Converting a digital control signal to an analog control signal

[Matt]

I have a circuit operating off 120VAC Line voltage, that turns on a
motor when a photo eye is broken. Simple really. The photo eye is
connected to a relay which switches power to the motor. I want to
replace the photo eye with a digital signal. However the relay is of
analog design. This would be easier if i could replace the relay with
one that accepts a digital control signal.

So my question is what is the best way to convert the digital control
signal (probably 24 VDC) to an analog one?

could I use an analog opto-isolator? Would it work?
can you apply a digital voltage to one side of an opto isolator
and switch on an analog voltage on the other side?

 

[David Wooff]

Matt,
Whether "digital" or "analogue", a voltage source is a voltage source (even
though it may have different characteristics such as source impedance etc.).
If your digital output can supply enough current to drive the opto-isolator
and the output of the opto isolator can sink enough current to drive your
relay coil then I think you have a solution. Be aware that you may need to
protect your opto isolator output if driving a coil (using a diode across
the coil for example - but check this as I'm a little rusty). Look at the
data sheets for your components to answer the above questions.

 

[Soeren]

Hi Matt,

So my question is what is the best way to convert the digital control
signal (probably 24 VDC) to an analog one?

Err, a digital signal is an analog signal without the "grey tones".

could I use an analog opto-isolator? Would it work?
can you apply a digital voltage to one side of an opto isolator
and switch on an analog voltage on the other side?

Why do you want an "analog voltage" ? After all, it is a relay you want
to control and it is quite digital in its behaviour

 

[petrus bitbyter]

I have a circuit operating off 120VAC Line voltage, that turns on a
motor when a photo eye is broken. Simple really. The photo eye is
connected to a relay which switches power to the motor. I want to
replace the photo eye with a digital signal. However the relay is of
analog design. This would be easier if i could replace the relay with
one that accepts a digital control signal.

So my question is what is the best way to convert the digital control
signal (probably 24 VDC) to an analog one?

could I use an analog opto-isolator? Would it work?
can you apply a digital voltage to one side of an opto isolator
and switch on an analog voltage on the other side?

A relay is a very digital thing by its nature. It can be on or off. Anything
in between is not wanted and has to last as short as possible. So if you
know the required voltage and the resistance of the relaycoil, you know what
"digital" signal you need to switch the relay. If you want more precise
suggestions you'll have to tell something more first. So what is the source
of your digital signal? Does you "electronic eye" has a light source or does
it use available light?

petrus

 

[[email protected]]

CLARE makes a bunch of TTL controlled relays in a 6 pin dip. I've used
them up to 1 amp. Do a web sarch, I forget the part number (though I
have some in the basement, you get stuck, I'll go dig them out)

 

[Bob Myers]

Hi Matt,



Err, a digital signal is an analog signal without the "grey tones".

Not really.

A digital signal is one whose value is interpreted directly as a
numeric quantity, via any of a number of encoding/decoding
methods. There are most certainly signals with more than two
possible states which are considered to be "digital" (for instance,
consider the signals produced by a modem, or the encoding used
in a digital television broadcast).

Conversely, an "analog" signal is just what the name implies - it
is a signal in which some basic parameter (voltage, frequency or
current, for instance) is to be directly interpreted as "analogous"
to the information being transmitted (e.g., in an "analog audio"
signal, the voltage of the signal is directly (although NOT necessarily
linearly) proportional to the amplitude of the sound being
transmitted.

Note that these distinctions have SOLELY to do with the
encoding and decoding ("modulation" and "demodulation,"
if you prefer) of information on the electrical signal. There is
actually NOTHING in terms of the basic physical characteristics
of a signal which causes it to be "analog" or "digital." Another way
to look at this is to say that, strictly speaking, there are no such
things as analog or digital signals - there is simply electricity (or
something similar, such as EM waves), and the distinction lies in
how that signal is supposed to be interpreted.

Confusion in this area leads to all sorts of utterly unwarranted
assumptions about just what "analog" and "digital" really mean,
and what the advantages and disadvantages of each system truly
are.

Bob M.

 

[Soeren]

Hi Bob,

Err, a digital signal is an analog signal without the "grey tones".

