555 Calculator

[Jim Thompson]

I suppose the more intelligent way to use that calculator is to enter
duty cycles that are arbitarily close to and above but not exactly equal
50%. Of course, you wouldn't know to do this if all you've ever done is
use calculators. Entering 50.3% , 1KHz, and C=0.001u, values obtained
with some fairly rough insight into 555 operation, returns RA=8.64K and
RB=716K- not std values according to my other formula derived
mysteriously from additional applications insight, almost as good as,
but a lot less fun than, the manual method.

Those of us who need a tight 50%... run oscillator at 2X frequency,
then div2.

...Jim Thompson

 

[Zak]

The standard 555 astable circuit will only give you duty cycles above
50%.
But you CAN get a 50% duty cycle with a simple mod to the circuit.
Put a diode in PARALLEL with the resistor that connects pin 6 and pin
7. Connect the cathode to pin 6, anode to pin 7.

What about a resistor from output to the cap? You may want to use a CMOS
555 for this.

Saves a resistor in any case.


Thomas

 

[Jasen Betts]

The standard 555 astable circuit will only give you duty cycles above
50%.
But you CAN get a 50% duty cycle with a simple mod to the circuit.
Put a diode in PARALLEL with the resistor that connects pin 6 and pin
7. Connect the cathode to pin 6, anode to pin 7.
With that diode in place, you can make the low duty cycle as low as you
want.
Using R1 and R2 equal should get you 50% duty cycle.
As a side note, you can make a fixed-frequency astable circuit with
continuously variable duty cycle by using a pot; connect the ends of
the pot to pins 6 and 8, and the wiper to pin 7, while keeping the
diode I mentioned previously.

put a 100 ohm resitor in series with the hot side of the pot. othewise with it
wound all the way down it'll be trying to discharge the powersupply.

 

[budgie]

---
Well, let's see...

For way less than a dollar you get an astable or a monostable
multivibrator, a couple of comparators with a nice internal pretty
much isothermal voltage divider that you can mess with if you want
to, a totem pole output that you can get a couple hundred milliamps
out of, an open collector NPN, and a RESET input.

And... you can get them anywhere, everybody makes them, and they're
not likely to stop making them anytime soon.

And... you can get it in CMOS.

Crap you say? LOL!

Crap repeatability.

 

[JR North]

Thanks for knocking yourself out on that
I used Jason Betts suggestion; subbed R1 with a 100 ohm trimmer and used
a 22NF. Works good; apx 46K on the trimmer gives 1ms width and pertneer
50 DC. Pulls some current through R1, but seems to be tolerable. Only
beef, the low ramp is stepped about .25V to 0V through half the off
width....how come?
JR

 

[JR North]

Poifect!
Thanks
JR

Looking for a calc that will solve for R1, R2, C in astable 50% DC with
entering desired freq (1KHZ)and DC(50%). All the calcs I find require
entering the component values to solve for freq and DC. Pain.
Thanks
JR

impossible with the normal 2-resistor circuit, to get 50% duty cycle the
easiest way is the 1 resistor circuit,

----+--- vcc
|
+-[R1]--------|--------+
| | |
| +--------+ |
| | | |
| | . . . .|. . . . |
| | . VCC(8) . |
| | . . |
| +--RES(4) OUT(3)--+--> out1 (totem pole)
| . 555 .
+-------TH(6) DIS(7)-----> out2 (open collector)
| . .
+-------TR(2) CV(5)--
C1 | . .
===== . GND(1) .
| . . . .|. . . .
| |
+-------------+
|
---+-- gnd


the frequecy is approximately 0.7/R1*C1

 

[Phil Allison]

"budgie"

There is a "standard" circuit for achieving 50% duty cycle with a 555, and
it
does include a diode.

** The "standard circuit" has no diode.

See " 50 % Duty Cycle Oscillator" - figure 14.

http://www.national.com/ds/LM/LM555.pdf

Needs a 14nF cap for 1000Hz square wave output.

Used it once before I decided 555's were crap.

** Usual load of mindless crapology.

Aka " budgie droppings ".




......... Phil

 

[Fred Bloggs]

I used Jason Betts suggestion; subbed R1 with a 100 ohm trimmer and used
a 22NF. Works good; apx 46K on the trimmer gives 1ms width and pertneer
50 DC. Pulls some current through R1, but seems to be tolerable. Only
beef, the low ramp is stepped about .25V to 0V through half the off
width....how come?

What is your Vcc and what is your load? The half-way point coincides
with Vcc/2 across the timing capacitor where the THRESH comparator is
just entering complete cutoff and the TRIG comparator is on the edge of
linear for low Vcc. If Vcc is small enough, this may reduce the pulldown
drive ever so slightly to cause a higher Vce,sat on the output.

 

[Fred Bloggs]

** The "standard circuit" has no diode.

See " 50 % Duty Cycle Oscillator" - figure 14.

http://www.national.com/ds/LM/LM555.pdf

Needs a 14nF cap for 1000Hz square wave output.

Not a standard circuit at all- you end up with odd values and bad
sensitivity to component values- difficult to bound against component
tolerances and possible to make everything hang. I don't see all that
many 14nf caps either.

 

[Phil Allison]

"Fred Bloggs" = a senile, BRAIN DEAD ****

Not a standard circuit at all-

** It is from the FUCKING NATIONAL SEMI data sheet -

dating from the 1970s !!!!

