555 timer questions

Hello,

If more than one circuits negative pulse output is going to a 555
monostable timers trigger input should there be something on each
circuits output to isolate them from each other? Perhaps a small
signal diode?

I also require a timer circuit for "once only" activation with no
retriggering. A search of old posts resulted in a 2000 post suggesting
the use of a 74HC4538 and to forget the 555 with its limitations. I
have yet to research the 74HC4538. Any suggestions on how to achieve
such a timer?

Cheers,

Andrew.

P.S. I have removed my valid Email address due to the swine who must
have collected it from previous posts and sold it. 100+ spams per day
and climbing.

 

[John Fields]

On Mon, 07 Jun 2004 03:50:45 GMT, (Rubicon) wrote:

Hello,

If more than one circuits negative pulse output is going to a 555
monostable timers trigger input should there be something on each
circuits output to isolate them from each other? Perhaps a small
signal diode?

I also require a timer circuit for "once only" activation with no
retriggering. A search of old posts resulted in a 2000 post suggesting
the use of a 74HC4538 and to forget the 555 with its limitations. I
have yet to research the 74HC4538. Any suggestions on how to achieve
such a timer?

---
You really haven't included enough information to get a good answer.

For instance, you don't say anything about the multiple input pulses
which are going to the single 555. That is, their amplitude and
duration (whether they last longer than the output pulse and where
they're coming from. Caps? TTL? A mechanical switch of some sort?

For your 4538 query, it would be nice to know whether by "once only"
you mean perpetual input pulse holdoff after the first output pulse or
whether you mean no retriggering before the chip has timed out.

--
John Fields

 

[John Fields]

On Tue, 08 Jun 2004 05:55:55 GMT, (Rubicon) wrote:

John,

Sorry for the lack of information.

Three triggering circuits to the 555, all operating off a 9V battery.
The first is an RF switch circuit with an NPN transistor and resistor
between V+ and the collector for the negative "pulse".

The second a simple sensor circuit again with a transistor/resistor
for the negative "pulse".

The third is a low voltage circuit. I have found an opamp/zener diode
based low voltage alarm circuit which might do if I eliminate the LED
and its resistor, replace the LM339 with an LM393 and the buzzer with
the same transistor/resistor method of activation.
http://www.electronics-lab.com/projects/sensors/023/

I understand that the 555 needs a negative pulse and that all three
circuits only provide the negative and not the quick return to
positive. Unfortunately I'm not sure of any other relatively easy way
to do this.
I think I'll need two timers here, the first for the RF circuit which
could be a 555 (or one half of the 4538) in monostable mode to allow
for retriggering after its timed out. The second timer though would
need to be a "perpetual input pulse holdoff after the first output
pulse" type to prevent the sensor and low voltage circuits from
constantly re-activating it.

Battery conservation is a factor but I haven't yet looked into methods
of doing this.

---
OK.

The first thing is that 'retriggerable' refers to being able to extend
the timeout of the one-shot while the output is still hot. That means
that if you've triggered a one-shot with a 1 second timeout and it's
only halfway through its timeout and you hit it with another trigger
pulse its timeout will be extended for 1 second after the application
of the new trigger without the output changing state. So, for those
two input pulses you'll get an output pulse which is 1-1/2 seconds
long. If it's _not_ retriggerable, then hitting it with additional
input pulses will have no effect on the timing until the output times
out, and then the first input after that that will trigger it again
for 1 second. 555's aren't capable of being retriggered without the
use of extra hardware, but '123s and 4538s are, with just wiring
changes.

The second thing is that by "perpetual input pulse holdoff after the
first output pulse" I meant that you'd get a single output pulse from
the timer no matter what happened on the input during or after it
timed out, and that after that you'd have to manually reset the
circuit to get another output pulse.

However, from your description, above, it sounds like you'd like one
output pulse whenever the input voltage goes below a certain level,
and the LM393 circuitry shown below should do that for you.

As far as sorting out your input pulse problem goes, you could do
something like this:
.
.
SENSORS . TIMER CIRCUITRY
.
.
+9V . +9V +9V
| . | |
[R] . [10k] [100K] +-----+
| . | | 3|__ |
+--> .>---[0.1ľF]---+-[<1N4148]---+-----------------+--O|TR |
| . | | |
C . | +-----+
--B . | 7555
E . |
| . |
GND . |
. |
. |
. |
+9V . +9V |
| . | |
[R] . [10K] |
| . | |
+--> .>---[0.1ľF]--+--[<1N4148]---------------------+
| . |
C . |
--B . |
E . |
| . |
GND . |
. |
. |
. |
. |
That low voltage sensor you linked to is a real POS |
in that it has no hysteresis and will probably |
chatter like crazy when it gets close to the |
threshold, but you can fix it by doing something |
like this: |
. |
. . . . . . . . . . . |
. |
+9V +9V . |
| | . |
[R3] [R4] . |
| | . +9V |
| +--[R2]--+ . | |
| | | . [10K] |
VIN>-[R1]--|--+--|+\ | . | |
| | >--+--> .>--[0.1ľF]--+--[<1N4148]--+
+-----|-/ .
| 1/2 LM393 .
|K .
[VREF] .
| .
GND .
.