Not really.

[Snip]

There is
actually NOTHING in terms of the basic physical characteristics
of a signal which causes it to be "analog" or "digital."

It might have escaped you, but that was exactly my point

Confusion in this area leads to all sorts of utterly unwarranted
assumptions about just what "analog" and "digital" really mean,
and what the advantages and disadvantages of each system truly
are.

I agree.

 

[John Fields]

On Fri, 07 May 2004 19:30:10 GMT, "Bob Myers"

....

There is
actually NOTHING in terms of the basic physical characteristics
of a signal which causes it to be "analog" or "digital." Another way
to look at this is to say that, strictly speaking, there are no such
things as analog or digital signals

---
I disagree.

Since any signal is an aggregation of particles, the appearance of
continuity is an illusion. The flow of current, for example, is
measured in amperes, where an ampere is defined as the movement, past
a fixed point, of a charge of one coulomb in one second. One coulomb
is further defined as a fixed quantity of electrons, (6.02E18) so it's
really all grits instead of jello. Electromagnetic radiation can also
be quantized down to photons, so it seems there is really no "analog"
at all, just larger or smaller bunches of particles moving around...
---

 

[Soeren]

Hi John,

I disagree.

Since any signal is an aggregation of particles, the appearance of
continuity is an illusion. [...] so it's
really all grits instead of jello. Electromagnetic radiation can also
be quantized down to photons, so it seems there is really no "analog"
at all, just larger or smaller bunches of particles moving around...

Georges Seurat would agree of course, but I newer liked his work anyway
<URL:http://www.artchive.com/artchive/S/seurat.html>

 

[John Fields]

Hi John,

I disagree.

Since any signal is an aggregation of particles, the appearance of
continuity is an illusion. [...] so it's
really all grits instead of jello. Electromagnetic radiation can also
be quantized down to photons, so it seems there is really no "analog"
at all, just larger or smaller bunches of particles moving around...

Georges Seurat would agree of course, but I newer liked his work anyway
<URL:http://www.artchive.com/artchive/S/seurat.html>

 

[Bob Myers]

I disagree.

OK - let's see if I can convince you otherwise...

Since any signal is an aggregation of particles, the appearance of
continuity is an illusion.

True - but then, nothing I said would require that a signal be
"continuous" or even "linear" to be considered "analog." Most
of what we currently think of as "analog signals" ARE continuous,
and I'd even go so far as to say that most are likely "linear" (in the
sense that equal changes over the permissible range of values always
correspond to equal changes in the information being transmitted).
But there is really no REQUIREMENT that this be so; we must be
careful not to confuse mere convention or tradition with formal
definition.

Consider, for example, an analog video signal describing an eight-step
"gray scale" bar pattern. Even if this signal truly were "discontinuous"
(i.e.., instantaneous transitions from one state to the next,, with the
states separated by a significant amplitude difference), this would not
prevent the signal from being considered "analog." The levels of the
signal, even though now completely discrete, still correspond directly
to the intended level of luminance in the displayed image. They are, in
other words "directly analogous" (which really is the source of the term
"analog" in the first place) to the information being transmitted.

And yet, a very similar signal can properly be considered to be "digital",
if it the case that the different levels are not to be interpreted directly
as
the "level" of the intended information, but rather as purely numeric
information. A perfect example here is the 8-VSB encoding used in the
U.S. digital TV standard. 3 bits of information per symbol are encoded
as one of eight possible levels in an amplitude-modulated system. The
states of the signal in this case cannot, though, be treated as directly
corresponding to the desired level of the input information, since there is
no way to tell within a given symbol itself where these bits go in the
data stream that's being transmitted and assembled. It IS, therefore, a
"digital" signal, despite "looking the same" (in at least one domain) as
the previous example.

Bob M.

 

[John Fields]

OK - let's see if I can convince you otherwise...


True - but then, nothing I said would require that a signal be
"continuous" or even "linear" to be considered "analog." Most
of what we currently think of as "analog signals" ARE continuous,

---
They really aren't, I think. If you buy the bit about that the
smallest part of _any_ signal being discrete is true, then you buy the
premise, which states that nothing which is composed of discrete
particles can be continuous must also be true, since the whole is the
sum of its parts and the smallest part comprising the whole is
discrete.