YOU DUMB AS DOG SHIT ASSHOLE !!!!!
-------------------------------------------------------

you end up with odd values

** As you do with any RC oscillator for a particular frequency

YOU DUMB AS DOG SHIT ASSHOLE !!!!!
----------------------------------------------------

and bad sensitivity to component values

** The alternatives offered are worse.

- difficult to bound against component tolerances and possible to make
everything hang

** I would sure like to see you HANG by you tiny balls

YOU DUMB AS DOG SHIT ASSHOLE !!!!!
---------------------------------------------------------

I don't see all that many 14nf caps either.

** ROTFLMAO !!!!!!!!!!!!!!

What a fucking asinine, shithead MORON !!!!!

Put a 22nF and a 39nF in series and what do you get ??

Duh ?????????????????????


F U C K W I T !!!




........ Phil

 

[John Fields]

Crap repeatability.

 

[John Fields]

Poifect!
Thanks
JR

 

[Fred Bloggs]

"Fred Bloggs" = a senile, BRAIN DEAD ****




** It is from the FUCKING NATIONAL SEMI data sheet -

dating from the 1970s !!!!

Yeah, so what, the point being?

YOU DUMB AS DOG SHIT ASSHOLE !!!!!
-------------------------------------------------------

YOU may consider "DOG SHIT" as an entity worthy of personal intellectual
comparison, but most people choose a higher standard.

** As you do with any RC oscillator for a particular frequency

YOU DUMB AS DOG SHIT ASSHOLE !!!!!
----------------------------------------------------






** The alternatives offered are worse.

Absolutely not the case, as anyone who successfully completed junior
high school could tell you.

** I would sure like to see you HANG by you tiny balls

YOU DUMB AS DOG SHIT ASSHOLE !!!!!
---------------------------------------------------------






** ROTFLMAO !!!!!!!!!!!!!!

What a fucking asinine, shithead MORON !!!!!

Put a 22nF and a 39nF in series and what do you get ??

Oooh yeah- that's convenient.

 

[JR North]

Vcc is 12V. I'm using it to drive a stepper driver board; L297/L298 for
a constant speed application. The RPM is critical, thus the 1KHZ req.
JR
..

 

[Fred Bloggs]

Vcc is plenty high enough then- makes me suspect your loading is going
through some kind of transition at the halfway point. If the step is
still there with no load, then it has to be the 555, but the effect is
coming from circuits not shown on the schematic.

 

[Phil Allison]

"Fred Bloggs" = a senile, BRAIN DEAD ****

Yeah, so what, the point being?

** Bloggs would NOT get the point of a 40 foot flag pole shoved up his fat
arse !!





** How strange ????

No reply at all from the DUMB AS DOG SHIT Bloog's **** brain.

Stuck for words on a crucial point ??

Absolutely not the case,

** What a FUCKING LIAR !!!!

Bloogs = another scum of the planet, charlatan & a damn LIAR .

Peeeeeeuuukeee !!

Oooh yeah - that's convenient.

** ROTFL !!

Bloggs is nothing but a braying, asinine, pig ignorant ass.

Heeee ...... haaaw , heeee ..... haaaw ..............




......... Phil

 

[Jim Thompson]

Yeah, so what, the point being?


YOU may consider "DOG SHIT" as an entity worthy of personal intellectual
comparison, but most people choose a higher standard.


Absolutely not the case, as anyone who successfully completed junior
high school could tell you.


Oooh yeah- that's convenient.

Every time I've completely forgotten Phil, somebody has to go and
respond to his crap. Why is that?

...Jim Thompson

 

[Michael A. Terrell]

Every time I've completely forgotten Phil, somebody has to go and
respond to his crap. Why is that?

Jim Thompson

Simple: They need a good laugh, and he is the best they can do on
Usenet. ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

** Bloggs would NOT get the point of a 40 foot flag pole shoved up his fat
arse !!

You are the only one stupid enough to let someone do that to you. On
the other hand, you would probably enjoy it.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Fred Bloggs]

Jim Thompson wrote:

Forget that idiot- he is clearly wrong about that hokus pokus 50% duty
circuit National publishes. I see no practical use for that touchy thing:
View in a fixed-width font such as Courier.

..
..
..
..
..
.. Vcc
.. |
.. RA TH=069*RA*C
.. |
.. DIS-RB--+ RA*RB RB-2RA
.. | TL= ------- * C *Ln( ------ )
.. | RA + RB 2RB-RA
.. ===
.. |
.. |
.. --- substituting RA*C=1.44*TH
..
..
..
.. TL 1.44*RB RB-2RA
.. -- = ------- *Ln( ------ )
.. TH RA + RB 2RB-RA
..
..
.. RA TL 1.44 1-2r
.. substituting r= -- => -- = -------*Ln( ---- )
.. RB TH 1 + r 2-r
..
..
.. TL ~
.. D=50% => -- = 1 , makes r=2.3 , RA and RB always in same ratio.
.. TH
..
..
.. TL
.. (--)
.. TH - r 4.32
.. Then S = ----- *( 1 + ------------)= -3.5
.. r 1 + r (1-2r)*(2-r)
..
..
..
..
..
.. For the case:
..
..
.. Vcc
.. |
.. RA
.. |
.. DIS---+
.. |
.. RB
.. |
.. ===
.. |
.. ---
..
.. TL RB 1
.. where RA<<RB or r->0, -- = ------ = ----
.. TH RA + RB 1+r
..
..
.. TL
.. (--)
.. TH - r
.. => S = ----- ->0 also
.. r 1 + r TL
.. (--)
.. TH
.. Example case of C=0.001u RA=10K RB=720K makes S =-0.014
.. r
..
.. Looks to be 3.5/0.014=250x better with initial accuracy.
..
..
.. The "standard ckt" is patently inferior.