Triggring a 555 timer in monostable mode with a compartor ou

[chuck]

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?

 

[David Eather]

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.

 

[Jasen Betts]

On 2010-05-23, chuck <chuckd87106@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@netfront.net ---

 

[chuck]

On May 23, 3:59 am, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-05-23, chuck <chuckd87...@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...@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.

 

[Jamie]

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.

 

[chuck]

On May 23, 2:09 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

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.

Thanks alot for the advice

Yes I was mistaken, the comparator is a 319.
I do understand the operation of the 555 as you explained.
What I cant understand is why the output of the 555 is on when VCC is
applied to the input of the timer. The input to the 555 comes from the
output of the 319 which is connected to a 1K resistor connected to
VCC.

 

[Jamie]

chuck wrote:

On May 23, 2:09 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

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.


Thanks alot for the advice

Yes I was mistaken, the comparator is a 319.
I do understand the operation of the 555 as you explained.
What I cant understand is why the output of the 555 is on when VCC is
applied to the input of the timer. The input to the 555 comes from the
output of the 319 which is connected to a 1K resistor connected to
VCC.
Hmm.

Well, that seems to be a little different than what you said before?

Any way, if you're using a simulator, start your DC voltage at 0, do
this in the simulation set up. Applying the VCC to the trigger when it
has been establish that the output is off, should not turn on the output
after that fact.

You could be experiencing a problem of the trigger getting an initial
sink pulse on power up.

To fix your problem, you could use a 100ohm R from the VCC to the RESET
pin and a .1uf from the RESET pin to common.. This will force a short
reset of the timer on initial power of Vcc. It will force it in the off
state.
You most likely have an issue with some capacitance that is causing a
short SINK pulse on the trigger during initiation. You need to make sure
the RESET input stays lower during the initiation state.

 

[chuck]

On May 24, 4:43 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

chuck wrote:
On May 23, 2:09 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

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.

Thanks alot for the advice

Yes I was mistaken, the comparator is a 319.
I do understand the operation of the 555 as you explained.
What I cant understand is why the output of the 555 is on when VCC is
applied to the input of the timer. The input to the 555 comes from the
output of the 319 which is connected to a 1K resistor connected to
VCC.

Hmm.
   Well, that seems to be a little different than what you said before?

Any way, if you're using a simulator, start your DC voltage at 0, do
this in the simulation set up. Applying the VCC to the trigger when it
has been establish that the output is off, should not turn on the output
after that fact.

   You could be experiencing a problem of the trigger getting an initial
sink pulse on power up.

  To fix your problem, you could use a 100ohm R from the VCC to the RESET
pin and a .1uf from the RESET pin to common..  This will force a short
reset of the timer on initial power of Vcc. It will force it in the off
state.
   You most likely have an issue with some capacitance that is causing a
short SINK pulse on the trigger during initiation. You need to make sure
the RESET input stays lower during the initiation state.

Thanks again for the advice,

I tried that, and unfortunately it didnt fix the problem. Still, when
the trigger input to the 555 comes from the output of the comparator
(or Vcc) the output of the 555 is high, but when I switch the trigger
input to Vcc directly, the output goes low (and stays that way untill
I decrease the input to 1/3 Vcc)

 

[John Fields]

On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck
<chuckd87106@gmail.com> wrote:

On May 24, 4:43 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

   You could be experiencing a problem of the trigger getting an initial
sink pulse on power up.

  To fix your problem, you could use a 100ohm R from the VCC to the RESET
pin and a .1uf from the RESET pin to common..  This will force a short
reset of the timer on initial power of Vcc. It will force it in the off
state.
   You most likely have an issue with some capacitance that is causing a
short SINK pulse on the trigger during initiation. You need to make sure
the RESET input stays lower during the initiation state.

Thanks again for the advice,

I tried that, and unfortunately it didnt fix the problem. Still, when
the trigger input to the 555 comes from the output of the comparator
(or Vcc) the output of the 555 is high, but when I switch the trigger
input to Vcc directly, the output goes low (and stays that way untill
I decrease the input to 1/3 Vcc)

---
Sound to me like what's happening is that there's a low-going
transient from the comparator triggering the 555 on power up.

Since the 555's RESET voltage is 1.2V max, at any supply voltage, then
what you need to do is hold the reset pin lower than 1.2V for as long
as it takes the transient to die away.

Assuming a 5V supply, 100 ohms and 0.1ľF gives a time constant of
2.7ľs from 0V to 1.2V, (the 555's RESET release voltage) which is way
too fast if what you're trying to do is keep a turn-on transient from
the comparator from triggering the 555.

