About 555 mode of operation

[kostas]

Hello

What thing determines the mode of operation of LM555? For example if a
555 is
triggered externally it is in monostable mode regardless if there is a
resistor
between pin 7 and 6?

Also if a 555 has connected pin 2 and 6 regardless of resistors again
it's in
astable mode, correct???

Thanks for your time

p.s. sorry for the crosspost in sci.electronics.basics but i need an
answer fast before i give my project

 

[Tim Wescott]

kostas wrote:

Hello

What thing determines the mode of operation of LM555? For example if a
555 is
triggered externally it is in monostable mode regardless if there is a
resistor
between pin 7 and 6?

Also if a 555 has connected pin 2 and 6 regardless of resistors again
it's in
astable mode, correct???

Thanks for your time

p.s. sorry for the crosspost in sci.electronics.basics but i need an
answer fast before i give my project

Yes. The 555 doesn't care where the trigger comes from, so connecting
it up puts it in astable mode.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Anthony Fremont]

"kostas" <asdas@.assfic.fdf> wrote

If it's not problem for you please have a look at the following link.

http://casemods.pointofnoreturn.org/pwm/circuit2.html

The guy says that the first 555 is astable and the second monostable
but
something isn't right with the connection of the second one.
Finally
is the second astable again. If this is true then the circuit is not
frequency stable when you adjust the duty cycle

The first is in true astable mode so that it triggers itself repeatedly.
It generates a constant stream of trigger pulses that feed into the
second 555. It is in one-shot or monostable mode (meaning that it isn't
triggering itself), but the time timing interval is shorter than the
period between trigger pulses feeding into it, so that it seems to
output a PWM signal whose duty cycle is varied by the pot.

Opinion: Clever enough, but I'd use a PIC (or micro of your choice)
because it's simpler, cheaper and generally will work better offering a
multitude of future options like temp sensors, automatic control, LCD
display etc.

 

[kostas]

Anthony Fremont wrote:

The first is in true astable mode so that it triggers itself repeatedly.
It generates a constant stream of trigger pulses that feed into the
second 555. It is in one-shot or monostable mode (meaning that it isn't
triggering itself), but the time timing interval is shorter than the
period between trigger pulses feeding into it, so that it seems to
output a PWM signal whose duty cycle is varied by the pot.

Opinion: Clever enough, but I'd use a PIC (or micro of your choice)
because it's simpler, cheaper and generally will work better offering a
multitude of future options like temp sensors, automatic control, LCD
display etc.

So finally the 555 if it's not triggered by itself (connection between
pin 2 and 6) is in monostable mode.


yes microcontrollers are better choice and give more freedom but i don't
know how to program one yet. I make my first steps with AVR

 

[Ken Smith]

In article <cs1ia5$n5v$1@usenet.otenet.gr>, kostas <asdas@.assfic.fdf> wrote:
[...]

If it's not problem for you please have a look at the following link.

http://casemods.pointofnoreturn.org/pwm/circuit2.html

The guy says that the first 555 is astable and the second monostable but
something isn't right with the connection of the second one. Finally
is the second astable again. If this is true then the circuit is not
frequency stable when you adjust the duty cycle

Follow it through for your self. A 555 always does this:

If TRG goes 1/3Vcc, the Q goes high.

If the THR goes above 2/3Vcc Q goes low.

Discharge is on when Q is low.

Notice what the RC circuits do and it should be clear.


--
--
kensmith@rahul.net forging knowledge

 

[Terry Pinnell]

"Anthony Fremont" <spam@anywhere.com> wrote:

"kostas" <asdas@.assfic.fdf> wrote in message
news:cs1mi3$c7v$1@usenet.otenet.gr...

yes microcontrollers are better choice and give more freedom but i
don't
know how to program one yet. I make my first steps with AVR

Keep at it, they are more than worth the effort to learn them. Learn to
program in assembler first, then use higher level languages if you wish.

I'm interested in why you recommend that? With no relevant Assembler
or PIC training, I've been taking the Basic route, and finding that
tough enough. (In fact I've temporarily shelved it - again!)

Although exaggerating to make my point, isn't your approach like
suggesting someone should learn how to dismantle and rebuild an auto
engine and its electronics before taking driving lessons?

--
Terry Pinnell
Hobbyist, West Sussex, UK