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Sweep Generator possible from 555 Astable circuit?

Discussion in 'Electronic Basics' started by royalmp2001, Mar 4, 2005.

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  1. royalmp2001

    royalmp2001 Guest

    Can anyone tell me if it is possible to make a square wave sweep
    generator from a standard 555 astable configuration. What mods are
    needed? If not what would be the easiest, simplest, cheapest way?
  2. I've been in the electronics field for a good while
    and managed to never come across a "square
    wave sweep generator" and I remain confounded
    as to what it might do. Could you elaborate?

    What gets swept? What waveforms result?
    I'm sure a 555 could be made to do something
    that might be given such a name, but there are
    too many possibilities without more details.
  3. But are you talking about using the 555 as a square wave generator
    that is swept by an external ramp, or about using the 555 to generate
    that ramp?

  4. He could probably use a double 555. Use one of them to create the sweep
    frequency and use the triangle shaped voltage on the capacitor as ramp.

    Use that ramp to control the fequency of the other 555 which is set to
    give square wave out.

    That will give us a swept frequency square wave output generator.

    You might need to buffer the voltage from the capacitor with an external
    transistor before you send it to the other 555.
  5. Chris

    Chris Guest

    First, you might want to look at a 555 tutorial web page:

    Looking at the circuit construction of the 555, you can see that
    there's three internal 5K resistors which set the upper and lower
    comparator threshold points, and also that the 2/3Vcc point is
    accessible at pin 5.

    That will lead you to something like this (view in fixed font or M$

    ` VCC
    ` +
    ` | VCC VCC
    ` .-. + +
    ` | | | |
    ` | | .---o------o---.
    ` '-' | 8 4 |
    ` | | |
    ` .---o------o7 |
    ` | | | |
    ` | - | |
    ` | ^ | | f(out)
    ` | | | 555 3o---------o
    ` V .-. .--o6 | 50% Duty Cycle
    ` - | | | | |
    ` | | | | | |
    ` | '-' | | |
    ` | | | | |
    ` '---o---o--o2 |
    ` | | |
    ` --- | 1 5 |
    ` --- '---o------o---'
    ` | | |
    ` === === |
    ` GND GND |
    ` |
    ` |
    ` V(c) |
    ` o---------------------'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    The basic circuit is made to give you an approximately 50% duty cycle
    square wave with the two diodes (use 1N4148s, choose Ra = Rb). The
    "trick" is V(c), which is where you apply a control voltage. Let's
    assume you have a 12VDC supply. You can apply an external control
    voltage at V(c) to adjust the voltage at pin 5. It shouldn't go above
    3/4Vcc, and shouldn't go below about 3.3V. That should easily give you
    a good sweep range with a single 555, especially if you've got a power
    supply well above the minimum 5V. For a 12VDC supply, your V(c) can be
    ramped from 9V (lowest f) to 3.3V (highest f) to give you your sweep.

    This is pretty basic and limited, but it's simple and cheap, and might
    do the job for you.

    Good luck
  6. John Fields

    John Fields Guest

    Any function generator (or function generator chip) with a square wave
    out and a VCF input can be used to generate a square wave which can be
    swept in frequency.
    The output frequency
    A rectangular waveform with a 50% duty cycle.
    If a swept square wave is the goal, There are only four possibilities
    using only the 555 or its CMOS brethren, three of which are varying
    the resistance of the timing resistor, the capacitance of the timing
    capacitor, or both, with the modulating voltage. In addition, the
    timer would have to be configured like this:

    +-O|T- OUT|--+--->OUT
    | | | |
    +-O|TH | |
    | +-------+ |

    and, if the 555 is used, note must be taken of the fact that its
    output _isn't_ rail to rail.

    The fourth, and simplest, possibility (albeit not the one which
    provide the greatest deviation) would be to use the CONTROL VOLTAGE
    input to sweep the output frequency.
  7. The 555 switches output states as the timing inputs (trigger and
    threshold) pass through 1/3 and 2/3 of its supply voltage. If you
    connect a capacitor to its timing inputs and want that capacitor to
    charge up and down at various rated (and change direction of charge
    each time the 555 output changes states,) you need a bi-directional
    (direction switchable) source of current for the capacitor that also
    varies the value of current over time to produce the sweep. A CA3080
    variable transconductance amplifier could perform both these tasks.
    Its differential input would monitor the output state of the 555 to
    switch current directions each time the trigger or threshold voltage
    passes through their respective boundary voltage, and its current set
    input would be used ot vary the magnitude if its output current. As a
    bonus, the timing capacitor voltage is a linear swept triangle wave.
    The only remaining problem is to come up with a circuit addition to
    sweep the current control pin on the 3080. A second 555 could do
  8. royalmp2001

    royalmp2001 Guest

    Sorry, Larry.

    I need to build a 555 astable circuit that is switch selectable between
    giving a
    1. Fixed square wave frequency (easy)
    2. Sweeping square wave that sweeps up and down continuously
    between two frequencies.