Here's a circuit for a 555 one-shot with a passive power-on reset:
(View in Courier)

..Vcc>-+---------+-----+-------+--------+----+
.. |R1 |R2 |R3 8| |R4 |
.. [10k] [10k][910k] +---+---+ [100k] |
..__ | C1 | Rt| 2|_ Vcc _|4 | |C3
..IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF]
.. | 6| | |C4 |
.. +---|TH 555| [100nF] |
.. | 7|_ |3 | |
.. +--O|D OUT|O---|----|--->OUT
.. +| | GND | | |
.. [1ľF] +---+---+ | |
.. Ct|C2 1| U1 | |
..GND>-----------------+-------+--------+----+

What we have to do is figure out the values of R4 and C4 which will
keep U1-4 below 1.2V for the time required to let the transient on
U1-3 die away.

Just for grins, lets say 10ms will do it and were running a 12V
supply.

Then we can write:

Vcc
t = RC ln ----------- = KRC
Vcc - Vth

Where T is the time required, in seconds,
R is the resistance required, in ohms,
C is the capacitance required, in farads,
Vcc is the supply voltage in Volts,
Vth is the threshold voltage, in Volts, and
ln is log to the base e.

if we solve for:

Vcc 12V
K = ln ----------- = ln ------------ = 0.105
Vcc - Vth 12V - 1.2v

then we can simplify to:


t = KRC


and solve for either R or C, knowing the other.

Let's say that we arbitrarily choose 1 megohm for R; then we can
rearrange and solve for C:

T 1e-2s
C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads.
KR 1.05e-1 * 1e6R

So now you have:

..Vcc>-+---------+-----+-------+--------+----+
.. |R1 |R2 |R3 8| |R4 |
.. [10k] [10k][910k] +---+---+ [1M] |
..__ | C1 | Rt| 2|_ Vcc _|4 | |C3
..IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF]
.. | 6| | |C4 |
.. +---|TH 555| [10nF] |
.. | 7|_ |3 | |
.. +--O|D OUT|O---|----|--->OUT
.. +| | GND | | |
.. [1ľF] +---+---+ | |
.. Ct|C2 1| U1 | |
..GND>-----------------+-------+--------+----+

If you need more time you can make either R4 or C4 bigger, and C3 is a
_must_ if you're using a bipolar 555 and should be connected directly
across pins 1 and 8. If you're using a CMOS 555 then it isn't needed.


JF

 

[John Fields]

On Fri, 28 May 2010 07:07:11 -0700 (PDT), chuck
<chuckd87106@gmail.com> wrote:

I appreciate your input,

I will try your suggestions and post the results. For now, I simply
buffered the input to the 555's trigger from the comparator, and it is
now working as it is supposed to. If it is a low-going transient on
the output of the comparator which is causing the problem, wouldn't it
die out on its own given enough time?

---
Yes, but if it stays low while the 555 is powering up it'll cause an
output pulse to occur.

 

[chuck]

On May 27, 6:52 am, John Fields <jfie...@austininstruments.com> wrote:

On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck



chuckd87...@gmail.com> wrote:
On May 24, 4:43 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
   You could be experiencing a problem of the trigger getting an initial
sink pulse on power up.

  To fix your problem, you could use a 100ohm R from the VCC to the RESET
pin and a .1uf from the RESET pin to common..  This will force a short
reset of the timer on initial power of Vcc. It will force it in the off
state.
   You most likely have an issue with some capacitance that is causing a
short SINK pulse on the trigger during initiation. You need to make sure
the RESET input stays lower during the initiation state.

Thanks again for the advice,

I tried that, and unfortunately it didnt fix the problem. Still, when
the trigger input to the 555 comes from the output of the comparator
(or Vcc) the output of the 555 is high, but when I switch the trigger
input to Vcc directly, the output goes low (and stays that way untill
I decrease the input to 1/3 Vcc)

---
Sound to me like what's happening is that there's a low-going
transient from the comparator triggering the 555 on power up.

Since the 555's RESET voltage is 1.2V max, at any supply voltage, then
what you need to do is hold the reset pin lower than 1.2V for as long
as it takes the transient to die away.

Assuming a 5V supply, 100 ohms and 0.1ľF gives a time constant of
2.7ľs from 0V to 1.2V, (the 555's RESET release voltage) which is way
too fast if what you're trying to do is keep a turn-on transient from
the comparator from triggering the 555.

