EIGHT BIT BINARY
QI’m writing to ask
your recommendation for a suitable
eight bit binary up-down counter. The output of
the counter will feed the inputs
of a DAC to produce a saw
tooth wave. The output of the
DAC will feed a voltage controlled resistor. I need to be
able to set the counter to start
at binary 50 and end at binary
200 instead of binary zero
and 255, respectively, because
the voltage controlled resistor
is not entirely linear at the
binary high and low number
values. The counter also needs
to have a reset pin to start it at
the pre-defined lower value.
— Al Lovecky
AFigure 8 does
what you want.
The 74F269 is a
power hog but it
QUESTIONS & ANSWERS
■ FIGURE 8
In your January ‘08 Q&A column,
you offered an excellent discussion on
power transformer secondary current
waveforms. Unfortunately, you didn’t
mention the most important bit of
wisdom that should be passed along:
the increase in secondary current rating
relative to final DC output current. I
wrote to TJ Byers about this issue as
well, but I think the information came at
a bad time.
Transformer secondary current is
greater than DC output current. An
example given by Frost ( www.at
instance, points out that a 5 VDC
2A power supply using a full-wave
bridge rectifier and capacitor input filter
actually yields 3.6A secondary AC
current and requires a transformer VA
rating almost double its DC output
wattage. The situation gets worse if
you need more voltage for regulator
headroom. This simple fact has led to
the demise of many power supplies
built by amateurs.
for Decade Engineering
Response: This result comes about
because RMS requires squaring the
pulse amplitude which makes it greater
than the average (root mean square).
The Mailbox Timer from the
January ‘08 Q&A column could also be
solved with a capacitor connected to
the battery when the door is closed and
switched to the transmitter when the
door is open. A single pole double
throw switch will do it. The capacitor
needs to be large enough to run the
transmitter two or three seconds.
Response: Yes, that will work but
you need to know the transmitter
current drain. If the transmitter current
was 10 ma, the allowable voltage drop
is one volt and the time is two seconds,
then the capacitance should be:
C=I*d T/dE = .01*2/1=.02 farad. Mouser
has a 0.1 farad, 5. 5 volt supercap
that would work; part number 878-
FS0H1042ZF. Connect two in series to
I am a ham radio op, using
preamps for two-way repeaters in
VHF/UHF communications. There is a kit
available right in the February issue
from Ramsey in their ad on page 21; RF
Preamp Kit #SA7 for $19.95. This needs
to be assembled, however, and needs a
power supply, as well.
There is another company called
MCM which has available SATV/CATV
preamps madeexactly for what
Nathanial needs in his question. Their
website is www.mcminone.com. On
page 282 of their 2007 catalog, they
have listed a ZVM-201 RF broadband
distribution amp for $21.50; frequency
range 50-550 MHz, gain 20db, noise
figure 3. 5 db.
This amplifier has its own internal
power supply and runs from 110V, AC
power and is housed in a metal box.
The connectors are type F6 which is the
type used in cable systems.
The part number for ordering from
MCM is 33-2845.
Dan Harger, W8BCI
Response: Thanks for the
feedback, Dan; this is certainly much
better than my one transistor amp.
However, if Nathanial uses the Terk
TV55 antenna, I don’t think a preamp
will be needed.
April 2008 27