Sweep Generator possible from 555 Astable circuit?

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?
Thanks

 

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.

 

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

 

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.

 

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

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

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 www.tech-chat.de

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

 

---
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 | |
| +-------+ |
+----[Rt]-----+
|
[Ct]
|
GND

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.

 

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.

http://www.intersil.com/data/fn/fn475.pdf

 

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.

 

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 www.tech-chat.de

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
http://www.futurlec.com.) 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.

 

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

 

Chris wrote:

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

Chris

 

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

 

Can you still get these?

http://www.chipcatalog.com/Intersil/ICL8038.htm

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
design.

Good Luck!
Rich

 

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
www.daqarta.com

 

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.