Here's a circuit for a 555 one-shot with a passive power-on reset:
(View in Courier)

.Vcc>-+---------+-----+-------+--------+----+
.     |R1       |R2   |R3    8|        |R4  |
.   [10k]     [10k][910k] +---+---+ [100k]  |
.__   |   C1    |   Rt|  2|_ Vcc _|4   |    |C3
.IN>--+-[100nF]-+-----|--O|T     R|O---+ [100nF]
.                     |  6|       |    |C4  |
.                     +---|TH  555| [100nF] |
.                     |  7|_      |3   |    |
.                     +--O|D   OUT|O---|----|--->OUT
.                    +|   |  GND  |    |    |    
.                   [1ľF] +---+---+    |    |        
.                   Ct|C2    1| U1     |    |    
.GND>-----------------+-------+--------+----+

What we have to do is figure out the values of R4 and C4 which will
keep U1-4 below 1.2V for the time required to let the transient on
U1-3 die away.

Just for grins, lets say 10ms will do it and were running a 12V
supply.

Then we can write:

                   Vcc
     t = RC ln ----------- = KRC
                Vcc - Vth

Where T is the time required, in seconds,
      R is the resistance required, in ohms,
      C is the capacitance required, in farads,
      Vcc is the supply voltage in Volts,
      Vth is the threshold voltage, in Volts, and
      ln  is log to the base e.

if we solve for:

               Vcc              12V
    K = ln ----------- = ln ------------ = 0.105  
            Vcc - Vth        12V - 1.2v

then we can simplify to:

    t = KRC

and solve for either R or C, knowing the other.

Let's say that we arbitrarily choose 1 megohm for R; then we can
rearrange and solve for C:

           T         1e-2s
     C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads.
          KR     1.05e-1 * 1e6R

So now you have:

.Vcc>-+---------+-----+-------+--------+----+
.     |R1       |R2   |R3    8|        |R4  |
.   [10k]     [10k][910k] +---+---+  [1M]   |
.__   |   C1    |   Rt|  2|_ Vcc _|4   |    |C3
.IN>--+-[100nF]-+-----|--O|T     R|O---+ [100nF]
.                     |  6|       |    |C4  |
.                     +---|TH  555|  [10nF] |
.                     |  7|_      |3   |    |
.                     +--O|D   OUT|O---|----|--->OUT
.                    +|   |  GND  |    |    |    
.                   [1ľF] +---+---+    |    |        
.                   Ct|C2    1| U1     |    |    
.GND>-----------------+-------+--------+----+

If you need more time you can make either R4 or C4 bigger, and C3 is a
_must_ if you're using a bipolar 555 and should be connected directly
across pins 1 and 8.  If you're using a CMOS 555 then it isn't needed.

JF

I appreciate your input,

I will try your suggestions and post the results. For now, I simply
buffered the input to the 555's trigger from the comparator, and it is
now working as it is supposed to. If it is a low-going transient on
the output of the comparator which is causing the problem, wouldn't it
die out on its own given enough time?

Thanks,
Chuck

 

[Jamie]

chuck wrote:

On May 27, 6:52 am, John Fields <jfie...@austininstruments.com> wrote:

On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck



chuckd87...@gmail.com> wrote:

On May 24, 4:43 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

You could be experiencing a problem of the trigger getting an initial
sink pulse on power up.

To fix your problem, you could use a 100ohm R from the VCC to the RESET
pin and a .1uf from the RESET pin to common.. This will force a short
reset of the timer on initial power of Vcc. It will force it in the off
state.
You most likely have an issue with some capacitance that is causing a
short SINK pulse on the trigger during initiation. You need to make sure
the RESET input stays lower during the initiation state.

Thanks again for the advice,

I tried that, and unfortunately it didnt fix the problem. Still, when
the trigger input to the 555 comes from the output of the comparator
(or Vcc) the output of the 555 is high, but when I switch the trigger
input to Vcc directly, the output goes low (and stays that way untill
I decrease the input to 1/3 Vcc)

---
Sound to me like what's happening is that there's a low-going
transient from the comparator triggering the 555 on power up.

Since the 555's RESET voltage is 1.2V max, at any supply voltage, then
what you need to do is hold the reset pin lower than 1.2V for as long
as it takes the transient to die away.

Assuming a 5V supply, 100 ohms and 0.1ľF gives a time constant of
2.7ľs from 0V to 1.2V, (the 555's RESET release voltage) which is way
too fast if what you're trying to do is keep a turn-on transient from
the comparator from triggering the 555.

