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'[OT]: Unstable 555?!'
2001\09\04@111911 by Anand Dhuru

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Hi guys,

I am trying to use the 555 configured as an astable, with a variable Ra (the resistance between 7 & 8), and measure the output pulsewidth (the ON time) on a 16F84.

The results I get pretty much match what the specs. of the 555 would expect, but there is a strange problem. The circuit is so unstable, if I just touch either pin 6, or 7, the readings go haywire. Same happens if I solder a long wire to place the resistance remotely as a sensor. I tried a bypass capacitor, a shielded wire for the remote resistance etc. but nothing seems to work.
My power supply, incidentally, is pretty well regulated, and I dont think I can suspect that.

Besides this problem the circuit works so well, I am reluctant to give it up in favour of some other scheme.

Any hints and advice would be highly appreciated.
Incidentally, I tried a 7555, the CMOS version as well; same results.

Regards,

Anand

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2001\09\04@125017 by Jim Paul

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Anand,

My off the top of my head response would be that body capacitance
is throwing everything off.   Try putting the whole circuit in a
metal box so the whole thing is sheilded.  That should do it.

And I don't know this for sure, but the CMOS version I think would
be even more affected by hand (body) capacitance because of the higher
impedances (lower current requirements) that the typical version.

                                              Regards,

                                                Jim





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2001\09\04@143709 by Fok, Ben

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If you are using a breadboard, I would suspect loose connection somewhere.
I suggest soldering all components onto a perfboard instead.  Pin 6 is for
the timing cap and it is sensitive to any capacitance change.  Pin 7 is the
internal bypass transistor.  If your body has any potential at all, which it
does, then you will offset the transistor's operation and change the period
of the output pulse.

Ben

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2001\09\04@195646 by Harold M Hallikainen

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       Actually, I generally find the 555 to be unstable. I prefer the CMOS
version, which is quite a bit better. The astable circuit I use is to tie
pins 2 and 6 together with a capacitor to ground. Put a resistor from pin
3 to pins 2 and 6. The output is on 3 with an open drain output available
on 7. Remember to tie the reset pin high.
       For an astable, though, I'd probably go with the 74HC14 hex schmitt
trigger. Put a resistor across one section and a capacitor from the input
to ground, and you've got an RC oscillator! And you've got 5 more
sections available for other stuff. You can also use CMOS schmitt NAND
gates if NANDS are more useful in the remainder of your circuit.

Harold



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2001\09\04@223025 by Gaston Gagnon

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What is the frequency of your astable ? Would it be near line frequency by any chance 50 or 60 Hz ?
Gaston

Anand Dhuru wrote:

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2001\09\04@223928 by Spehro Pefhany

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At 08:55 PM 9/3/01 +0530, you wrote:

>Incidentally, I tried a 7555, the CMOS version as well; same results.

Be sure to bypass pin 5 to ground with something like 10nF. Don't leave
it open.

Best regards,

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2001\09\05@130903 by Anand Dhuru

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Thanks for the various suggestions, guys. Unfotunately, I'm still with the original problem after trying out all of them.

Perhaps if I explain the application, it would be easier to see a solution.

The 555 is configured as an astable. Ra (the one between pins 7 & 8) is actually a resistance ladder of 10 discrete resistors. This assembly is to be used as a sensor in a water tank. The idea is, as the water level rises, it would progressively short out each resistance, and thus the 'on-time', or the pulse width would change. This change does not have to be linear; I can have a look up table in the PIC. Also, I am not interested in measuring absolute quantities; just a percentage, in steps of 10%.
Also, for this to work, the total resistance of Ra (all 10 inseries) has to be high enough that the water looks like an appreciable short to each element. Therefore, I need about 100k for each; works great on the breadboard, but goes crazy whenever pins 6 or 7 are touched.

I have tried values from .1 to .001 for the C, and the arithmetic still holds suprisingly well. Any way around this problem at all?

Regards,

Anand Dhuru

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2001\09\05@133438 by SkinTech

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Well, I haven't followed the previous suggestions, but it seems to me that a
megohm is pretty high impedance w.r.t. a human finger. When you touch the
pin(s), you couple a lot of 50Hz (60Hz) noise into the chip, which modulates
the internal comparators inputs all over the place. Of course it goes
beserk!
I suspect that in your actual app you will need to use really well screened
cabling (how long, physically, will the resistance string be?).
How about this alternative: a tube with an ultrasonic 'transmitter' at the
top, alongside a receiver. The us is pulsed, and you time how long it takes
until the reflection from the water surface is received. Divide by two & the
speed of sound, and you get water level.
That also takes care of the problem that your water 'resistance' is very
much dependent on whatever  is solved into the water, which can probably
vary unpredictable.

Cheer up,

Jan Didden

{Original Message removed}

2001\09\05@142914 by Byron A Jeff

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On Wed, Sep 05, 2001 at 07:41:52PM +0200, SkinTech wrote:
> Well, I haven't followed the previous suggestions, but it seems to me that a
> megohm is pretty high impedance w.r.t. a human finger. When you touch the
> pin(s), you couple a lot of 50Hz (60Hz) noise into the chip, which modulates
> the internal comparators inputs all over the place. Of course it goes
> beserk!
> I suspect that in your actual app you will need to use really well screened
> cabling (how long, physically, will the resistance string be?).

