Forums

simple oneshot(2)

Started by George Herold April 16, 2014
Hi all,  OK a silly basic question.
I'm doing some digital stuff.  
I wanted a one shot to reset some things, but didn't want to put in an entire chip
(74HC123.. or whatever the number is.)
So I just used a cap, resistor, diode and inverter.
https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG
Is this OK?  

Thanks,
George H.
(This didn't appear the first time I posted it...
trying again.)
On Wed, 16 Apr 2014 08:28:32 -0700, George Herold <gherold@teachspin.com>  
wrote:

> Hi all, OK a silly basic question. > I'm doing some digital stuff. > I wanted a one shot to reset some things, but didn't want to put in an > entire chip > (74HC123.. or whatever the number is.) > So I just used a cap, resistor, diode and inverter. > https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG > Is this OK? > > Thanks, > George H. > (This didn't appear the first time I posted it... > trying again.)
didn't look at the pic, but first thought, "What happens when the unexpected happens?" Like power up, up then off then up again, or very slow up. those kinds of things.
On Wednesday, April 16, 2014 11:58:56 AM UTC-4, RobertMacy wrote:
> On Wed, 16 Apr 2014 08:28:32 -0700, George Herold <gherold@teachspin.com> > > wrote: > > > > > Hi all, OK a silly basic question. > > > I'm doing some digital stuff. > > > I wanted a one shot to reset some things, but didn't want to put in an > > > entire chip > > > (74HC123.. or whatever the number is.) > > > So I just used a cap, resistor, diode and inverter. > > > https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG > > > Is this OK? > > > > > > Thanks, > > > George H. > > > (This didn't appear the first time I posted it... > > > trying again.) > > > > didn't look at the pic, but first thought, "What happens when the > > unexpected happens?"
Oh just an RC high pass and diode to ground to "eat up" the reverse voltage spike.
> > Like power up, up then off then up again, or very slow up. those kinds of > things.
Hmm that shouldn't be a problem. It only "sees" internal sharp pulses. I was more worried about too much current in the diode, or if the -0.3 volt from the diode clamp would be an issue. It seems like this must be (or must have been*) either a standard trick.. or something that is frowned upon. George H. *does anyone do pieces parts digital circuits anymore? Or is it all FPGA's and their ilk.
On Wed, 16 Apr 2014 09:10:27 -0700, George Herold <gherold@teachspin.com>  
wrote:

>> ..snip... > *does anyone do pieces parts digital circuits anymore? > Or is it all FPGA's and their ilk. >
Naw, today the 'component' is an iPod, or such. people are writing apps as the product and think in terms of a super complex system as the component. Whole new ballgame.
Dit is een meerdelig bericht in de MIME-indeling.

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	charset="utf-8"
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"George Herold"  schreef in bericht =
news:e9caced1-21a9-4345-9eb3-bad0b02c6584@googlegroups.com...=20

Hi all,  OK a silly basic question.
I'm doing some digital stuff. =20
I wanted a one shot to reset some things, but didn't want to put in an =
entire chip
(74HC123.. or whatever the number is.)
So I just used a cap, resistor, diode and inverter.
https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG
Is this OK? =20

Thanks,
George H.
(This didn't appear the first time I posted it...
trying again.)

If you need a clean reset puls like a 74HC121 or similar circuit will =
provide, this is *not* OK. You will need some more components like in =
the schematic below.

      ||          D2          |\
  ----||----+--->|---+---+----| >O------
      ||    |        |   |    |/
        C1  |       .-.  |     HCT14
            -       | | ---
            ^ D1   R| | --- C2
            |       '-'  |
            |        |   |
   ---------+--------+---+---------------
created by Andy=C2=B4s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de=20

There are some thing you have to mention:
- The starting pulse need to be able to provide the required power i.e. =
provide enough current during it=E2=80=99s on-time to load C2.
- So C1>>C2
- D1 eats the negative pulse on the falling edge of the input pulse.
- D2 isolates the one shot from the input once it has started.
- R*C2 determines the resulting pulsewidth (along with the thresholds of =
the 74HC14).
- To trigger this one shot again, it requires a new rising edge i.e. a =
new pulse on its input.
- C1 and D1 can be omitted if you can couple the input pulse directly. =
But in this case the timing will start when the input pulse has gone.