Here's a circuit for a 555 one-shot with a passive power-on reset:
(View in Courier)

.Vcc>-+---------+-----+-------+--------+----+
. |R1 |R2 |R3 8| |R4 |
. [10k] [10k][910k] +---+---+ [100k] |
.__ | C1 | Rt| 2|_ Vcc _|4 | |C3
.IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF]
. | 6| | |C4 |
. +---|TH 555| [100nF] |
. | 7|_ |3 | |
. +--O|D OUT|O---|----|--->OUT
. +| | GND | | |
. [1ľF] +---+---+ | |
. Ct|C2 1| U1 | |
.GND>-----------------+-------+--------+----+

What we have to do is figure out the values of R4 and C4 which will
keep U1-4 below 1.2V for the time required to let the transient on
U1-3 die away.

Just for grins, lets say 10ms will do it and were running a 12V
supply.

Then we can write:

Vcc
t = RC ln ----------- = KRC
Vcc - Vth

Where T is the time required, in seconds,
R is the resistance required, in ohms,
C is the capacitance required, in farads,
Vcc is the supply voltage in Volts,
Vth is the threshold voltage, in Volts, and
ln is log to the base e.

if we solve for:

Vcc 12V
K = ln ----------- = ln ------------ = 0.105
Vcc - Vth 12V - 1.2v

then we can simplify to:

t = KRC

and solve for either R or C, knowing the other.

Let's say that we arbitrarily choose 1 megohm for R; then we can
rearrange and solve for C:

T 1e-2s
C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads.
KR 1.05e-1 * 1e6R

So now you have:

.Vcc>-+---------+-----+-------+--------+----+
. |R1 |R2 |R3 8| |R4 |
. [10k] [10k][910k] +---+---+ [1M] |
.__ | C1 | Rt| 2|_ Vcc _|4 | |C3
.IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF]
. | 6| | |C4 |
. +---|TH 555| [10nF] |
. | 7|_ |3 | |
. +--O|D OUT|O---|----|--->OUT
. +| | GND | | |
. [1ľF] +---+---+ | |
. Ct|C2 1| U1 | |
.GND>-----------------+-------+--------+----+

If you need more time you can make either R4 or C4 bigger, and C3 is a
_must_ if you're using a bipolar 555 and should be connected directly
across pins 1 and 8. If you're using a CMOS 555 then it isn't needed.

JF


I appreciate your input,

I will try your suggestions and post the results. For now, I simply
buffered the input to the 555's trigger from the comparator, and it is
now working as it is supposed to. If it is a low-going transient on
the output of the comparator which is causing the problem, wouldn't it
die out on its own given enough time?

Thanks,
Chuck
Yes it would how ever, if it takes longer to dissipate than the timer

RESET circuit event last, you'll still have your problem..

The timer RESET must last longer than it does to get the trigger
level above 1/3 of VCC.

 

[chuck]

On May 28, 4:19 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

chuck wrote:
On May 27, 6:52 am, John Fields <jfie...@austininstruments.com> wrote:

On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck

chuckd87...@gmail.com> wrote:

On May 24, 4:43 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

  You could be experiencing a problem of the trigger getting an initial
sink pulse on power up.

 To fix your problem, you could use a 100ohm R from the VCC to the RESET
pin and a .1uf from the RESET pin to common..  This will force a short
reset of the timer on initial power of Vcc. It will force it in the off
state.
  You most likely have an issue with some capacitance that is causing a
short SINK pulse on the trigger during initiation. You need to make sure
the RESET input stays lower during the initiation state.

Thanks again for the advice,

I tried that, and unfortunately it didnt fix the problem. Still, when
the trigger input to the 555 comes from the output of the comparator
(or Vcc) the output of the 555 is high, but when I switch the trigger
input to Vcc directly, the output goes low (and stays that way untill
I decrease the input to 1/3 Vcc)

---
Sound to me like what's happening is that there's a low-going
transient from the comparator triggering the 555 on power up.

Since the 555's RESET voltage is 1.2V max, at any supply voltage, then
what you need to do is hold the reset pin lower than 1.2V for as long
as it takes the transient to die away.

Assuming a 5V supply, 100 ohms and 0.1ľF gives a time constant of
2.7ľs from 0V to 1.2V, (the 555's RESET release voltage) which is way
too fast if what you're trying to do is keep a turn-on transient from
the comparator from triggering the 555.

