Is it some sort of "Latch" I need?

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Hello everyone,

This is all 12VDC, BTW. I have a very simple situation where a signal is  
making an LED light up. I have to work with the signal I'm getting, which  
is initially off, then on, then off, then on again. And I need to negate  
the final 'on again' so in other words, rather than having this light go  
off and then on again, I don't want it to come back on after it goes out.  
In case it matters, at present it comes on for about 4 seconds, then goes  
out for about 300ms, then comes back on permanently.
I just want it to *stay off* rather than coming back on after the 300ms  
of zero volts. Sorry if that's over-explaining it!
I think some kind of latch would do it, but it's so long ago I studied  
electronics I can't remember. Any help appreciated, thanks.

Re: Is it some sort of "Latch" I need?
On Friday, May 4, 2018 at 4:13:50 AM UTC+10, Al wrote:
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You could do something with good old-fashioned 4000-series CMOS - which Motorola (now ON-Semiconductor) sold as MC14000 parts.

Unfortunately, the "then comes back on permanently" part of the specification makes it an insoluble problem.

What you are implying is that the signal starts off low, goes high to 4 seconds, goes low for 300msec, than stays high forever.

In reality, it presumably eventually goes low again, so it can subsequently go high.

The simplest way of doing what you seem to want is to use a non-retriggerable  monstable, set up for a period of slightly longer than 4.3 seconds, to control your light.

http://www.ti.com/lit/ds/symlink/cd4098b.pdf

can be set up to be non-retriggerable.

The outputs clearly won't drive your LED directly but could drive the gate of MOSFET which could.

--  
Bill Sloman, Sydney

Re: Is it some sort of "Latch" I need?
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Is this your airbag warning light?

--  
     ?

Re: Is it some sort of "Latch" I need?
On 5/3/2018 11:43 PM, Al wrote:
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This should work:
https://www.dropbox.com/s/ztr0zbvlbeqlsfn/Latch.png?dl=0

Re: Is it some sort of "Latch" I need?
Pimpom wrote:
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I think he means the source is 12V or it's open, so it needs a pull-down  
resistor.  You have pul-ups on the gates at the left.




Re: Is it some sort of "Latch" I need?
On 5/4/2018 8:02 PM, Tom Del Rosso wrote:
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The OP says he has to work with a "signal.... which is initially  
off, then on, then off ....."

"on for...4 seconds.....out for 300ms....then on permanently"

"after the 300ms of zero volts".

I take this to mean initially low, then high for 4 sec, then low  
for 0.3 sec, then continuously high for an indefinite period  
until the whole process is terminated.

This is represented in the lower right side of my diagram. Let's  
see what the OP has to say about it.

Re: Is it some sort of "Latch" I need?
On Friday, May 4, 2018 at 1:18:53 PM UTC-4, Pimpom wrote:
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Yes, it is the "on permanently" part of the description that I find odd.  I don't know of any input signal that is like that.  After all, "permanent" is a long time.  

How would the "process" be terminated?  What exactly is your "process"?  

Rick C.  

Re: Is it some sort of "Latch" I need?
On 5/4/2018 10:52 PM, snipped-for-privacy@gmail.com wrote:
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Not *my* process. I meant until the OP stops doing whatever he's  
doing with the gadget - maybe switch the whole thing off.

Re: Is it some sort of "Latch" I need?
On Fri, 04 May 2018 22:48:45 +0530, Pimpom wrote:

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Correct.


As I said earlier, for some reason you have the output inverted. Or I'm  
going mad. Or both!


Re: Is it some sort of "Latch" I need?
Al wrote:
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So it is driven high and low - not just open or closed to the +12V  
source to drive a lamp?


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That's because his circuit drives an LED to +12V whereas you seem to  
have a lamp with one side grounded, or do you?




Re: Is it some sort of "Latch" I need?
On 04/05/18 17:22, Pimpom wrote:
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This is a beautiful drawing. May I ask, what software do you use?

werner Dahn


Re: Is it some sort of "Latch" I need?
On 5/4/2018 8:19 PM, aioe usenet wrote:
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CircuitMaker 2000. It's an old program as the name suggests.
I do a virtual print of the schematic and save it as a pdf file,  
then capture a screenshot of the pdf display and save it as a  
bitmap image after cropping as desired.

Sometimes I capture the CAD display directly but it's not  
antialiased and raster "jaggies" are visible on curves and  
slanting lines.

It's not for everyone. Drawing the schematic and preparing it for  
uploading took much longer than designing the circuit in my head.

Re: Is it some sort of "Latch" I need?
On Fri, 04 May 2018 21:49:31 +0700, aioe usenet wrote:

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It's over-elaborate IMHO, though. If it were me, I'd just use a 555 in  
single-shot mode.  



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Re: Is it some sort of "Latch" I need?
On Friday, May 4, 2018 at 7:17:33 PM UTC-4, Cursitor Doom wrote:
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I think you would need two of them.  Do you have a way of making the 555 trigger only once and never again?  Or will you make the pulse longer than the input pulse masking the "glitch"?  The spec is it only happens once I guess.  

Rick C.  

Re: Is it some sort of "Latch" I need?
On 5/5/2018 4:47 AM, Cursitor Doom wrote:
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It's always interesting to see if the same function can be  
performed by a simpler circuit.

Apart from the LED and its series resistor which make up the load  
and are not a part of the solution, my design uses three  
resistors, two capacitors and a diode. One resistor (R3) can be  
eliminated if the input is permanently connected.

True, a basic 555 monostable needs only one resistor and one  
capacitor. But it needs a negative-going trigger while the OP's  
signal is positive-going. This requires some form of inverter at  
the input which adds complexity and increases parts count.

The OP specifies only approximate input pulse lengths. Some  
margin, maybe a lot, will have to be added to the monostable time  
constant to accommodate that uncertainty as well as tolerances in  
component values. The output pulse length will then not conform  
to the input pulse length.

Finally, there are other factors which may or may not be  
important depending on the application: My design draws  
practically no current in the idle state whereas a 555 uses  
~100mW at 12V. It also produces huge current spikes while  
switching states. Both the idle and spike currents may be reduced  
by using the CMOS versions of 555, but they're still orders of  
magnitude greater than those associated with a 4011.

Re: Is it some sort of "Latch" I need?
On Saturday, 5 May 2018 05:08:24 UTC+1, Pimpom  wrote:
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You may be able to do it with just a comparator, depending on as yet unspecified details. RC on one input means it passes the input signal until a certain time when it locks out. Add diode to discharge when power off.


NT

Re: Is it some sort of "Latch" I need?
On 5/5/2018 6:01 PM, snipped-for-privacy@gmail.com wrote:
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May be possible, but will it be any simpler than my design -  
i.e., use fewer parts? And will the output match the input pulses  
precisely without careful adjustment? I'm always interested in  
any improved alternative to my own ideas.

Re: Is it some sort of "Latch" I need?
On Saturday, 5 May 2018 14:40:43 UTC+1, Pimpom  wrote:
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1 comparator, 1 R, 1 C.

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whether you need to adjust it depends on timings & tolerances. In some apps it won't be critical anyway. Only the OP knows what best suits his unstated requirements.


NT

Re: Is it some sort of "Latch" I need?
On 5/6/2018 12:49 AM, snipped-for-privacy@gmail.com wrote:
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What about setting the threshold?


Re: Is it some sort of "Latch" I need?
On Saturday, 5 May 2018 20:42:55 UTC+1, Pimpom  wrote:
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What about it are you asking?
Obviously the comparator's threshold moves analoguely over time, if you know what the specs are for the input signal you can make that work.


NT

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