Yes there are a lot of issues at that high a resistance. Also you're going to
get huge delay times, or if you use a small cap very very unstable delays.

I think you'll find that at that resistance you're very nearly out of spec.
Go and doublecheck the datasheet.

> How about this alternative: a tube with an ultrasonic 'transmitter' at the
> top, alongside a receiver. The us is pulsed, and you time how long it takes
> until the reflection from the water surface is received. Divide by two & the
> speed of sound, and you get water level.

Ouch! that's a really complicated system. It's several orders of magnitude
above the original.

Let me take stab at a simpler one: If you have clearance above the tank you
can do it with a simple float. Attach a pole that gets floated and place a
magnet on the pole. Then put a tower with magnetic reed switches at regular
intervals next to the pole. As the magnet passes each switch it'll switch.
Another possibility is to have magnets at the same intervals along the pole
so that when the float is at the bottom of the tank all of the switches are
engaged and as the float rises fewer and fewer magnets engage the remaining
switches.

You can then go right back to the 555 but with much lower resistances. If
you couple a different resistance across each switch then when that switch
engages, it'll change the frequency. Or you can simply read the digital
inputs of the switches directly.

Depending on the tank material, you may be able to implement this same kind
of scheme along the edge of the tank. Another possibility is if the tank
is transparent then the float can carry a mirror and you detect the mirror
using a LED photodector pair...

> That also takes care of the problem that your water 'resistance' is very
> much dependent on whatever  is solved into the water, which can probably
> vary unpredictable.

Very much agreed on that.

BAJ

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2001\09\05@155537 by Barry Gershenfeld

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The high resistance is a big problem as others have noted.  And if
touching it with your body is a problem then it will be a problem
touching it with a big water tank.  Although you can "ground" the
circuit to the tank and that will help.  You would also get more
noise if the tank had a pump or other electrical object immersed
in it.

How about you go with the same "ladder" but have each contact
run through a transistor or CMOS gate?  The water resistance
would now switch the gate.  You could read each
"bit" with your PIC or if you wanted to keep the original
scheme you could arrange for the gate outputs to switch
lower-valued resistors in and out of your oscillator and still
be able to measure it the original way.

One other problem:  When you run an electric current through
water you will electrolyze one or both of the metal contacts,
as well as turning some of the water to hydrogen and oxygen.
Maybe you can get away with the tiny currents required to
switch CMOS but otherwise you'll have to arrange to have "AC"
(a reversing current or a capacitively coupled square wave)
going through the water to minimize this effect.

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2001\09\06@024202 by Vasile Surducan

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Anand, youll be surprised to find I had almost the same problem using a
555 for a microwave PIN based chopper. After the whole chip and
passive components reach the thermal equilibrium the frequency was stable
but 10 % different from the computed one. It was neither the 555, nor
the capacitor or the connection to passive devices, and not the supply.
However there are some limits for both capacitor and resistor values, I
hope you are in permited area with your values.
I've tried a whole lot of 555 with the same problem. After 10 minutes from
power on, the frequency becomes stable.
I've recommend you instead of 555, 1/4 of CD4093 in astable mode. R can be
greatest than in 555 astable mode and the circuit is stable.
Vasile

On Tue, 4 Sep 2001, Anand Dhuru wrote:

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2001\09\06@115428 by Harold M Hallikainen

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On Tue, 4 Sep 2001 22:52:03 +0530 Anand Dhuru <.....ardhuruKILLspamspam@spam@VSNL.COM> writes:
> Thanks for the various suggestions, guys. Unfotunately, I'm still
> with the original problem after trying out all of them.
>
> Perhaps if I explain the application, it would be easier to see a
> solution.
>
> The 555 is configured as an astable. Ra (the one between pins 7 & 8)
> is actually a resistance ladder of 10 discrete resistors. This
> assembly is to be used as a sensor in a water tank. The idea is, as
> the water level rises, it would progressively short out each
> resistance, and thus the 'on-time', or the pulse width would change.
> This change does not have to be linear; I can have a look up table
> in the PIC. Also, I am not interested in measuring absolute
> quantities; just a percentage, in steps of 10%.
>

       OK, here's another approach (I just love redesigning other people's
stuff!). How about putting a fully insulated probe down into the water
tank. The water acts as a grounded plate of a capacitor while the
conductor in the insulated probe is the first. As the water goes up, the
"plate area" increases, increasing the capacity between the probe and
ground.
       A similar trick is used to measure water contamination in oil pipelines.
Here, an insulated probe is stuck in the metal pipe. The instrument
measures the capacity between the probe and the pipe. Due to the
variation in dielectric constant between oil and water, the typical
capacity varies between 20 pF (oil) and 200 pF (water).

Harold


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Lighting control for theatre and television at http://www.dovesystems.com

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