Here's a circuit for a 555 one-shot with a passive power-on reset:
(View in Courier)

.Vcc>-+---------+-----+-------+--------+----+
.     |R1       |R2   |R3    8|        |R4  |
.   [10k]     [10k][910k] +---+---+ [100k]  |
.__   |   C1    |   Rt|  2|_ Vcc _|4   |    |C3
.IN>--+-[100nF]-+-----|--O|T     R|O---+ [100nF]
.                     |  6|       |    |C4  |
.                     +---|TH  555| [100nF] |
.                     |  7|_      |3   |    |
.                     +--O|D   OUT|O---|----|--->OUT
.                    +|   |  GND  |    |    |    
.                   [1ľF] +---+---+    |    |        
.                   Ct|C2    1| U1     |    |    
.GND>-----------------+-------+--------+----+

What we have to do is figure out the values of R4 and C4 which will
keep U1-4 below 1.2V for the time required to let the transient on
U1-3 die away.

Just for grins, lets say 10ms will do it and were running a 12V
supply.

Then we can write:

                  Vcc
    t = RC ln ----------- = KRC
               Vcc - Vth

Where T is the time required, in seconds,
     R is the resistance required, in ohms,
     C is the capacitance required, in farads,
     Vcc is the supply voltage in Volts,
     Vth is the threshold voltage, in Volts, and
     ln  is log to the base e.

if we solve for:

              Vcc              12V
   K = ln ----------- = ln ------------ = 0.105  
           Vcc - Vth        12V - 1.2v

then we can simplify to:

   t = KRC

and solve for either R or C, knowing the other.

Let's say that we arbitrarily choose 1 megohm for R; then we can
rearrange and solve for C:

          T         1e-2s
    C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads.
         KR     1.05e-1 * 1e6R

So now you have:

.Vcc>-+---------+-----+-------+--------+----+
.     |R1       |R2   |R3    8|        |R4  |
.   [10k]     [10k][910k] +---+---+  [1M]   |
.__   |   C1    |   Rt|  2|_ Vcc _|4   |    |C3
.IN>--+-[100nF]-+-----|--O|T     R|O---+ [100nF]
.                     |  6|       |    |C4  |
.                     +---|TH  555|  [10nF] |
.                     |  7|_      |3   |    |
.                     +--O|D   OUT|O---|----|--->OUT
.                    +|   |  GND  |    |    |    
.                   [1ľF] +---+---+    |    |        
.                   Ct|C2    1| U1     |    |    
.GND>-----------------+-------+--------+----+

If you need more time you can make either R4 or C4 bigger, and C3 is a
_must_ if you're using a bipolar 555 and should be connected directly
across pins 1 and 8.  If you're using a CMOS 555 then it isn't needed..

JF

I appreciate your input,

I will try your suggestions and post the results. For now, I simply
buffered the input to the 555's trigger from the comparator, and it is
now working as it is supposed to. If it is a low-going transient on
the output of the comparator which is causing the problem, wouldn't it
die out on its own given enough time?

Thanks,
Chuck

Yes it would how ever, if it takes longer to dissipate than the timer
RESET circuit event last, you'll still have your problem..

  The timer RESET must last longer than it does to get the trigger
level above 1/3 of VCC.- Hide quoted text -

- Show quoted text -

Thanks to both of you for the help,

I learned a valuable lesson on the importance of using de-coupling
capacitors. Things worked just fine as soon as I put the 100nF cap
between pin 1 and pin 8. Should I get in the habit of always using
them on the Vcc pin on ICs?

 

[krw@att.bizzzzzzzzzzzz]

On Fri, 28 May 2010 23:15:54 -0700 (PDT), chuck <chuckd87106@gmail.com> wrote:

On May 28, 4:19 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
chuck wrote:
On May 27, 6:52 am, John Fields <jfie...@austininstruments.com> wrote:

On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck

chuckd87...@gmail.com> wrote:

On May 24, 4:43 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

  You could be experiencing a problem of the trigger getting an initial
sink pulse on power up.

 To fix your problem, you could use a 100ohm R from the VCC to the RESET
pin and a .1uf from the RESET pin to common..  This will force a short
reset of the timer on initial power of Vcc. It will force it in the off
state.
  You most likely have an issue with some capacitance that is causing a
short SINK pulse on the trigger during initiation. You need to make sure
the RESET input stays lower during the initiation state.

