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About 555 mode of operation

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

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  1. kostas

    kostas Guest

    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
     
  2. Tim Wescott

    Tim Wescott Guest

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

    kostas Guest


    Thanks for the reply

    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 Smith

    Ken Smith Guest

    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]

    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.
     
  5. 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. kostas

    kostas Guest

    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. 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. 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. 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. 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.
     
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