problem using a 555 - help!

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I'm trying to use a 555 to generate a reset pulse ~1ms on power application.
Have the "usual" configuration with timing cap from pin 2/6 to pin 1 (ground),
charging resistor pins 2/6 to Vcc pin 8.  As this is a once-off pulse, DISCHARGE
pin is open (no function required).  RESET is at Vcc, CONTROL is open/bypassed
to gnd.

Schematic at
http://i224.photobucket.com/albums/dd163/skyline2000au/555.jpg

There is also a reverse-biased diode across the timing resistor to discharge the
cap when Vcc is removed, to assure that the cap starts below 1/3Vcc.

According to the 7555 sheet, on application of Vcc this *should* give a high
output until pin 6 reaches 2/3Vcc, then low output.  That assumes that the chip
gets its ducks in line before pins 2/6 rise past 1/3Vcc, which shouldn't be a
challenge.

In my test, the output is low from the outset.  No blip.

Apart from using a 555, what am I doing wrong?  I could have done this with a
bipolar, two resistors and a cap but I wanted a decent fall time :-(.

Re: problem using a 555 - help!


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application.
(ground),
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DISCHARGE
open/bypassed
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http://i224.photobucket.com/albums/dd163/skyline2000au/555.jpg
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discharge the
high
chip
be a
with a

I'm not sure why your circuit won't work. I've pasted two more conventional
approaches below. (You'll need to view them in "Courier" font. Copy and
paste into Notepad if necessary.)

        Power-up Reset Circuit
  +V o--+----+----+----+--------+
        |    |    |    |        |
        \    \    |    |       _|_+
  Rtrig /    / Rt |    |       === 100uF
   10K  \    \ 10K|8   |4       |
        /    /  +-+----+-+      |
        \    \  |Vcc  RST|      |      ~1mS
        |    | 2|        |3     |      Reset
        +-------+TR   OUT+-----------o Pulse
        |    | 7| (7)555 |      |      Out
        |    +--+DIS     |      |
        |    |  |        |      |
        |    +--+THR     +      |
        |    | 6|        |7     |    |
        |    |  |        |      |  --+--  = Connected
 Ctrig _|_  _|_ +-+----+-+      |    |
  0.01 ---  ---  1|    |5       |
        |    |Ct  |   _|_       |    |
        |    |0.1 |   --- 0.1uF |  -----  = Not connected
        |    |    |    |        |    |
 0V o---+----+----+----+--------+
               T=1.1RtCt



                +V
                 o
                 |
                 \
           Rrst  /
           150K  \
                 /
                 \        |\          ~1mS
                 |        |  \        Reset
                 +----+---+    >o---o Pulse
                 |    |   |  /        Out
                 |    \   |/
                 |    /
           Crst _|_   \ Rdis
           0.01 ---   / 2M2
                 |    \       T=0.7RC
                 |    |  (A Schmitt Trigger is best,
                 +----+   but any gate will work. To
                 |        use a non-inverting gate,
                -+-       connect Rrst to 0V and Crst/
                GND       Rdis to +V.)


Hope this helps,
... Steve



Re: problem using a 555 - help!



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There are several easy to find 555 application notes on the web which
describe in detail how to tailor pulse width to whatever you might need.



Re: problem using a 555 - help!



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Indeed there are, and I probably have them all here - including a Signetics
paper one from the 80's.  None of these provide a direct solution to the
requirement, although the configuration I trialled - a hybrid of astable and
monostable - seemed that it *should* work.

The requirement is that this unit MUST generate one and only one reset pulse
(active high) once Vcc is applied to the circuit.  Apart from controlling Vcc to
the module I have no other control inputs possible.  Also, the footprint
available won't permit a DIP14 package, hence the selection of a 555 as a
possible solution.

Re: problem using a 555 - help!



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The traditional method of generating a reset pulse is to use a simple C/R
across Vdd & GND, if sharp transitions are important a Schmidt trigger can
be constructed with a couple of transistors and a few resistors.



Re: problem using a 555 - help!



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I appreciate that.  But in the same footprint I can squeeze the 555.

The circuit I am using matches fig 9b on page 8 of the Philips AN170, *except*
(don't you love the way that always appears in places like this) instead of a
switch shunting the cap as the start control I am relying on the cap being
discharged below 1/3Vcc whn Vcc is first applied to the curcuit.

Incidentally I placed a 10uF cap across the 10nF and the circuit works fine,
giving ~2.2secs of high pulse.  So at some point between the two values it would
seem that the 555 is not getting its act together before the cap charges to
1/3Vcc - about 1mS - which sounds incredible.  Will be playing with more values
to home in on the critical area, but it means the circuit's repeatibility is in
question.

Thanks for your ongoing input.

Re: problem using a 555 - help!



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Why not use Maxim's series of microprocessor supervisors. They're available in
SOT-23,
or SC-70 packages, and you don't need an electrolytic cap that takes up valuable
real estate.
http://para.maxim-ic.com/cache/en/results/13538.html

And they're low power.

Microchip has a range as well.
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID90%08&mid11%&lang=en&pageId79%

dmm


Re: problem using a 555 - help!



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would
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SOT-23,
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valuable real estate.
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Fair coment David.  Unfortunately I have the pcb's to hand for the 555 effort,
and also 555's are on the shelf here.  I tend to think (aka design) with the
bits on hand unless I can see a major issue with them or a major advantage in
going another way.

