Controlling Resistance Digitally - what kinds of components

[Patrick Keenan]

Hello All,
I've recently posted on another topic, and thanks for the varied replies,
which gave me the information I needed.

I've another question regarding digital control of resistance. I have an
application where I'd like to add digitally controlled resistance to
pre-existing devices, where I can't modify anything except the resistance
control itself. I can't alter the circuit in any other way. For an
example, think of a "vintage" wah-wah pedal - in this case, pots do wear out
so it's acceptable to replace that part - but that part only. One can't
redo the circuit to match the pot, as that could alter the tone...

I'm aware that there are digital potentiometers, but I have been finding
that in a number of cases these just are not available in the values I might
need to match the existing potentiometer, and/or may not be able to handle
the current or voltage that might be present (such as in a guitar
amplifier).

So my question would be, what other kinds of resistive devices could be used
to provide a digitally controllable resistance?

Any hints on this are greatly appreciated.

Thanks!
Patrick Keenan

 

[Jasen Betts]

On 2008-09-18, Patrick Keenan <test@dev.null> wrote:

Hello All,
I've recently posted on another topic, and thanks for the varied replies,
which gave me the information I needed.

I've another question regarding digital control of resistance. I have an
application where I'd like to add digitally controlled resistance to
pre-existing devices, where I can't modify anything except the resistance
control itself. I can't alter the circuit in any other way. For an
example, think of a "vintage" wah-wah pedal - in this case, pots do wear out
so it's acceptable to replace that part - but that part only. One can't
redo the circuit to match the pot, as that could alter the tone...

I'm aware that there are digital potentiometers, but I have been finding
that in a number of cases these just are not available in the values I might
need to match the existing potentiometer, and/or may not be able to handle
the current or voltage that might be present (such as in a guitar
amplifier).

So my question would be, what other kinds of resistive devices could be used
to provide a digitally controllable resistance?

a motorised dual gang pot in a servo circuit ?

an ORP7 (or similar LDR) illuminated by a LED. (pwm at some ultrasonic rate)

an ordinary pot driven by an off-the shelf R.C. servo.

conbine two digital pots to give the range you want.






Bye.
Jasen

 

[Patrick Keenan]

"Bob Masta" <N0Spam@daqarta.com> wrote in message
news:48d3a031.2039183@news.sysmatrix.net...

On 19 Sep 2008 02:32:49 GMT, Jasen Betts
jasen@xnet.co.nz> wrote:

On 2008-09-18, Patrick Keenan <test@dev.null> wrote:
Hello All,
I've recently posted on another topic, and thanks for the varied
replies,
which gave me the information I needed.

I've another question regarding digital control of resistance. I have
an
application where I'd like to add digitally controlled resistance to
pre-existing devices, where I can't modify anything except the
resistance
control itself. I can't alter the circuit in any other way. For an
example, think of a "vintage" wah-wah pedal - in this case, pots do wear
out
so it's acceptable to replace that part - but that part only. One can't
redo the circuit to match the pot, as that could alter the tone...

I'm aware that there are digital potentiometers, but I have been finding
that in a number of cases these just are not available in the values I
might
need to match the existing potentiometer, and/or may not be able to
handle
the current or voltage that might be present (such as in a guitar
amplifier).

So my question would be, what other kinds of resistive devices could be
used
to provide a digitally controllable resistance?

a motorised dual gang pot in a servo circuit ?

an ORP7 (or similar LDR) illuminated by a LED. (pwm at some ultrasonic
rate)

an ordinary pot driven by an off-the shelf R.C. servo.

conbine two digital pots to give the range you want.

All good ideas. One unusual addition is to use a
CMOS switch to apply PWM to a fixed resistor,
again at some ultrasonic rate. At low duty cycles
there is less current through the resistor, so its
value appears to be larger. This works especially
well for circuits like wah pedals that are
essentially low-pass anyway.

For new designs, there is the advantage that you
can have as many variable Rs as you want... and
_they_all_track_perfectly with a single PWM
signal. Works great for phaser/flanger circuits,
where each stage adds another sliding notch to the
output spectrum.

Credit for this idea goes to Don Lancaster's "CMOS
Cookbook"... strongly recommended!

Thanks!

As it happens, I have an old copy of the CMOS Cookbook downstairs, which
some years ago I read until it started to fall apart. Yes, it's an
excellent book.

-pk

Bob Masta

DAQARTA v4.00
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!

 

[Ted Pavlic]

On Sep 18, 12:03 pm, "Patrick Keenan" <t...@dev.null> wrote:

So my question would be, what other kinds of resistive devices could be used
to provide a digitally controllable resistance?

Something that hasn't been mentioned (and may not be practical in your
application) is to use a FET in Ohmic mode. If you keep the drain-
source voltage low (say, less than the FET threshold voltage), then
the analog voltage at the gate changes the channel resistance. You can
use a DAC (plus divider, even) to drive the gate voltage.

There is a nice example on page 140 of Horowitz and Hill's _The Art of
Electronics_ (2nd edition). They give two variable-gain circuit
examples in Figure 3.33 (in the second, they show how to get around
the threshold voltage restriction). In their examples, they use a
trick to get a LINEAR relationship between gate-voltage and channel
resistance. By putting some resistance R between drain and gate and
that same resistance between gate and control signal, you get rid of
the quadratic term.

Just thought I'd offer that for completeness.

Best! --
Ted

 

[John Fields]

On Thu, 18 Sep 2008 12:03:31 -0400, "Patrick Keenan" <test@dev.null>
wrote:

Hello All,
I've recently posted on another topic, and thanks for the varied replies,
which gave me the information I needed.

I've another question regarding digital control of resistance. I have an
application where I'd like to add digitally controlled resistance to
pre-existing devices, where I can't modify anything except the resistance
control itself. I can't alter the circuit in any other way. For an
example, think of a "vintage" wah-wah pedal - in this case, pots do wear out
so it's acceptable to replace that part - but that part only. One can't
redo the circuit to match the pot, as that could alter the tone...

I'm aware that there are digital potentiometers, but I have been finding
that in a number of cases these just are not available in the values I might
need to match the existing potentiometer, and/or may not be able to handle
the current or voltage that might be present (such as in a guitar
amplifier).

So my question would be, what other kinds of resistive devices could be used
to provide a digitally controllable resistance?

Any hints on this are greatly appreciated.

---
View in Courier:

A7>-----------+
|
IN>------+--[ASW]---------+--->OUT
| | |
GND>---+-|----+ |
| | |
A6>----|-+----+ |
| | | |
| +--[ASW]---[R]---+
| | | |
+-|----+ |
| | |
A5>----|-+----+ |
| | | |
| +--[ASW]--[2R]---+
| | | |
+-|----+ |
| | |
A4>----|-+----+ |
| | | |
| +--[ASW]--[4R]---+
| | | |
+-|----+ |
| | |
A3>----|-+----+ |
| | | |
| +--[ASW]--[8R]---+
| | | |
+-|----+ |
| | |
A2>----|-+----+ |
| | | |
| +--[ASW]--[16R]--+
| | | |
+-|----+ |
| | |
A1>----|-+----+ |
| | | |
| +--[ASW]--[32R]--+
| | | |
+-|----+ |
| | |
A0>----|-+----+ |
| | | |
| +--[ASW]--[64R]--+
| |
+------+

Where 'ASW' is an analog switch, or a solid state relay, or even a
mechanical relay if you need the low contact resistance.

JF