# About 555 mode of operation

Discussion in 'Electronic Design' started by kostas, Jan 11, 2005.

1. ### kostasGuest

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???

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

2. ### Tim WescottGuest

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

3. ### kostasGuest

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

4. ### Ken SmithGuest

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[/QUOTE]

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.

5. ### Anthony FremontGuest

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.

6. ### kostasGuest

Anthony Fremont wrote:

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

7. ### Anthony FremontGuest

Yes, if I understand you. Monostable just means that 1 trigger pulse ->
1 timing cycle. Astable is when the trigger is somehow (via pin 6 in
your circuit) reactivated by the completion of the timing cycle.
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.

8. ### John PopelishGuest

The threshold input detects when the voltage is greater than 2/3 of
the supply voltage by setting the output to a low. The trigger input
detects when the voltage is below 1/3 of the supply voltage and resets
the output to a high. The discharge pin turns on a switch to the
negative rail. when the output is low. If both the trigger in
threshold pins watch the same voltage and the discharge pin is
involved in changing that voltage, the circuit is probably an astable,
since there are two levels involved in making decisions, and a means
to alter the direction of change in that signal. If only the
threshold or trigger are involved in monitoring a timing voltage, the
circuit is probably a monostable.
There are lots of possibilities.

9. ### Terry PinnellGuest

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?

10. ### Anthony FremontGuest

IMO, the problem with starting on a high level language is that it
insulates you from how the hardware actually works. For example,
PULSEIN doesn't give me clue me on what a CCP module is or how it
operates. Assembler gives me the advantage of fully appreciating the
time scale of the real world (uS in my own micro development world ;-)
I think it's more like learning to crawl, then learning to walk and
finally run. ;-)

I don't have anything against high level languages, I just think that
assembler should be learned first. I strongly believe that only then
can you really take full advantage of a high level language on your
chosen micro.