petrus bitbyter


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<HTML><HEAD></HEAD>
<BODY dir=3Dltr>
<DIV dir=3Dltr>
<DIV style=3D"FONT-SIZE: 12pt; FONT-FAMILY: 'Calibri'; COLOR: #000000">
<DIV><FONT face=3D"Courier New">"George Herold"&nbsp; schreef in bericht =

news:e9caced1-21a9-4345-9eb3-bad0b02c6584@googlegroups.com... =
</FONT></DIV>
<DIV><FONT face=3D"Courier New"></FONT>&nbsp;</DIV>
<DIV><FONT face=3D"Courier New">Hi all,&nbsp; OK a silly basic=20
question.</FONT></DIV>
<DIV><FONT face=3D"Courier New">I'm doing some digital stuff.&nbsp; =
</FONT></DIV>
<DIV><FONT face=3D"Courier New">I wanted a one shot to reset some =
things, but=20
didn't want to put in an entire chip</FONT></DIV>
<DIV><FONT face=3D"Courier New">(74HC123.. or whatever the number=20
is.)</FONT></DIV>
<DIV><FONT face=3D"Courier New">So I just used a cap, resistor, diode =
and=20
inverter.</FONT></DIV>
<DIV><FONT=20
face=3D"Courier =
New">https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG</FONT></DIV>
<DIV><FONT face=3D"Courier New">Is this OK?&nbsp; </FONT></DIV>
<DIV><FONT face=3D"Courier New"></FONT>&nbsp;</DIV>
<DIV><FONT face=3D"Courier New">Thanks,</FONT></DIV>
<DIV><FONT face=3D"Courier New">George H.</FONT></DIV>
<DIV><FONT face=3D"Courier New">(This didn't appear the first time I =
posted=20
it...</FONT></DIV>
<DIV><FONT face=3D"Courier New">trying again.)</FONT></DIV>
<DIV><FONT face=3D"Courier New"></FONT>&nbsp;</DIV>
<DIV><FONT face=3D"Courier New">If you need a clean reset puls like a =
74HC121 or=20
similar circuit will provide, this is *not* OK. You will need some more=20
components like in the schematic below.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3D"Courier New">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
||&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
D2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |\</FONT></DIV>
<DIV><FONT face=3D"Courier New">&nbsp; ----||----+---&gt;|---+---+----|=20
&gt;O------</FONT></DIV>
<DIV><FONT face=3D"Courier New">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
||&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; =
|&nbsp;&nbsp;=20
|&nbsp;&nbsp;&nbsp; |/</FONT></DIV>
<DIV><FONT face=3D"Courier =
New">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
C1&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; .-.&nbsp;=20
|&nbsp;&nbsp;&nbsp;&nbsp; HCT14</FONT></DIV>
<DIV><FONT=20
face=3D"Courier =
New">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
-&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | | ---</FONT></DIV>
<DIV><FONT=20
face=3D"Courier =
New">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
^ D1&nbsp;&nbsp; R| | --- C2</FONT></DIV>
<DIV><FONT=20
face=3D"Courier =
New">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; '-'&nbsp; |</FONT></DIV>
<DIV><FONT=20
face=3D"Courier =
New">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=20
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp; |</FONT></DIV>
<DIV><FONT face=3D"Courier New">&nbsp;&nbsp;=20
---------+--------+---+---------------</FONT></DIV>
<DIV><FONT face=3D"Courier New">created by Andy=C2=B4s ASCII-Circuit =
v1.24.140803 Beta=20
<A href=3D"http://www.tech-chat.de">www.tech-chat.de</A> </FONT></DIV>
<DIV><FONT face=3D"Courier New"></FONT>&nbsp;</DIV>
<DIV><FONT face=3D"Courier New">There are some thing you have to=20
mention:</FONT></DIV>
<DIV><FONT face=3D"Courier New">- The starting pulse need to be able to =
provide=20
the required power i.e. provide enough current during it=E2=80=99s =
on-time to load=20
C2.</FONT></DIV>
<DIV><FONT face=3D"Courier New">- So C1&gt;&gt;C2</FONT></DIV>
<DIV><FONT face=3D"Courier New">- D1 eats the negative pulse on the =
falling edge=20
of the input pulse.</FONT></DIV>
<DIV><FONT face=3D"Courier New">- D2 isolates the one shot from the =
input once it=20
has started.</FONT></DIV>
<DIV><FONT face=3D"Courier New">- R*C2 determines the resulting =
pulsewidth (along=20
with the thresholds of the 74HC14).</FONT></DIV>
<DIV><FONT face=3D"Courier New">- To trigger this one shot again, it =
requires a=20
new rising edge i.e. a new pulse on its input.</FONT></DIV>
<DIV><FONT face=3D"Courier New">- C1 and D1 can be omitted if you can =
couple the=20
input pulse directly. But in this case the timing will start when the =
input=20
pulse has gone.</FONT></DIV>
<DIV><FONT face=3D"Courier New"></FONT>&nbsp;</DIV>
<DIV><FONT face=3D"Courier New">petrus bitbyter</FONT></DIV>
<DIV><FONT face=3D"Courier New"></FONT>&nbsp;</DIV>
<DIV><FONT face=3D"Courier =
New"></FONT>&nbsp;</DIV></DIV></DIV></BODY></HTML>