Thanks again for the advice,

I tried that, and unfortunately it didnt fix the problem. Still, when
the trigger input to the 555 comes from the output of the comparator
(or Vcc) the output of the 555 is high, but when I switch the trigger
input to Vcc directly, the output goes low (and stays that way untill
I decrease the input to 1/3 Vcc)

---
Sound to me like what's happening is that there's a low-going
transient from the comparator triggering the 555 on power up.

Since the 555's RESET voltage is 1.2V max, at any supply voltage, then
what you need to do is hold the reset pin lower than 1.2V for as long
as it takes the transient to die away.

Assuming a 5V supply, 100 ohms and 0.1ľF gives a time constant of
2.7ľs from 0V to 1.2V, (the 555's RESET release voltage) which is way
too fast if what you're trying to do is keep a turn-on transient from
the comparator from triggering the 555.

Here's a circuit for a 555 one-shot with a passive power-on reset:
(View in Courier)

.Vcc>-+---------+-----+-------+--------+----+
.     |R1       |R2   |R3    8|        |R4  |
.   [10k]     [10k][910k] +---+---+ [100k]  |
.__   |   C1    |   Rt|  2|_ Vcc _|4   |    |C3
.IN>--+-[100nF]-+-----|--O|T     R|O---+ [100nF]
.                     |  6|       |    |C4  |
.                     +---|TH  555| [100nF] |
.                     |  7|_      |3   |    |
.                     +--O|D   OUT|O---|----|--->OUT
.                    +|   |  GND  |    |    |    
.                   [1ľF] +---+---+    |    |        
.                   Ct|C2    1| U1     |    |    
.GND>-----------------+-------+--------+----+

What we have to do is figure out the values of R4 and C4 which will
keep U1-4 below 1.2V for the time required to let the transient on
U1-3 die away.

Just for grins, lets say 10ms will do it and were running a 12V
supply.

Then we can write:

                  Vcc
    t = RC ln ----------- = KRC
               Vcc - Vth

Where T is the time required, in seconds,
     R is the resistance required, in ohms,
     C is the capacitance required, in farads,
     Vcc is the supply voltage in Volts,
     Vth is the threshold voltage, in Volts, and
     ln  is log to the base e.

if we solve for:

              Vcc              12V
   K = ln ----------- = ln ------------ = 0.105  
           Vcc - Vth        12V - 1.2v

then we can simplify to:

   t = KRC

and solve for either R or C, knowing the other.

Let's say that we arbitrarily choose 1 megohm for R; then we can
rearrange and solve for C:

          T         1e-2s
    C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads.
         KR     1.05e-1 * 1e6R

So now you have:

.Vcc>-+---------+-----+-------+--------+----+
.     |R1       |R2   |R3    8|        |R4  |
.   [10k]     [10k][910k] +---+---+  [1M]   |
.__   |   C1    |   Rt|  2|_ Vcc _|4   |    |C3
.IN>--+-[100nF]-+-----|--O|T     R|O---+ [100nF]
.                     |  6|       |    |C4  |
.                     +---|TH  555|  [10nF] |
.                     |  7|_      |3   |    |
.                     +--O|D   OUT|O---|----|--->OUT
.                    +|   |  GND  |    |    |    
.                   [1ľF] +---+---+    |    |        
.                   Ct|C2    1| U1     |    |    
.GND>-----------------+-------+--------+----+

If you need more time you can make either R4 or C4 bigger, and C3 is a
_must_ if you're using a bipolar 555 and should be connected directly
across pins 1 and 8.  If you're using a CMOS 555 then it isn't needed.

JF

I appreciate your input,

I will try your suggestions and post the results. For now, I simply
buffered the input to the 555's trigger from the comparator, and it is
now working as it is supposed to. If it is a low-going transient on
the output of the comparator which is causing the problem, wouldn't it
die out on its own given enough time?

Thanks,
Chuck

Yes it would how ever, if it takes longer to dissipate than the timer
RESET circuit event last, you'll still have your problem..

  The timer RESET must last longer than it does to get the trigger
level above 1/3 of VCC.- Hide quoted text -

- Show quoted text -

Thanks to both of you for the help,

I learned a valuable lesson on the importance of using de-coupling
capacitors. Things worked just fine as soon as I put the 100nF cap
between pin 1 and pin 8. Should I get in the habit of always using
them on the Vcc pin on ICs?

Unless Vcc and Gnd are both on planes, yes. If they're on planes, and wiring
to those planes is *short*, you can get away without decoupling caps on each
part. You should have some larger caps somewhere on the board, too.