Make sure you short all the inputs and the output of the unused
comparator to ground.

The resistor values in the places I left unspecified will determine
the drain on your battery, and you'll need to choose them with that in
mind as well as with what the highest impedances you can use and still
have the circuit operate reliably will be.

Also, if you're trying to sense when the 9V goes low you'll need to
connect the VIN end of R1 to ground and arrange for the voltage
divider R1 R2 R4 to cause the voltage on the comparator's
+ input to go lower than the voltage on the - input when the battery
voltage falls to the voltage you've chosen for the alarm point.

A pretty good choice for a reference would be something like a common
LM 185/285/385, where you could get down to around a 10ľA constant
drain on the battery to keep the reference's output voltage constant,
but just make sure that that 10ľA is flowing when the battery gets
down to the low voltage you want to sense!

--
John Fields

 

On Sun, 06 Jun 2004 13:57:00 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Mon, 07 Jun 2004 03:50:45 GMT, (Rubicon) wrote:

Hello,

If more than one circuits negative pulse output is going to a 555
monostable timers trigger input should there be something on each
circuits output to isolate them from each other? Perhaps a small
signal diode?

I also require a timer circuit for "once only" activation with no
retriggering. A search of old posts resulted in a 2000 post suggesting
the use of a 74HC4538 and to forget the 555 with its limitations. I
have yet to research the 74HC4538. Any suggestions on how to achieve
such a timer?

---
You really haven't included enough information to get a good answer.

For instance, you don't say anything about the multiple input pulses
which are going to the single 555. That is, their amplitude and
duration (whether they last longer than the output pulse and where
they're coming from. Caps? TTL? A mechanical switch of some sort?

For your 4538 query, it would be nice to know whether by "once only"
you mean perpetual input pulse holdoff after the first output pulse or
whether you mean no retriggering before the chip has timed out.

--
John Fields

John,

Sorry for the lack of information.

Three triggering circuits to the 555, all operating off a 9V battery.
The first is an RF switch circuit with an NPN transistor and resistor
between V+ and the collector for the negative "pulse".

The second a simple sensor circuit again with a transistor/resistor
for the negative "pulse".

The third is a low voltage circuit. I have found an opamp/zener diode
based low voltage alarm circuit which might do if I eliminate the LED
and its resistor, replace the LM339 with an LM393 and the buzzer with
the same transistor/resistor method of activation.
http://www.electronics-lab.com/projects/sensors/023/

I understand that the 555 needs a negative pulse and that all three
circuits only provide the negative and not the quick return to
positive. Unfortunately I'm not sure of any other relatively easy way
to do this.
I think I'll need two timers here, the first for the RF circuit which
could be a 555 (or one half of the 4538) in monostable mode to allow
for retriggering after its timed out. The second timer though would
need to be a "perpetual input pulse holdoff after the first output
pulse" type to prevent the sensor and low voltage circuits from
constantly re-activating it.

Battery conservation is a factor but I haven't yet looked into methods
of doing this.

Thankyou,

Andrew.

 

[John Fields]

On Wed, 09 Jun 2004 05:23:26 GMT, (Rubicon) wrote:

John,

Thankyou for the detailed reply.

Just a couple more questions if you have time.

Why the 100K resistor on the first ASCII sensor/timer circuit and not
the others and also what does the K by the VREF on the non-inverting
comparator with hysteresis mean? Am I missing something obvious there?

---
The 100K resistor is the DC return for the diodes/pullup for the 7555
and the 'K' is the 'Kathode' of the [Zener-like] voltage reference.

--
John Fields

 

John,

Thankyou for the detailed reply.

Just a couple more questions if you have time.

Why the 100K resistor on the first ASCII sensor/timer circuit and not
the others and also what does the K by the VREF on the non-inverting
comparator with hysteresis mean? Am I missing something obvious there?

Thanks,

Andrew.



On Mon, 07 Jun 2004 15:20:10 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Tue, 08 Jun 2004 05:55:55 GMT, (Rubicon) wrote:


John,

Sorry for the lack of information.

Three triggering circuits to the 555, all operating off a 9V battery.
The first is an RF switch circuit with an NPN transistor and resistor
between V+ and the collector for the negative "pulse".

