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- Posted on
- Roger Dewhurst
September 21, 2009, 11:07 am
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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.
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
I am sure that there are other ways of achieving this.