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>>> QUESTIONS
Voltage Problem?
We would like to plug our credit
card terminal into one of our Vodavi
Starplus STSe telephone system's SLT
ports so that we can use the system's
"off hook preference" feature to find
another unused line if the usual line is
already in use. However, the credit card
terminal needs more power than the
22V provided by the KSU's single line
port which drops to 8V when in use.
Sometimes the terminal says "no line"
and other times it does connect but
cannot communicate because something inside the terminal is causing a
clicking noise as if it were rapidly connecting and disconnecting — although
it does send the DTMF dialing signals in
spite of the clicking. (Our phone
company provides 50V which drops to
12V, which works fine for the terminal.
We have tried our fax machine on the
same KSU port we're trying to connect
the terminal to and it works very well,
so we know the port is programmed
correctly.) I have tried simply adding a
9V battery in series and it solved the
original problem 100% — no clicking,
no "no line" message, and the KSU
found an unused line just fine.
However, the phone company no
longer was able to recognize the DTMF
dialing which is not surprising, but it
proved that the problem is indeed
caused by too low voltage. (We could,
of course, simply get an extra phone
line, but people in Nuts & Volts land try
to patronize the phone company as a
last resort!) Can anyone suggest a
circuit that would increase the power
going from the KSU to the terminal
without disturbing the signals?
#9111 Leroy Sensenig
Penn Yan, NY
Aquatic Sounds Detector
I intend to build an instrument
similar to the one described in the June
‘ 11 Nuts & Volts article, Build a Bat
78 September 2011
Detector by Jonathan Berber. The
instrument I hope to build will be used
in an aquatic environment — both fresh
and marine waters — to monitor sounds
produced by fish and other aquatic life.
I would like to get more information or
at least get pointed in a direction where
I could find help with the design and
development of this project.
1.) Does anyone know of others
doing this type of aquatic acoustic
observation and the type of instruments being used in their program?
2.) Can anyone recommend
software to be used on a laptop for this
type of monitoring?
3.) What type of hydrophones
should be used in fresh water and in
ocean water?
I hope someone can provide help
or leads to get me started.
#9112 Richard W. Kotsch
Indian, AK
Increase Resolution
How do I increase the resolution of
a variable resistor (pot)? I have to move
the knob so slightly that I can't fine-tune
or properly adjust the resistance in the
circuit. Any type of solution would be
appreciated.
#9113
Michael Cimkowski
East Rockaway, NY
DC to AC Power
I’m looking for a small inverter
circuit that can supply 200 mA, 12
VDC input, and 9 VAC output. A small
and compact circuit would be ideal.
#9114 Tom Farkas
Shoreline, WA
>>> ANSWERS
[#7113 - July 2011]
Resistance Amplifier
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 range.
Consider the non-inverting voltage
amplifier of Figure 1. Normally, we
construct it with V connected to
ground, but we don't have to. To
compute the voltage at A for any
output, observe that the voltage A-V is
across R while the voltage Out-V is
across X+R, and the currents are the
same through both resistors, so:
;;;
; ;;;;; ;;;
Skipping all of the tedious algebra
while solving for Out — and realizing
that it is an op-amp's job to keep its two
inputs equal by changing its output —
thus A = VRef. We get:
So, if you want the output to be
10V when X is zero, VRef must be 10V.
Skipping more algebra while solving for
the ratio of X to R when Out is zero,
we get:
;;;; ;;;; ; ; ;; ;; ;;;;;
; ; ; ; ; ;;;; ; ;;;;;
So, with V = 12V and VRef = 10V,
the multiplier N is 5, i.e., the output is
zero when X is five times R. Alternately,
a V of 11V gets a multiplier, N, of 10.
Though some op-amps are rail-to-rail,
they still have difficulty getting all the
way there. It's best to use a bi-polar
