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From: chuck on 22 May 2010 21:04
Im trying to tigger a 555 timer with a comparator. Im using the 555
timer in a monostable configuration with the supply voltage at 7
volts. When the input to the 555 comes via the high output of the
comparator at 7 volts, the output of the 555 timer is high instead of
low. But when the input to the 555 comes via the power supply at 7
volts, the output of the 555 timer is low as expected until the input
is changed from 7 volts to ground. Is there two little available
output current from a 317 comparator? It seems I had it working
properly at one time, but maybe I was using a CMOS 555, Im note
sure.............would it matter?
From: David Eather on 22 May 2010 21:40
On 23/05/2010 11:04 AM, chuck wrote:
> Im trying to tigger a 555 timer with a comparator. Im using the 555
> timer in a monostable configuration with the supply voltage at 7
> volts. When the input to the 555 comes via the high output of the
> comparator at 7 volts, the output of the 555 timer is high instead of
> low. But when the input to the 555 comes via the power supply at 7
> volts, the output of the 555 timer is low as expected until the input
> is changed from 7 volts to ground. Is there two little available
> output current from a 317 comparator? It seems I had it working
> properly at one time, but maybe I was using a CMOS 555, Im note
> sure.............would it matter?

Trace out a circuit diagram from what you actually have running (not
what you think you have running) - perhaps that will help you find the
fault. If not post the diagram somewhere and post the link here and
someone will help.
From: Jasen Betts on 23 May 2010 05:59
On 2010-05-23, chuck <chuckd87106(a)gmail.com> wrote:
> Im trying to tigger a 555 timer with a comparator. Im using the 555
> timer in a monostable configuration with the supply voltage at 7
> volts.

there's several different ways to get a 555 to be a monostable
which one are you using?

> When the input to the 555 comes via the high output of the
> comparator at 7 volts, the output of the 555 timer is high instead of
> low. But when the input to the 555 comes via the power supply at 7
> volts, the output of the 555 timer is low as expected until the input
> is changed from 7 volts to ground. Is there two little available
> output current from a 317 comparator? It seems I had it working
> properly at one time, but maybe I was using a CMOS 555, Im note
> sure.............would it matter?

Many comparitors don't go high on their own, try adding a resistor (eg 1K)
from the comparitor output to the +7V

--- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: chuck on 23 May 2010 13:27
On May 23, 3:59 am, Jasen Betts <ja...(a)xnet.co.nz> wrote:
> On 2010-05-23, chuck <chuckd87...(a)gmail.com> wrote:
>
> > Im trying to tigger a 555 timer with a comparator. Im using the 555
> > timer in a monostable configuration with the supply voltage at 7
> > volts.
>
> there's several different ways to get a 555 to be a monostable
> which one are you using?
>
> > When the input to the 555 comes via the high output of the
> > comparator at 7 volts, the output of the 555 timer is high instead of
> > low. But when the input to the 555 comes via the power supply at 7
> > volts, the output of the 555 timer is low as expected until the input
> > is changed from 7 volts to ground. Is there two little available
> > output current from a 317 comparator? It seems I had it working
> > properly at one time, but maybe I was using a CMOS 555, Im note
> > sure.............would it matter?
>
> Many comparitors don't go high on their own, try adding a resistor (eg 1K)
> from the comparitor output to the +7V
>
> --- news://freenews.netfront.net/ - complaints: n...(a)netfront.net ---

Thanks for your input, but I did actually take the output across the
pull up resistor attached to the output of the comparator and th 7
volt rail supply.
From: Jamie on 23 May 2010 16:09
chuck wrote:
> Im trying to tigger a 555 timer with a comparator. Im using the 555
> timer in a monostable configuration with the supply voltage at 7
> volts. When the input to the 555 comes via the high output of the
> comparator at 7 volts, the output of the 555 timer is high instead of
> low. But when the input to the 555 comes via the power supply at 7
> volts, the output of the 555 timer is low as expected until the input
> is changed from 7 volts to ground. Is there two little available
> output current from a 317 comparator? It seems I had it working
> properly at one time, but maybe I was using a CMOS 555, Im note
> sure.............would it matter?
First off, a 317 is a voltage regulator, not a comparator. But maybe
that was a wording mistake?

It would be nice to know what kind of comparator you are actually
using ?

Most comparators pull to the common with an open collector NPN and
have no pull up R. You may want to employ a pull R on that line..

The Trigger input requires a pull to common of 1/3 of VCC or less to
activate it, it uses a PNP logic on the input.. Having your comparator
in the low state is simply turning on the trigger and setting the output
to a High state.

Having the (Vcc) rail connected to it, is actually not triggering
anything, so it remains where it is and when the threshold is reached,
it'll turn off the output, in case the output isn't yet off from some
prior trigger or when ever the threshold reaches the 2/3 Vcc level.

Only a pull to common and 1/3 or less (VCC) on the trigger will
actually make the trigger do it's thing! (+) volts has no effect other
than pulling it up to remove stray signals.

The Threshold on the other hand, works the other way around.. It does
nothing when it gets pulled to common and operates when it sees at least
2/3 or more of the Vcc supply.. That will put the timer in the off
state, as long as the trigger isn't being held in it's operating region.


And remember this, if you hold the trigger in its operating region
longer than the one shot calculations on the threshold, the output will
remain on until you get that trigger out of the on region, at which
point, the internal Flip Flop will then flip to the next step to turn
off the output and discharge the cap. That is, if the Threshold is still
at 2/3 or more of Vcc at that time.

If you have digested what I have explain to you, I think this should
open up some shadows on how the 555 works and things it can do.

In short, you need to reverse of your logic on the comparator and use
a cap to isolate the trigger if the trigger may stay on longer than the
one shot calculations. That is up to you..


Hope that help.

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