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When a 555 is wired as a 1 Hz square wave oscillator and pin 4 goes high
does pin 3 go high at the same instant or does it go high half a cycle
later?

R

Re: 555


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Ah, the good old 555 - still knocking around.



Re: 555



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Ah yes, three 5K resistors in series to the comparator inputs.
Now where did they get the 555 name from?

Amazing how many varied uses the device can be put into.

For the OP, at 1 Hz you won't even need an oscilloscope to answer your
question.
Sounds a bit exam'ish to me?

Re: 555


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Not really. A question from one who is 100% self educated, if that is
the right expression, in this field.  Exams?  Geology, OK. Zoology, OK,
Botany, OK, Electronics, no.  Sometimes there is someone here who wants
to pick the brains of others, not just to score points.

R

Re: 555



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Pin 4 is the 'reset'.

This may help you:

http://www.jaycar.com.au/images_uploaded/LM555.PDF

jerd



Re: 555


@lust.ihug.co.nz:

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a
your
OK,
wants

OK, I must admit it is an interesting question.
Pin 4 of course is the reset input, and raising it to VCC will allow the
capacitor to begin charging.
The way I see it, pin 3 will be low during the reset state and go high
immediately when reset is released as the cap will be below 1/3 VCC.
Pin 3 will then return low once the cap gets to 2/3 VCC.
The cap will then be discharged until it returns to 1/3 VCC, pin 3 goes
high and so on.

The time to initially get to 2/3 VCC will be longer than the time to
charge from 1/3 to 2/3 VCC in normal astable mode,
Since you are not charging with a constant current source, this will not
be twice the time of normal oscillation, but a little bit quicker as the
cap will charge to 1/3 VCC quicker than it will from 1/3 to 2/3.

Grab the Fairchild datasheet, it has a nice clear table showing the pin
3 behaviour at various voltage levels about the chip.
http://www.fairchildsemi.com/ds/LM/LM555.pdf

Ray


Re: 555


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This is what I was thinking of.

Two 555s with pin 3 of the first connecting to pin 4 of the second.

The first has a lower frequency that the second.  The result should be a
short repeating series of pulses.  Each series is separated by a
distinct pause.

I am sure that there are other ways of achieving this.

R

Re: 555



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Use a 556



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