    All signals generated internally with no external control signal having
    to be applied.
  9. John Fields

    John Fields Guest

  10. Chris

    Chris Guest

    If you've got two 555s, you can get something like what you're talking
    about like this (view in fixed font or M$ Notepad):

    ` VCC VCC
    ` + +
    ` | VCC VCC | VCC VCC
    ` .-. + + .-. + +
    ` | | | | | | | |
    ` | | .--o----o--. | | .--o----o--.
    ` '-' | 8 4 | '-' | 8 4 |
    ` | | | | | |
    ` .----o-------o | .---o------o7 |
    ` | | | | | | | |
    ` | - | | | - | |
    ` | ^ D | | | ^ D | | f(out)
    ` | | | 555 3o N.C. | D | | 555 3o--o
    ` V D .-. .--o6 | V .-. .--o6 |
    ` - | | | | | - | | | | |
    ` | | | | | | | | | | | |
    ` | '-' | | | | '-' | | |
    ` | | | | | | | | | |
    ` o----o----o--o2 | VCC '---o---o--o2 |
    ` | | | | + | | |
    ` | --- | 1 5 | | --- | 1 5 |
    ` | --- '--o----o--' | --- '--o----o--'
    ` | | | N.C. |/ | | |
    ` | === === .---| Q === === |
    ` | GND GND | |> GND GND |
    ` | | | _/ |
    ` | | o--------o/ o--------'
    | | |
    '-----------------------' .-. SW1
    | |
    | |1K
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    This circuit will work better at higher Vcc. The left 555 determines
    the sweep frequency, and the right one controls the oscillating
    frequency as before. Transistor Q1 acts like a voltage follower to
    buffer the cap voltage on the first 555, and applies it to the control
    pin of the second when the switch is closed.

    This is a kludgy circuit, has limited range, does not sweep frequency
    in a linear manner, has limited and fixed sweep range, and generally
    isn't the best way to do this at all, but there it is. It might be
    suitable for a buzzer/siren-type circuit, depending on the values of R
    and C chosen.

    Good luck
  11. A better way would be to buy a cheapo signal generator chip, like the
    (obsolete) intersil ICL8038 (which you can actually still get from The problem with that one is that you need to
    drive the VCO input at between Vcc+0.2 (that's right, ABOVE Vcc) and 2/3
    * Vcc - 2 to get the full range. Since it really wants at least 10V
    input (and works better at +-15V) you are then stuck with building a 30V
    power supply and driving the VCO input using a wide voltage rail to rail
    opamp (or some other clever circuit), and also dropping the voltage on
    the actual chip a bit.

    However, assuming you've done that, it's then easy to build your sweep
    generator. The datasheet has plans for it.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  12. People seem to all be suggesting using pin 5, but that's not likely
    to allow much frequency variation (and I've always thought using
    that pin for frequency control as messy).

    The proper way to use the 555 as a VCO is to feed current into
    the pin 2/6/7 junction. So making that resistor to the positive
    supply a constant current generator of some type that is voltage controlled
    allows for a much wider sweep range.

  13. Chris

    Chris Guest

    Chris wrote:

    And inadvertently edited out that this is the concept from Roger
    Johansson's post below. Sorry -- credit where due.

  14. John Fields

    John Fields Guest

  15. royalmp2001

    royalmp2001 Guest

    John Fields,
    The answers to your further questions are
    1. The fixed frequency is 30KHz
    2. The two frequency ideally would be 10Hz to 30KHZ. Sweep rate not
    critical, maybe one cycle up and down every 2 seconds. And no it does
    not have to be a linear sweep.
  16. John Fields

    John Fields Guest

    OK. Three more questions: Do you want the sweep to (A) go from 10Hz to
    30kHz and then back to 10Hz and then repeat continuously or (B) do you
    want the sweep to start at 10Hz, go to 30kHz, then start over again at
    10Hz abruptly, and what kind of frequency accuracy are you looking

  17. royalmp2001

    royalmp2001 Guest

    Thanks for the questions, John Fields.

    I need the frequency to cycle from 10Hz upto 30KHz then down to 10 then
    back up to 30K, etc
    Accuracy is not critical at all, I'll take whatever is feasible with
    this kind of circuit, even if it can't go all the way down to 10Hz.
    Thanks, John
  18. Rich Grise

    Rich Grise Guest

    Can you still get these?

    I did one of them once, although I didn't do the whole 1000X sweep range -
    the thing that stands out in my mind was tweaking R11 and R12 to try to
    get that little point off the top of the "sine wave".

    You could probably extend the sweep range with some clever current-source

    Good Luck!
  19. Bob Masta

    Bob Masta Guest

    If you are not limited to the 555, the standard way to do this
    in old hardware music synths was a voltage controlled
    oscillator (VCO) consisting of a current source that charged
    a capacitor, a threshold detector, that triggered a short one-shot,
    and a transistor to dump the capacitor. The control voltage
    set the charging current, hence the frequency. These VCOs can
    be made incredibly linear over wide ranges. (You need to add a
    small resistor in series with the cap, below the threshold detector
    junction, to compensate for the one-shot time.)

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  20. The national LF155 datasheet has a schematic for a 3 decade VCO made out
    of an LF356 and an LM319. I don't know how linear it is, or how fast
    it'll track the input frequency.

    Datasheets are a wonderful resource. It would be nice if there was a
    catalog of snippets of circuits in datasheets that could be searched for
    building blocks. Some industrious web designer should take that on.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
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