Sweep Generator possible from 555 Astable circuit?

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.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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$ Notepad):

` 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 Chris

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?

Michael

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.

--
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.

--
1. What frequency?

2. What are the two frequencies, what's the sweep rate, and does it
have to be a linear sweep?

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 that.

--
John Popelish

between

having

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 '-' | GND 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 Chris

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.

--
Regards,
   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.

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.

--
Roger J.

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

Chris

--
Regardless of what you might have thought, It's actually not a bad way
to do it.  Have you ever worked out what the frequency variation would
be?

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.

Michael

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.

--
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
for? 


      30kHz

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

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 dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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.

--
Regards,
   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.

--- Nice one but, unfortunately, the OP's asking for from 10Hz to 30kHz, and the one on the data sheet goes from 10Hz to 10kHz with a control voltage of zero to 30VDC!

He only needs a period of 2 seconds for his tuning voltage, so that shouldn't be too hard for the VCO to follow, and I'm thinking along the lines of a two opamp triangle wave generator to generate the sweep and a 7555 for the VCO.

Feeding the control voltage input with the sweep voltage and wiring the chip up as an astable with a 50% duty cycle output would be kind of interesting in that, leaving out the output buffer and the MOSFET, the 7555 looks essentially like this:

Vcc>---------+ | [R1] +--------[Rt]-------+ | | | TH>----------------+--|+\ +------+ | | | | >---|R Q|--+--->OUT Vc>----------+-----|--|-/ | | | | | | [R2] | | | | | | | +-----|--|+\ | _| __ | | | >---|S Q| TR>----------------+--|-/ +------+ | | [R3] [Ct] | | GND>---------+-----+

The input divider resistors, R1,R2,R3, are all the same value, so with no external voltage on Vc the circuit will oscillate between 1/3 and

2/3 Vcc with an output period of about 1.4RtCt.

If we were now to connect Vc to a voltage source (the sweep voltage generator) which varied between close to 0V and close to 5V, we would be able to vary the output frequency over a very wide range, the highest frequency occurring with Vc close to 0V and the lowest frequency occurring with Vc close to +5V.

That's scheme 1, but I don't have any numbers yet.

Scheme 2 leaves the the input divider alone, but varies the timing resistor resistance 3000:1 with the sweep voltage input. That's not as far-fetched as it sounds, since with the saming timing cap, if we could get 30kHz with 1000 ohms we ought to be able to get 10Hz with

3 megohms. Since

T = 1.4RC,

For 30kHz and 1000 ohms we'd need

T 3.3E-5 C = ------ = -------- ~ 23.8nF 1.4R 1400

and just to check 10Hz:

T 0.1s R = ------ = -------------- = 3 megohms 1.4C 1.4 * 23.8nF

Implementation might be something as simple as this: +5 | [R1] | +----+ E | Vc---[R]----B Q1 [R2] C | +----+ | [R3] | +--->TO 7555 PINS 2 AND 6 | [Ct] | GND

Where Q1 is an N-Channel FET or a PNP bipolar, R1 and R3 make up the

1000 ohm high-freq resistance and R1, R2, and R3 make up the 3M low frequency resistance with the transistor cut off. Kind of iffy, though. Maybe an LDR and an LED or an opto? Dunno yet.

Scheme three is a high side voltage controlled current source with a

3000:1 current range feeding the timing cap. 1µA low freq current out and 3mA high freq current out? Or 10µA -> 30mA? Or 5 and 15? Sounds pretty good and less klunky than scheme 1, and _lots_ less klunky than scheme 2...

Time will tell. :-)

-- John Fields