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# how can I multiply resistance?

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I need to take an incoming resistance
(up to c 200Ω), and then multiply it's varying
signal while keeping the "secondary" isolated.
I know, two resistors in series = R1 + R2, and
two in parallel is the (R1 + R2) / 2. I'm just
wondering though, if I have a 150Ω input, is
it possible to convert it to, say, 1500Ω ???
It's a coil varying a timer, FYI...

I cannot visualize what you are trying to do. If the 150Ω varying resistance controls a voltage or a current in an active circuit, you can do something with an opamp which will make an equivalent voltage or current while being controlled by a higher value resistor, like 1500Ω. Is that what you are talking about?

Linearity is an issue, but a FET can be treated as a variable resistor. You need to "sense" the resistance you are wanting to boost and use a programable voltage to drive the FET.

Not sure how to apply this to your case though.

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I want to multiply the change of resistance within
a coil when the coil is affected by magnetism. The coil
is about 160Ω, and I get a variance of up to 50Ω, which
is what I want to multiply. The variance is set into an
astable/monostable timer that drives a servo. I'm seeking
more motion in the servo from the little bit of motion by
the magnet. If I simply parallel a resistor, I will increase
the overall resistance, which will only change the servo's
position, and cut the range of motion in half. With the
FET thing, how would I incorporate it into the resistor &
coil, since they are between pins 6 & 7 of the 555? I
tried the VCO thing with the timer but couldn't get the
results desired, by manipulating the CV input (pin 5)
with varying voltages which should have moved the
servo (it made @30 degrees of motion far CCW). The
resistance of the coil set into the timer (monostable)
causes an ideal result, but I want the stylus being
driven by the servo to move a greater range (and I
have tried all sorts of resisitor values all over the timer).

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When a magnet is moving near a coil, it generates a voltage. This voltage will confuse an ohm meter. Are you certain that the resistance is changing? Perhaps you are just measuring the voltage that the coil is generating.

If the voltage is actually what you want to amplify, this gets a lot easier.

I originally started with the voltage, but couldn't
get the circuit to work, so I tested the resistance,
which does vary, and with this variant in series with
another resistor (that charges the cap for it's single
shot output), it actually does move the servo. I can
simply tap on the table and the servo jitters a few
times to each side until the suspended magnet quits
moving (it's for a seismograph). I'm just seeking a
means to make it more sensitive without enlarging the
coil (1000' of 32AWG) anymore due to size restriction.
Another idea (actually three ideas) I had, was if I
open up the servo and parallel a resistor across the
entire potentiometer, or two of them between the
wiper and the ends, or one in series with the wiper,
so the motor will have to move more to reach it's
destination as directed by the input pulse width.
What do you think?

Please post a photo of this magic coil that changes resistance. A magnetic field does not change the resistance of copper.

The coil is almost 1000' of 32AWG "magnet"
wire, wrapped neatly 22 layers around a 3/4" PVC
pipe about 1.25" long. A neodymium magnet rests
slightly within the coil (maybe I will borrow a camera).
Believe me, it does change the resistance! If it
doesn't then my meters are fooling me, but either
way, a basic astable/monostable circuit drives the
servo's signal. Coil is in series w/ R4 in between
pins 6/7 & vcc of the monostable timer (I'm using a
556 but can't remember offhand the pins on it that
correspond to the 555 chip which is the pins I'm
noting). Even if it doesn't change the resistance,
it still moves the servo accordingly with the motion
of the magnet !! I'm just trying to "sensitize" the
two rectifier diodes from vcc to pin5 of monostable.
The resistance only changes when the magnet
is in motion! I've tried various resistors and a pot
to "find" the most ideal value (with the 160Ω coil in
series!) as a replacement for R4.
<<<note...my prior posting noted the resistor/coil
between 6 & 7, (the astable part), I actually
mean between 6/7 and VCC! (monostable part) >>>

https://www.electro-tech-online.com/attachments/servodrv-gif.5798/

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I see what it's doing. Yes your meters are fooling you, but wait, this is good news.

Move the magnet with the meter in 2V or 200mV position. If you get 100mV or more, this is plenty to work with.

You can apply the voltage generated by the coil, through a capacitor, to the CV pin (5) of U2. I suggest a capacitor value of around 100µF. If this isn't enough variation, it needs to be amplified, but this will be easy.

I only get (with an incredible amount
of motion), a maximum of about 30mv.
But again, I tried the Vin to pin 5, and
the servo stayed far CCW no matter
how much I limited or gave it up to 5v.
That's why I gave up on the VCO...
Also, with a VCO, the board will need
an amp (more draw/space), and if I
could just "sensitize" the servo, I'm
already in a good spot. Before we go
to the VCO, I would think if I add a
small resistor to the servo pot's wiper,
it would make the motor move farther
(exponentially) for the same pulsewidth.
I just don't know if the servo's control
circuitry measures both sides of the
pot all the time, or just the side it is
"approaching" when in motion. Do you
know? Otherwise, I was thinking of
placing an appropriately valued resisitor
across the entire pot (to "lower" the
pot's resistance slightly) then wherever
the wiper is (since I'm not using the
entire range of the servo) it will have
to move a little farther to reach the
right value <or raise the pot's R?>
(I'm guessing that the servo's pot is
likely biasing + & -, and the servo's
board is likely only reading the +v
from the pot's wiper).

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If you change R4 so the servo sits in the middle, then small signals on the CV side should move it a little. Whether you amplify this voltage, or you sensitize the servo, this is much more effective than putting your 160 ohm coil in series with 15k.

If you change R4 so the servo sits in the middle, then small signals on the CV side should move it a little. Whether you amplify this voltage, or you sensitize the servo, this is much more effective than putting your 160 ohm coil in series with 15k.

I did change R4 (I think it's c 4.7k) so the servo
is in the center, before I added my coil. I even have a
pot to adjust it's center. Basically, what I'm looking
to do is build a "Servo Stretcher", as the internet coins
a relatively inexpensive device on the market. I just
want the whole "sensor" part of the seismograph on
one board, without the stretcher(s?) in the midst of
the servo's wiring. How else, does anybody know, I
could exponentially expand the pw (I still have
to stay in the same range and keep center). So, say
1.5ms is center, and I send it 1.6ms, I want the servo
to react as if it's getting 1.7ms pw. Conversely, if it
gets 1.4ms, react as if it's getting 1.3ms pw. My only
thought aside of integrating a stretcher(s?) onto my
board, is by "fooling" the servo's pot. Does anybody
know how I could (in any other way), exponentially
expand the pw that I'm sending to the servo while
maintaining the servo's center point?

I know, you wrote of changing the value of R4 so
the CV is more sensitive, but my other problem is if I
do the VCO, CV will also be getting negative voltage
from the coil. I would have to not only amplify this
signal, but build a circuit (which I do have) that will
take the coil's Vout and center it at a few volts while
either lowering or raising it's Vout depending on the
coil's polarity and generation of voltage. This circuit
requires a + and a - voltage supply, which I don't
have (so build another circuit). Now, with my reduced
space, I've probably tripled? the circuits space, and
since it's battery operated, I'm quadrupling? the draw,
compared to what I have so far. There's just got to
be an easier way to make the servo swing farther, on
the circuit I have that actually operates ideally, aside
of the sensitivity, when it comes to servo motion.

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Fooling the servo's pot should work, but I would expect stability problems unless the servo power supply is rock solid.

If you put a capacitor in series with the coil and CV, it would automatically zero the offset. Since the offset of the coil is zero, they should be matched automatically. Any amplifier would be AC coupled (albeit at a very low cutoff frequency).

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