We watch movies and perceive them as being continuous, yet they're
presented to us one frame at a time (one pixel at a time if you're
watching TV or if you're reading this), and the pixels we see (which
escape each frame) are themselves composed of discrete numbers of
photons being allowed to pass through or being blocked by bunches of
individual dye molecules which are made up of bunches of individual
atoms which are made up of...
---

and I'd even go so far as to say that most are likely "linear" (in the
sense that equal changes over the permissible range of values always
correspond to equal changes in the information being transmitted).
But there is really no REQUIREMENT that this be so; we must be
careful not to confuse mere convention or tradition with formal
definition.

---
I agree in the sense that if a zillion photons impinging on a surface
caused an effect to occur which, if measured, would increase by a
factor of two if the measurement were made when two zillion photons
impinged on that same surface, then that relationship might be termed
linear. In other words, Y=kX.
---

Consider, for example, an analog video signal describing an eight-step
"gray scale" bar pattern. Even if this signal truly were "discontinuous"
(i.e.., instantaneous transitions from one state to the next,, with the
states separated by a significant amplitude difference), this would not
prevent the signal from being considered "analog." The levels of the
signal, even though now completely discrete, still correspond directly
to the intended level of luminance in the displayed image. They are, in
other words "directly analogous" (which really is the source of the term
"analog" in the first place) to the information being transmitted.

And yet, a very similar signal can properly be considered to be "digital",
if it the case that the different levels are not to be interpreted directly
as
the "level" of the intended information, but rather as purely numeric
information. A perfect example here is the 8-VSB encoding used in the
U.S. digital TV standard. 3 bits of information per symbol are encoded
as one of eight possible levels in an amplitude-modulated system. The
states of the signal in this case cannot, though, be treated as directly
corresponding to the desired level of the input information, since there is
no way to tell within a given symbol itself where these bits go in the
data stream that's being transmitted and assembled. It IS, therefore, a
"digital" signal, despite "looking the same" (in at least one domain) as
the previous example.

---
OK, but...

I don't think there's any way we can get around quantization (and its
attendand requirement for particulate quantities) unless you can,
somehow, show that there are infinitessimally subdividable states
which exist between the smallest infinitessimaly divisible states
which can't be further divided but which must instead be smeared.

 

[Bob Myers]

---
They really aren't, I think. If you buy the bit about that the
smallest part of _any_ signal being discrete is true, then you buy the
premise, which states that nothing which is composed of discrete
particles can be continuous must also be true, since the whole is the
sum of its parts and the smallest part comprising the whole is
discrete.

True; eventually, you run into discrete "things" at SOME level.
My point was, though, that the consideration of something as
either "discrete" or "continuous" actually has very little to do with
whether it is "analog" or "digital", despite most "analog" types of
communications generally being at least treated as "continuous,"
and most "digital" types being treated as discrete.

We watch movies and perceive them as being continuous, yet they're
presented to us one frame at a time (one pixel at a time if you're
watching TV or if you're reading this), and the pixels we see (which
escape each frame) are themselves composed of discrete numbers of
photons being allowed to pass through or being blocked by bunches of
individual dye molecules which are made up of bunches of individual
atoms which are made up of...

Sure; the distinction between "discrete" and "continuous" at pretty
much any level has to do with the limitations of human perception.
On the other hand, re the above, if you really want to get down to the
smallest levels, we're going to run into the whole wave/particle duality
thing - and I don't EVEN wanna go THERE...

---

I agree in the sense that if a zillion photons impinging on a surface
caused an effect to occur which, if measured, would increase by a
factor of two if the measurement were made when two zillion photons
impinged on that same surface, then that relationship might be termed
linear. In other words, Y=kX.

But "linear" is also irrelevant. There are a number of signalling
methods which are generally considered to be "analog" by any
reasonable, PRACTICAL definition of the word, and yet do
not involve linear encoding of the information. (Video is again
possibly the most obvious example; the transformation from
light levels to signal amplitude is REQUIRED to be non-linear
in just about all standards, for some very good reasons.)

