# voltage to resistance convertor?

Discussion in 'Electronic Basics' started by Andrew Howard, Apr 14, 2005.

1. ### Andrew HowardGuest

If I have a source of variable voltage ( a sweep generator for example),
is there any way to create a source of varying resistance in proportion to
the voltage?

About the only way I can think of off the top of my head is a light/LDR
combination, but I doubt that would be very efficient or accurate.

Anyone have any better ideas? Preferably simpler ones, even if it is
slightly less accurate. Or maybe someone knows of a resistance sweep
generator...

Thanks
Andrew Howard

2. ### Larry BrasfieldGuest

If your resistance range is not too large, and the
signals you will impress across it is small enough,
that function with an op-amp controlling it.

What input and output range to you need, over
what frequency range, with what distortion?

Another approach is to use a multiplier IC, hooked
up as a divider, with voltage input and current fed
back from the output to the input. But again, the
required specs are necessary to proceed.

3. ### Andrew HowardGuest

The resistance, I will need between 0 and 2k5.

Distortion isn't really that important, I'll be using it to control
something that isn't all that precise anyway.

The input voltage can be between 0V and 9V. I can probably regulate it
to anything within that.

In case you are wondering, it is for a Guitar FX pedal I am semi-designing.

Thanks again,
Andrew Howard

4. ### Guest

Maxim and Microchip have digital potentiometers that may be useful.

Fred.

5. ### Andrew HowardGuest

Ah... but are they available in Australia?

I'll look into it though
Andrew Howard

7. ### Larry BrasfieldGuest

....
[MOSFET array cut as unlikely to be suitable.]
There will be accuracy and current capacity issues with
the low end of that range. What a designer will need
to know, (at least), is: What currents or voltages will be
applied to the resistance?
That will certainly help.
Right now, your requirement appears to be:
1. Accept an input voltage, 'Vx', ranging between 0 V and 9 V,
on one port, consisting of pins VxIn and VxRef.
2. Provide a "resistance", 'Ro', on another port, consisting of
pins RoA and RoB.
3. If current is passed thru Ro, the voltage across it should
be Vx * 2500/9, within some unknown tolerance. There is
no presently known limit on what that current might be.
4. If a voltage is applied across Ro, the current passing thru
it should be 9/(2500 * Vx), within some unknown tolerance.
There is no presently known limit on what that voltage might be.
5. The accuracy implied by items 3 and 4 applies only over the
frequency range 0.2 Hz to 20 Hz. Outside that range, any
voltage or current result is acceptable.
6. The connection, if any, between the Vx port and Ro port, is
unspecified. They might be required to be isolated from each
other for an unbounded voltage difference. Or it may be fine
for either RoA or RoB to be connected to VxRef.

I state the above to clarify what is missing. For any of the
unknown bounds and limits, smaller will be easier and if
it can be unipolar rather than having to be bipolar, the
design may be simplified. Not having to isolate the ports
will greatly simplify the problem.
What sort of circuit is going to use the synthesized
resistance? Can it accept a voltage or current instead?
There may be an easier way to solve the real problem
than synthesizing a resistance.
You're welcome.

8. ### Andrew HowardGuest

This is the circuit that needs the variable resistance. It is the basis for
a simple phaser. Apparently it works by using the fact that the emiiter is
180 degrees out of phase with the collector (or something). The varying
resistance has been labelled R1.
VCC
+ (9V)
|
|-----'
.-. .-.
47K| | | |2K2
| | | |
'-' '-' 1uF
| | +[/
1uF | '----[|------'
\] | |/ [\ | |
IN o------|]---o---| BC548 | | 1uF
/]+ | |> .-. | \]
| .----' | |<-----|]----o OUT
o | | | | |2K5 /]+
| .-. .-. | '-'R1
=== 22K| | | | | | o
GND | | | |2K2'---' |
'-' '-' ===
|-----' GND
|
===
GND

(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

I have decided that I would prefer a sine wave, if that makes any
difference to anything. Both the phaser circuit and the sine wave generator
will be using the same power supply.

I am fairly new to this, so I am unsure about what you mean by having an
isolate resistance, or the difference between unipolar and bi polar. I am
guessing that unipolar means that it doesn't go below 0V (changing DC?) and
that bipolar means otherwise (AC?). If so, wouldn't it be possible to bias
an AC waveform into DC, or am I getting confused. I have not yet figured out
what I would be using to generate the sine wave, so if anyone has any
suggestions, that would be great.
Also, I am not sure how to figure out the current needed, and I have
never dealt with this type of circuit before, so I can't really guess. It
would probably be in the low mA range though.

Thanks for the help
Andrew Howard

9. ### BanGuest

If you want to take digital potentiometer for this task, it should be
possible, but it will require a SPI or similar control signal, not a voltage
input. If you modify the circuit, you might come to a solution with a
multiplyer, but this is not a trivial case either.
You can use the LDR/LED approach, but the lowest resistance will not be 0,
but maybe 400 to 1k, so you get less range into the clean sound, where the
circuit acts merely as a follower. It can be tried out easily, there are
even combinations in a 8DIP readily available, I do not remember the part
Some manufacturers make motor pots, I saw some surplus ones go for only 6
bucks, but only 10k, not 2k5. (k is not in capital letters, BTW).

