before this question can be answered.
What data rate (throughput) is
required? The maximum data rate
usually found with a computer serial
port is 115 Kbit or 230 Kbit. Will this
link be half or full duplex? What type
of modulation scheme will be used?
The answers to these two questions will be used to determine the
bandwidth required for the wireless
link. What is the purpose of this link?
Is this for experimentation with radio
communications (the same thing as
wireless communications) or is there a
serious application being implemented? If the latter, I'd seriously consider
using currently manufactured IEEE
803.xx wireless Ethernet modems and
high gain directional antennas. Three
to five miles might be achievable.
I was playing with the former over
30 years ago. Thanks for bringing
back memories of fun times in early
computer experimentation. Check out
some of the earlier Circuit Cellar
articles published in Byte Magazine.
Douglas Hall, CPBE
80 December 2011
When I saw my answer published
in September, I realized that I had
made a big mistake. I missed two very
important words in the original
question: "inversely proportional." So,
my circuit may not be the solution that
Mr. Prigge is looking for.
The circuit is a form of constant
current source. One end of R, point A,
is held at VRef while the other end is
connected to V, so there is a constant
voltage difference across a constant
resistance, and thus a constant current
through R which must also flow
through Rx to generate a theoretically
linear voltage response.
My inner math geek says that a linear equation takes the form of Y = M X
+ B, where M is the slope of the line —
(VRef - V)/R in this case (the current
through R) — and B is the Y intercept
(where the line crosses the Y axis):
VRef, in this case, and so the formula
also shows this solution to be linear.
That's the ideal theory, but what
can go wrong? There are a lot of mysteries (to me) about op-amps and I'll
leave out a bunch to concentrate on
just three things that apply to all op-amp circuits and may impact this one.
Reprinted from September
I was wondering if someone
knows how to design a circuit to
measure resistance. I would like to
jumper select the input range, run it
through a zero and span circuit,
and output a 0-10 VDC signal inversly
proportional to the selected resistance
As the resistance goes down to the
bottom of the range, the output would
increase to the maximum ( 10 volts).
As the resistance increases to the
maximum of the range, the voltage on