But if it works with 10uF, why not with the original 10nF value?

Re: problem using a 555 - help!



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would
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values
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in
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SOT-23,
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valuable real estate.
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A slow risetime Vcc would likely have a deleterious effect on the timing of the
555 output.
First I'd check the risetime of the output of your intended power supply and
ensure that the voltage doesn't
make any unintended changes, like dropping to lower voltages while on its way to
its final voltage.
You could check if the 555 is affected by using a signal generator with a 5V
sawtooth waveform to power
a sample 555 and check at what point the trigger is made. I'd check some from
other manufacturers as well,
especially if you are intending to manufacture a number of your project.

dmm

Re: problem using a 555 - help!



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*except*
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would
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values
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in
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in SOT-23,
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valuable real estate.
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555 output.
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ensure that the voltage doesn't
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to its final voltage.
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sawtooth waveform to power
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other manufacturers as well,
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Just to provide background, this module will live between two existing
production assemblies.

DRIVING DEVICE <-> MODULE <-> TARGET DEVICE

The "target" device requires a reset pulse to initialise an on-board counter,
and the driving device contains no spare directly controllable lines.  The
module's only role is to generate that reset pulse, and the only way to achieve
this with existing pinouts and without redesigning and reworking driving device
is a "power-on reset pulse generator".

The Vcc supply to the driving device's output connector (where the module
connects) is switched by a  relay.  Vcc caps are on the relay input side except
for a couple of .01's on the load device and one on the module.  The rise time
on the Vcc on the module is quite fast.  Bear in mind the reset pulse duration
isn't critical - the target of 1 or 2mS was established simply to allow software
delays in the driving device to outlast it, and to be an order of magnitude (or
more) slower than any Vcc rise issues.

I only hope that relay bounce won't intervene.  But again, the "10uF works but
10nF doesn't" bit is what has upset my thinking.

The requirement is for about a dozen of these modules, so I can reasonably
settle on a single manufacturer's 555 at this stage, but your point is noted re
any variation with brand.

Re: problem using a 555 - help!


to keyboard and composed:

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Can you replace the relay with a transistor switch, at least for
troubleshooting purposes?

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- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: problem using a 555 - help!



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software
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Certainly can, Franc.  Will try that out later on and report if anything
different noted.

Re: problem using a 555 - help!



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achieve
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device
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except
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software
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(or
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That didn't take long.  I actually used another 555 as the switched source for
Vcc, and the module behaved as planned.  Went back to the CRO and I could see
the Vcc bounce on the relay-driven configuration - although I had been
*triggering* the CRO on Vcc I hadn't been *viewing* Vcc, only the reset pulse
and the target counter.  

Apparently the bounce causes the 555 some issues.  Certainly even with the
bounce the module circuit works as predicted with 10uF, so I tried some other
values and found that 100nF would reliably give a ~10mS blip.

As bounce can be unreliable especially from unit to unit, I will probably go
with a more conservative 1uF and simply allow a longer software delay on
startup.

Thanks to all who offered suggestions.

Re: problem using a 555 - help!



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It occurred to me that the 555 might be suffering the same problem that
makes the logic need a power on reset - unpredictable operation while Vcc
stabilises.

A radical solution, admittedly a bit like using an A-bomb to kill an ants
nest, use an 8-dil PIC 12F series MCU to generate the pulse, the PIC has
built in power on delay specifically to avoid the problem I suspect you
might be having, if you need a longer delay for Vcc to stabilise, its only a
few lines of code delay loop and the same again to programme precise pulse
duration, its a little more expensive than a 555 but some of the cost will
be offset by not needing any external components.



Re: problem using a 555 - help!



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Ian, I don't know if you had seen my later posts when you despatched this.  The
problem *appears* to be caused by contact bounce on the relay which switches the
already_establiished_and_rock_steady Vcc to the subject module.  When I use an
RC combination which takes longer to charge from 0 to 1/3Vcc than the bounce
takes to settle (worst case) I get good and repeatable behaviour.

I had actually considered those hard_to_source 8-pin minilogic gates as a simple
way to achieve my requirement -  I often use XOR gates to generate pulses.
However the ubiquitous nature of the 555 won me over, and particularly as they
are on the shelf here.

But certainly an 8-pin micro would have been a workable solution too.

Re: problem using a 555 - help!


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the
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simple
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     555s are notorious for (re)triggering on the narrowest of spikes.
My old Yellow Light car alarm was based on 555s and turning off the
parking lights after arming it would make it trigger. I had to install a
choke in series with its 12V feed to fix it. A mate of mine in
Wollongong made a sprinkler timer with 555s and it would switch on by
itself when he saw lightning in Sydney, 80km away.
     If the supply is bypassed really well and you've got an appropriate
RC network on the trigger input so tiny contact-caused spikes can't get
through, then it'll probably work OK.
     My 3c worth (it was 2c, but inflation pushed up the price).





Re: problem using a 555 - help!



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Yes, retriggering in the normal sense is "managed" by the configuration (see URL
in my original post).  The Vcc supply itself is solid and thoroughly bypassed,
but of course the weak point in my scheme  is always going to be the relay which
applies that rail to the 555 and its RC circuit.

With the revised value of C, the proto module is now happily behaving itself.

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