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On Wednesday, April 16, 2014 8:28:32 AM UTC-7, George Herold wrote:
> Hi all, OK a silly basic question. > > I'm doing some digital stuff. > > I wanted a one shot to reset some things, but didn't want to put in an entire chip > > (74HC123.. or whatever the number is.) > > So I just used a cap, resistor, diode and inverter. > > https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG > > Is this OK?
If the inverter weren't CMOS, and you can tolerate multiple edges near the end of the delay, yes. For CMOS, slow-slewing inputs are highly discouraged (can cause high power consumption) unless the gate is one that has hysteresis (i.e. a Schmitt inverter). At/near the logic threshold, a slow ramp voltage might result in power supply noise getting amplified (and that means lots of hash on the output), so hysteresis is your friend. Two thresholds, so you're never near both of 'em.
On Wednesday, April 16, 2014 8:28:32 AM UTC-7, George Herold wrote:

> I wanted a one shot to reset some things, but didn't want to put in an entire chip > > (74HC123.. or whatever the number is.) > > So I just used a cap, resistor, diode and inverter. > > https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG
Either a '555 or a SN74LVC1G123 (both in SO-8 nowadays) would work. In CMOS, a slow-slewing input that approaches threshold will result in lots of hash (because power supply noise gets amplified), and possibly in high power consumption, UNLESS you have a Schmitt gate. The hysteresis of a Schmitt trigger solves both problems. Switching to TTL solves the transient power consumption issue.
On Wednesday, April 16, 2014 8:28:32 AM UTC-7, George Herold wrote:

> I wanted a one shot to reset some things, but didn't want to put in an entire chip > > (74HC123.. or whatever the number is.) > > So I just used a cap, resistor, diode and inverter. > > https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG
Either a '555 or a SN74LVC1G123 (both in SO-8 nowadays) would work. In CMOS, a slow-slewing input that approaches threshold will result in lots of hash (because power supply noise gets amplified), and possibly in high power consumption, UNLESS you have a Schmitt gate. The hysteresis of a Schmitt trigger solves both problems. Switching to TTL solves the transient power consumption issue. (having posting problems, hope this isn't a repeat send!)
On Wed, 16 Apr 2014 08:28:32 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>Hi all, OK a silly basic question. >I'm doing some digital stuff. >I wanted a one shot to reset some things, but didn't want to put in an entire chip >(74HC123.. or whatever the number is.) >So I just used a cap, resistor, diode and inverter. >https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG >Is this OK? > >Thanks, >George H. >(This didn't appear the first time I posted it... >trying again.)
This is a prop-delay one-shot: https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Prop_Delay_One-shot.JPG That one makes a pulse that is one gate delay wide, but you can add any delay element in series with the lower inverter: coax, RC, more gates, or just a cap from the inverter output to ground, if you're feeling tacky. It's all "logic legal" electrically. Use an XOR gate, and it fires on both input edges. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Wed, 16 Apr 2014 17:24:06 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

>On Wed, 16 Apr 2014 08:28:32 -0700 (PDT), George Herold ><gherold@teachspin.com> wrote: > >>Hi all, OK a silly basic question. >>I'm doing some digital stuff. >>I wanted a one shot to reset some things, but didn't want to put in an entire chip >>(74HC123.. or whatever the number is.) >>So I just used a cap, resistor, diode and inverter. >>https://www.dropbox.com/s/flnonc9rgwmerhw/DSCF0082.JPG >>Is this OK? >> >>Thanks, >>George H. >>(This didn't appear the first time I posted it... >>trying again.) > >This is a prop-delay one-shot: > >https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Prop_Delay_One-shot.JPG > >That one makes a pulse that is one gate delay wide, but you can add >any delay element in series with the lower inverter: coax, RC, more >gates, or just a cap from the inverter output to ground, if you're >feeling tacky. > >It's all "logic legal" electrically. >
Found of any old text book.
>Use an XOR gate, and it fires on both input edges.
...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.