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'[PIC]: LM335'
2001\02\07@043209 by Peter May

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I gave up on the DS18S20's and went to a simple LM335. It says in the data
sheet I can run a long cable to it and use it remotely. In the finished
product it will be within a couple of inches of the PIC but for bench
testing I want about 20 feet of cable. Would this introduce much noise into
the analogue input?

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2001\02\07@052658 by Vasile Surducan

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I don't know this device but if you plan to connect to an analogic input
from a source with low output impedance through 20 feet of cable you have
to use a shielded cable either a switched pair gnd-signal.
Vasile


On Wed, 7 Feb 2001, Peter May wrote:

{Quote hidden}

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2001\02\07@054604 by Thys Van Tonder

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Hi there Peter

The LM335 gives you 10mV/°K. So you better use shielded cable. I used the
LM35 with about 4m
of cable and it worked fine for our application, a bit of switching noise
from the motors we were probing. You can average out your reading in the
software.

Regards
Thys

{Original Message removed}

2001\02\07@061628 by Vasile Surducan

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Thys, can you tell me how do you scale the output signal of LM35 to obtain
full AD pic resolution and how do you convert for displaing celcius
degrees. AT 0C LM35 output is about 2.7315V and depends of biasing
current. At 100C will be about 3.73V For a 0..5V AD input, probably you
need a diferential amplifier or for a pic16F877 to set -ref at 2.73 and
+ref at 3.73 but in this case you will not have 10 bit AD resolution.
Is there any more simple method to do that without external amplifiers
and of course without any one wire or i2c temperature measurement devices.
Thank's very much,
Vasile

On Wed, 7 Feb 2001, Thys Van Tonder wrote:

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> {Original Message removed}

2001\02\07@063120 by Peter May

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Well what I do is my reff voltages are 0 and 5 volts. I store the analogue
input into a word. I feed the word value to a computer and calculate the
voltage from there. Multiply that by 1000 and then dived that by 10 gives
you the degrees Kelvin. Take of 273.15 and that should give you degrees
Celsius.

{Original Message removed}

2001\02\07@070520 by info

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We achieved what you want but needed a dual op-amp!
We used a dual op-amp and used one side to offset the reference of the
sensor to 0 volts and then used the other side of the op-amp to apply
enough gain to get 100 degrees at 4 volts. This gives me a 40 mV per
degree input to the ADC. This range means a simple right shift of the
ADRESH and you have the temp in centigrade in the register. We wanted to
simplify it further by replacing the op-amp, any ideas?
Scott



In message <spam_OUTPine.LNX.4.20.0102071236130.4083-100000TakeThisOuTspamL30.itim-cj.ro>,> Vasile Surducan <.....vasileKILLspamspam@spam@L30.ITIM-CJ.RO> writes
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>> {Original Message removed}

2001\02\07@074545 by Thys Van Tonder

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Vasile

The LM35 gives you an output of 10mV/°C so you do not have to convert Kelvin
to Celcius. So
you get 2 bits/°C. If you use a opamp X5 you get 50mV/°C and 5v at 100°C
(Maybe X5 is a bit too match) The code can do the rest.
Hope it helps you?

Thys



{Original Message removed}

2001\02\08@021020 by Vasile Surducan

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The same solution for scaling I've used at
http://geocities.com/vsurducan/c520.htm
But is not the simplest way for interfacing...
Vasile

On Wed, 7 Feb 2001, info wrote:

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> >> {Original Message removed}

2001\02\08@021034 by Vasile Surducan

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Peter, I presume you've done that using a C compiler isn't it ?
If you have something in asm to share I would be interested...
Thank's Vasile


On Wed, 7 Feb 2001, Peter May wrote:

> Well what I do is my reff voltages are 0 and 5 volts. I store the analogue
> input into a word. I feed the word value to a computer and calculate the
> voltage from there. Multiply that by 1000 and then dived that by 10 gives
> you the degrees Kelvin. Take of 273.15 and that should give you degrees
> Celsius.
>
> {Original Message removed}

2001\02\08@022916 by Vasile Surducan

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Thank's Thys but my problem is to simplify the hardware.
You have right, if 0...5V is 1024 bites 10mV is 2.048 bit.
But this is right only if the input signal is scaled from 0 to 5V
I don't want to scale usind OA the output of lm35.
This tehnique I've used for interface the lm35 with oldest 7107
http://www.geocities.com/vsurducan/4temp.htm
I was thinking to the same method with pic, changing both references
at desired values.
But my test shows it wouldn't work.
Or probably we talking about different versions of LM35 ?
Vasile


On Wed, 7 Feb 2001, Thys Van Tonder wrote:

{Quote hidden}

> {Original Message removed}

2001\02\08@043459 by Peter May

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Sorry. No I did it in software written in Delphi....