10. ### Roger JohanssonGuest

The circuit looks faulty as you have drawn it.
Look at the signal path from the collector, through a 1uF, to the wiper
of R1, through a 1uF, to the output.
That is a very low impedance path, practically a shortcircuit for AC
signals. The collector signal will be at the output in full force, no
matter how R1 is set.

Now you want to mix into that signal the opposite phase from the
emitter, by using R1, and what effect do you want to achieve by that?

I think the signal from the emitter will hardly be stronger that the
collector signal, so the result will be an attenuation of the signal,
which could be achieved a lot simpler, if that is what you want.

Maybe you have made a mistake in the drawing, so the connection between
the collector output cap to the wiper of R1 should be erased?

Then we will at least get something meaningful, a circuit which can
move between positive phase, through nothing, to negative phase, by
moving the wiper of R1.

If we replace R1 with a pair of FET transistors we can voltage control
how much positive or negative phase goes to the output. Or by using
analog gates, type 4066 series, the same effect can be achieved.

But basically I think the whole project seems badly concieved and
designed. What you really need is to start over from the beginning,
make it clear what you want to achieve, and ask for help to design a
circuit to do that.

11. ### Bob MastaGuest

There is a nifty way to do this with plain old 4016 analog
switches. You use the input voltage to control the duty
cycle of a high-frequency oscillator. The 4016 (or 4066, etc)
plus some minimum resistance is used as the variable
resistor. The effective resistance (amount of current that
flows for any voltage) is then proportional to the duty
cycle. The beauty of this is that once you build up the
PWM control circuit, all stages can be made to track
very closely, and adding another stage only involves

Don Lancaster discussed this in his CMOS Cookbook.
I have used it (many years ago) to create a phaser
effect with multiple stages to give more notches.
All you have to worry about is getting the PWM
clock above the audio range.

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

12. ### Larry BrasfieldGuest

[Brasfield wrote:]
I concur with Roger on the misconnection between
the top end of the pot and its wiper.
Sorry, but I cannot understand what you mean by that.
Are you saying you want the resistance versus control
voltage function to be a sinusoid? Or that you intend
to use sinusoids as the control voltage?
That pretty much resolves the isolation issue. The
control voltage can be assumed to share the same
You've roughly got the bipolar/unipolar distinction, except
bipolar means can have either of two signs and unipolar
means can have only one sign or be zero. A bipolar signal
could have DC content or not, and it is meaningful to speak
of the AC content of a unipolar signal. The AC/DC concept
and the unipolar/bipolar concept are not the same.
There are many ways to do that. Digitally synthesized
sine generators are pretty cheap these days.
Yes. But I am not inclined to help go down the synthesized
resistance route. (I concur with Roger on that as well.)

It would be easier to build a voltage controlled "phaser"
than to synthesize a voltage controlled resistance to put
into that circuit you drew.

For example, by differencing the output of two multipliers,
one fed with (Vmax - Vcontrol) and the other fed with
(Vcontrol), (where Vmax is the fullscale multiplier input),
you would get the effect you are after. You can get a
good multiplier in an 8 pin package.

13. ### Larry BrasfieldGuest

(Sheepishly), the same function can be accomplished with
a single 4-quadrant multiplier. One input is the audio to be
subjected to "phaser" manipulation, the other input is biased
to be +/- fullscale at the extremes of the control input range,
and the audio output is the multiplier output. With suitable
biasing, it could all be done with a single supply and an 8-pin
multiplier such as the AD835. See

You're welcome.

14. ### Andrew HowardGuest

Thanks for the information.

I have decided in the end that it is getting too complex. I'll still
probably tinker around with it, and try out your suggestion of the
multiplier. I'll probably try and find complete circuits of DIY guitar FX
pedals, at least until I gain a bit more knowledge on some of this stuff.

I have learnt some things though, and learning about electronics is one of
the main reasons I try to make guitar effects pedals.

Thanks again,
Andrew Howard

15. ### Roger JohanssonGuest

You can easily find many schematics of phasers on the web.

Try googling for
guitar phaser schematic
or
guitar phaser circuit diagram

A few useful links for you:
http://www.harmony-central.com/Effects/Articles/Phase_Shifting/
http://www.4qdtec.com/phasr.html
http://www.geocities.com/j4_student/mxrdodphase.gif
http://www.geocities.com/j4_student/schematics.htm
http://www.geofex.com/schemata.htm
http://www.geofex.com/FX_images/p180plus.gif
http://www.geofex.com/newatgeo.htm
http://www.diystompboxes.com/pedals/index2.html
http://www.mif.pg.gda.pl/homepages/tom/schematics.htm
http://www.mif.pg.gda.pl/homepages/tom/files/dod595.jpg