The second a simple sensor circuit again with a transistor/resistor
for the negative "pulse".

The third is a low voltage circuit. I have found an opamp/zener diode
based low voltage alarm circuit which might do if I eliminate the LED
and its resistor, replace the LM339 with an LM393 and the buzzer with
the same transistor/resistor method of activation.
http://www.electronics-lab.com/projects/sensors/023/

I understand that the 555 needs a negative pulse and that all three
circuits only provide the negative and not the quick return to
positive. Unfortunately I'm not sure of any other relatively easy way
to do this.
I think I'll need two timers here, the first for the RF circuit which
could be a 555 (or one half of the 4538) in monostable mode to allow
for retriggering after its timed out. The second timer though would
need to be a "perpetual input pulse holdoff after the first output
pulse" type to prevent the sensor and low voltage circuits from
constantly re-activating it.

Battery conservation is a factor but I haven't yet looked into methods
of doing this.

---
OK.

The first thing is that 'retriggerable' refers to being able to extend
the timeout of the one-shot while the output is still hot. That means
that if you've triggered a one-shot with a 1 second timeout and it's
only halfway through its timeout and you hit it with another trigger
pulse its timeout will be extended for 1 second after the application
of the new trigger without the output changing state. So, for those
two input pulses you'll get an output pulse which is 1-1/2 seconds
long. If it's _not_ retriggerable, then hitting it with additional
input pulses will have no effect on the timing until the output times
out, and then the first input after that that will trigger it again
for 1 second. 555's aren't capable of being retriggered without the
use of extra hardware, but '123s and 4538s are, with just wiring
changes.

The second thing is that by "perpetual input pulse holdoff after the
first output pulse" I meant that you'd get a single output pulse from
the timer no matter what happened on the input during or after it
timed out, and that after that you'd have to manually reset the
circuit to get another output pulse.

However, from your description, above, it sounds like you'd like one
output pulse whenever the input voltage goes below a certain level,
and the LM393 circuitry shown below should do that for you.

As far as sorting out your input pulse problem goes, you could do
something like this:
.
.
SENSORS . TIMER CIRCUITRY
.
.
+9V . +9V +9V
| . | |
[R] . [10k] [100K] +-----+
| . | | 3|__ |
+--> .>---[0.1ľF]---+-[<1N4148]---+-----------------+--O|TR |
| . | | |
C . | +-----+
--B . | 7555
E . |
| . |
GND . |
. |
. |
. |
+9V . +9V |
| . | |
[R] . [10K] |
| . | |
+--> .>---[0.1ľF]--+--[<1N4148]---------------------+
| . |
C . |
--B . |
E . |
| . |
GND . |
. |
. |
. |
. |
That low voltage sensor you linked to is a real POS |
in that it has no hysteresis and will probably |
chatter like crazy when it gets close to the |
threshold, but you can fix it by doing something |
like this: |
. |
. . . . . . . . . . . |
. |
+9V +9V . |
| | . |
[R3] [R4] . |
| | . +9V |
| +--[R2]--+ . | |
| | | . [10K] |
VIN>-[R1]--|--+--|+\ | . | |
| | >--+--> .>--[0.1ľF]--+--[<1N4148]--+
+-----|-/ .
| 1/2 LM393 .
|K .
[VREF] .
| .
GND .
.

Make sure you short all the inputs and the output of the unused
comparator to ground.

The resistor values in the places I left unspecified will determine
the drain on your battery, and you'll need to choose them with that in
mind as well as with what the highest impedances you can use and still
have the circuit operate reliably will be.

Also, if you're trying to sense when the 9V goes low you'll need to
connect the VIN end of R1 to ground and arrange for the voltage
divider R1 R2 R4 to cause the voltage on the comparator's
+ input to go lower than the voltage on the - input when the battery
voltage falls to the voltage you've chosen for the alarm point.

A pretty good choice for a reference would be something like a common
LM 185/285/385, where you could get down to around a 10ľA constant
drain on the battery to keep the reference's output voltage constant,
but just make sure that that 10ľA is flowing when the battery gets
down to the low voltage you want to sense!

--
John Fields

 

Thanks,

Andrew.




On Tue, 08 Jun 2004 07:53:11 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 09 Jun 2004 05:23:26 GMT, (Rubicon) wrote:


John,

Thankyou for the detailed reply.

Just a couple more questions if you have time.

Why the 100K resistor on the first ASCII sensor/timer circuit and not
the others and also what does the K by the VREF on the non-inverting
comparator with hysteresis mean? Am I missing something obvious there?

---
The 100K resistor is the DC return for the diodes/pullup for the 7555
and the 'K' is the 'Kathode' of the [Zener-like] voltage reference.

--
John Fields