I don't think there's any way we can get around quantization (and its
attendand requirement for particulate quantities) unless you can,
somehow, show that there are infinitessimally subdividable states
which exist between the smallest infinitessimaly divisible states
which can't be further divided but which must instead be smeared.

Sure - but again, mere "quantization" has nothing to do with
whether or not we consider a signal to be "analog" or "digital" in
any practical sense. Fundamentally, both analog and digital
methods of encoding information into ANY real-world signal run
into what is really the same limit, just viewed from two different
perspectives. That limit is the inability of the receiving device or
entity to distinguish "adjacent" meaningful states (symbols) in the
information stream, due to the corrupting effects of what must
generally be called "noise in the channel." It doesn't matter what
the source of that "noise" is - there is ALWAYS a fundamental
limit to the number of states that can be reliably distinguished, and
so on the information capacity of the channel. That's the basic
notion behind the Shannon equation for information capacity, which
(despite typically giving a result in "bits/second") is NOT talking
about either "analog" or "digital" methods specifically. (Here,
"bits" is being used in the information theory sense as the smallest
possible unit of information - the answer to a yes/no question. It
is a concept which is as applicable to "analog" systems as to "digital,"
although admittedly the linkage is not as intuitive or obvious in the
"analog" case.)

My bottom-line point is that the TERMS "analog" and "digital"
really just apply to different methods of encoding or interpreting
information. They are not the same as "linear" or "continuous," or
"discrete" or "quantized," respectively. Each of these words has
a perfectly good meaning already, and we're just getting sloppy
in our thinking when we confuse them.

Bob M.

 

[John Fields]

True; eventually, you run into discrete "things" at SOME level.
My point was, though, that the consideration of something as
either "discrete" or "continuous" actually has very little to do with
whether it is "analog" or "digital", despite most "analog" types of
communications generally being at least treated as "continuous,"
and most "digital" types being treated as discrete.



Sure; the distinction between "discrete" and "continuous" at pretty
much any level has to do with the limitations of human perception.
On the other hand, re the above, if you really want to get down to the
smallest levels, we're going to run into the whole wave/particle duality
thing - and I don't EVEN wanna go THERE...



But "linear" is also irrelevant. There are a number of signalling
methods which are generally considered to be "analog" by any
reasonable, PRACTICAL definition of the word, and yet do
not involve linear encoding of the information.

---
Yes, but that wasn't my point. My point was that linear relationships
can exist even in a fully quantized system, and in my example "k"
would be equal to one, so plotting "Y" VS "X" would yield a straight
line proceeding at 45° from 0,0 into quadrant I of the Cartesian
plane.
---

Video is again
possibly the most obvious example; the transformation from
light levels to signal amplitude is REQUIRED to be non-linear
in just about all standards, for some very good reasons.)

---
Yes, of course.
---

Sure - but again, mere "quantization" has nothing to do with
whether or not we consider a signal to be "analog" or "digital" in
any practical sense. Fundamentally, both analog and digital
methods of encoding information into ANY real-world signal run
into what is really the same limit, just viewed from two different
perspectives. That limit is the inability of the receiving device or
entity to distinguish "adjacent" meaningful states (symbols) in the
information stream, due to the corrupting effects of what must
generally be called "noise in the channel." It doesn't matter what
the source of that "noise" is - there is ALWAYS a fundamental
limit to the number of states that can be reliably distinguished, and
so on the information capacity of the channel. That's the basic
notion behind the Shannon equation for information capacity, which
(despite typically giving a result in "bits/second") is NOT talking
about either "analog" or "digital" methods specifically. (Here,
"bits" is being used in the information theory sense as the smallest
possible unit of information - the answer to a yes/no question. It
is a concept which is as applicable to "analog" systems as to "digital,"
although admittedly the linkage is not as intuitive or obvious in the
"analog" case.)

---
Using "bits per second" was a good move, IMO, because that way the
smallest possible piece of information will always be defined in terms
of the noise floor, and if the noise floor should diminish to
nothingness, then a bit would, indeed, be a single, lone, discrete,
photon!-)
---