555 timer triggering

[chuck]

Thanks for the replys.
I am using the monostable configuration that is shown in the datasheet
(unsure of manufacture but I think its a universal configuration), it
uses two external caps between pin 5 and 6 and a resistor between pin
7 and 8. A falling edge is supposed to trigger a high output from the
555 timer but doesn't when the input to the 555 is taken across the
pull up resistor of the compartor.

 

[chuck]

On May 23, 11:19 am, chuck <chuckd87...@yahoo.com> wrote:

Thanks for the replys.
I am using the monostable configuration that is shown in the datasheet
(unsure of manufacture but I think its a universal configuration), it
uses two external caps between pin 5 and 6 and a resistor between pin
7 and 8. A falling edge is supposed to trigger a high output from the
555 timer but doesn't when the input to the 555 is taken across the
pull up resistor of the compartor.

Actually, the 555 isn't triggered until the falling edge reaches 1/3
Vcc.

 

[John Fields]

On Wed, 26 May 2010 16:11:02 -0700 (PDT), chuck
<chuckd87106@gmail.com> wrote:

On May 23, 11:19 am, chuck <chuckd87...@yahoo.com> wrote:
Thanks for the replys.
I am using the monostable configuration that is shown in the datasheet
(unsure of manufacture but I think its a universal configuration), it
uses two external caps between pin 5 and 6

---
That's wrong.

It uses one cap from pin 5 to ground and another from pin 6 to ground,
but the one on pin 5 you can pretty much get rid of.
---

and a resistor between pin
7 and 8. A falling edge is supposed to trigger a high output from the
555 timer but doesn't when the input to the 555 is taken across the
pull up resistor of the compartor.

---
It's not supposed to be taken across the pullup, it's supposed to be
taken from the end of the resistor connected to the output of the
comparator and cap-coupled to the 555 if the low-going output from the
comparator lasts longer than the 555 output pulse.
---

Actually, the 555 isn't triggered until the falling edge reaches 1/3
Vcc. ^
on pin 2

---
That's right.

JF