Re: Driving 555 IC with a pulse stream

From: John Fields (jfields_at_austininstruments.com)
Date: 11/27/04 

Date: Fri, 26 Nov 2004 19:31:38 -0600

On Sat, 27 Nov 2004 00:50:10 GMT, "Jag Man"
<Jag_Man653R-E-MOVE@hotmail.com> wrote:

>Thanks again, Bill. Just out of curiosity, though, why does it work
>when I simply connect pin 4 to the same voltage seen by pin 2? That
>is, connected this way the output of the 555 can indeed be set shorter
>than the pulse width at 2/4. Curious.
               
When the driving signal pulls RESET and TRIGGER both low at the same
time, RESET will dominate and the output pf the 555 will go low
unconditionally. Then when the driving signal starts going high, it
will release the reset when it climbs through about 0.7V (1.2V max).
When the reset is released the driving signal will still be below
2/3Vcc, so it will trigger the chip, forcing the output high and
starting the charging of the timing cap. Then, when the driving
signal goes higher than 2/3Vcc, the trigger will be released and the
555 will time out according to the values of the timing resistor and
capacitor. What's interesting about triggering the 555 this way (for
your application) is that there's no need to cap couple to the trigger
input since you want the thing to start and finish timing out when the
RESET/TRIGGER signal goes high. What you _do_ have to be careful
about, though, is that your input signal doesn't go low while the 555
is timing out. But, for your application, I don't think that can
happen.
      

-- 
John Fields

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