-----Original Message-----
From: pic microcontroller discussion list
[EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU]On Behalf Of Vasile Surducan
Sent: Wednesday, 7 February 2001 10:41 PM
To: PICLISTspamspam_OUTMITVMA.MIT.EDU
Subject: Re: [PIC]: LM335


Peter, I presume you've done that using a C compiler isn't it ?
If you have something in asm to share I would be interested...
Thank's Vasile


On Wed, 7 Feb 2001, Peter May wrote:

{Quote hidden}

the
> > LM35 with about 4m
> > of cable and it worked fine for our application, a bit of switching
noise
{Quote hidden}

data
{Quote hidden}

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2001\02\09@040116 by Peter L. Peres

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>simplify it further by replacing the op-amp, any ideas?
>Scott

Remove the opamp and increase the resolution of the A/D using another PIC
pin to output 'noise' produced in software, which can be added to the A/D
input or to the bias circuit of the LM335. Then average the readings in
software. With 0-5V A/D you have ~20mV/AD step. You need to improve the
resolution four times to resolve 1/2 degree. That means that you need to
average 16 measurements.

Now the challenge: How can the A/D tris control and the A/D output pin be
manipulated to output the 'noise' directly on the A/D pin before switching
it to A/D mode and measuring. Assuming that the LM335 is connected through
a T circuit with two 110R resistors in series and a 0.01 uF capacitor
being the stem of the T to gnd. There should be no need for other external
components.

Peter

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2001\02\09@043322 by Vasile Surducan

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Well Peter, what you write here looks as a perpetuum mobile with drag
chain...
You can't biasing LM35 just like specification said. You can't increase
the ad resolution for 5 times adding noise...
Hardware is to switch from 2.7-3.7V range into 0-5V range.
Don't forget LM335 is almost the same like a usual zenner so it has
a low output impedance. Trying to modulate low impedance output signal
with noise through a small capacitor is impossible.
Vasile



On Thu, 8 Feb 2001, Peter L. Peres wrote:

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2001\02\10@154646 by Peter L. Peres

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Vasile, thank you for the compliment with the perpetuum mobile, but I
think that it works. I have good resons for that since I once wrote an A/D
that works like that and attained 10 bits resolution in the range that
interested me (near 2V input).

In short, you turn the PIC pin to output for a certain (short) time and
output a 1 or a 0. The T filter circuit acts as the integrator. Then the
PIC pin is turned input and A/D conversion is done. The small charge
change in the 0.01 uF capacitor induced by the output pulse biases the LSB
in the desired direction. By using 16 different combinations (length and
polarity) of output pulse and 16 measurements a resolution improvement of
4 bits should be possible. It is possible that the value of the capacitor
will need to be changed somewhat. I'll need to simulate the circuit to
tell ;-)

Peter

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2001\02\12@015929 by Vasile Surducan

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Hi Peter,
I hope you are not obset about my inocent joke...
But you could tell me you've allready test the method.
Sound's very interesting this integration biasing method.
So, input signal near 0 C must be mixed first with a negative voltage (
negative pulse ) analogic read it, input signal near 100 C  mixed with
a positive voltage (positive pulse ) again analogic reading and probably
a diference between this two measurement gave us the modified slope.
But this means, variable lenght pulse are required and first a low
resolution measurement is required to know where is the real signal...
Sound's too complicated.
Vasile


On Sat, 10 Feb 2001, Peter L. Peres wrote:

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2001\02\13@191453 by Peter L. Peres

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>Sound's very interesting this integration biasing method.
>So, input signal near 0 C must be mixed first with a negative voltage (
>negative pulse ) analogic read it, input signal near 100 C  mixed with
>a positive voltage (positive pulse ) again analogic reading and probably
> a diference between this two measurement gave us the modified slope.
>But this means, variable lenght pulse are required and first a low
>resolution measurement is required to know where is the real signal...
>Sound's too complicated.
>Vasile

Maybe I did not make myself clear enough, I am sorry. I used a 16C711 with
A/D (8 bits) and improved the resolution of the existing A/D by another
two bits using the method I have described.

Peter

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