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Converting a digital control signal to an analog control signal

Discussion in 'Electronic Design' started by Matt, May 5, 2004.

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  1. Matt

    Matt Guest

    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?
  2. David Wooff

    David Wooff Guest

    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.
  3. Soeren

    Soeren Guest

    Hi Matt,

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

    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 :)
  4. 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?

  5. Guest

    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)
  6. Bob Myers

    Bob Myers Guest

    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

    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

    Bob M.
  7. Soeren

    Soeren Guest

    Hi Bob,

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

    I agree.
  8. John Fields

    John Fields Guest

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

    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...
  9. Soeren

    Soeren Guest

    Hi John,

    Georges Seurat would agree of course, but I newer liked his work anyway ;)
  10. John Fields

    John Fields Guest

  11. Bob Myers

    Bob Myers Guest

    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,
    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

    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
    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.
  12. John Fields

    John Fields Guest

    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

    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...
    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.
    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.
  13. Bob Myers

    Bob Myers Guest

    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. (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.)
    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.
  14. John Fields

    John Fields Guest

    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
    Yes, of course.
    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,
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