backup capable of powering
the unit continuously through power failures of up to 12
hours? Because of its small size and large power density, a
lithium-ion cell would be preferable.
A truly nice bonus would be if it was
capable of running off solar cells so
it could be truly eternal and free of
the power company. Thank you for
your help.
might be even better. As usual, I can send anyone a
programmed 12F675 for $5; just send your request to me
at russlk@yahoo.com (that is my spam filter address). The
schematic is Figure 7. If a single 20 mA LED is used, the
MOSFET Q1 is not needed. With Q1, the LED could draw
up to five amps to the detriment of battery life.
Figure 8 is the power supply; the battery is nominal
7. 2 VDC, maintained at 8.2V for a full charge. The battery
is rated 2. 4 Ah and the load is 30 mA so the light will
operate for 80 hours with no input power. If the solar
panel is used for the only power, consider that the daily
power used is .03A X 24H = 0.72 AH. The solar panel is
active for about four hours, so the panel output has to be
0.72/4 = 180 mA at minimum. Two of the 100 mA rated
Jameco solar panels in parallel should work as long as
there are not extended rainy days. The panels no doubt
produce some power even on cloudy days. NV
— Tom Holloway
AIn the October issue, page 38, Jon McPhalen wrote a very good article on this very subject. I was mostly
interested in the mechanics of
making the candle because the
software was over my head. My
approach using Basic is rather simple:
I vary the brightness and rate of
brightness change using the
RANDOM command. I first had only
one loop of FOR-NEXT, but the
pattern was easily discerned so I
added a second loop; see Figure 6.
A third loop with triple RANDOM
February 2011 27
