Searching \ for 'Using 12C67x and a CdS cell' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: techref.massmind.org/techref/index.htm?key=using+12c67x+cds
Search entire site for: 'Using 12C67x and a CdS cell'.

Truncated match.
PICList Thread
'Using 12C67x and a CdS cell'
1999\03\30@120832 by wft

face
flavicon
face
Having discussed bit and pieces of monitoring a CdS cell with PICsters
, I now want to elicit final suggestions.

The CdS cell varies from 9K to 500K ohms resistance.
To take a sample of the cell resistance 3 times per second,
should I use the CdS to charge up a cap and read that
with an A/D?  If so, what is the minimum cap size?
OR should I charge a cap until it crosses the digital threshhold
of a digital input and time that?  Same size cap?

I think I require a resolution of about 1 part in 25.

Any and all responses are gratefully endured...

Gus

--
Y2K   The biggest block party on the planet
http://www.frii.com/~wft/Y2K.html    Let's party ! ! ! ! !

Gus Calabrese    Lola Montes      WFT Electronics
4337 Raleigh Street      Denver, CO 80212
303 964-9670......voicemail         spam_OUTwftTakeThisOuTspamfrii.com
http://www.frii.com/~wft

Alternate:   791 High Street     Estes Park, CO  80517
if no success with .....wftKILLspamspam@spam@frii.com, try ....
wftspamKILLspambigfoot.com             then .....wftKILLspamspam.....eudoramail.com

1999\03\30@123724 by Wagner Lipnharski

picon face
hmm, why should you charge a cap?
Why not just connect the CdS in series with a low ppm resistor and to a
stable Voltage Reference, so you could measure the voltage over the CdS?
For a more linear response, why not use a constant current through the
CdS? 5uA would generate 2.5V when the CdS is 500kOhms and 45mV when it
is 9kOhms, kind of 5mV/kOhm.

Gus Calabrese wrote:
{Quote hidden}

1999\03\30@134635 by Gerhard Fiedler

picon face
At 10:00 03/30/99 -0700, Gus Calabrese wrote:
>should I use the CdS to charge up a cap and read that
>with an A/D?  If so, what is the minimum cap size?
>OR should I charge a cap until it crosses the digital threshhold
>of a digital input and time that?  Same size cap?

just to add this to what wagner said, this method was primarily invented to
read analog values if you =don't= have an adc (you measure the time it
needs to charge up to a certain threshold).

ge

1999\03\30@135045 by Andy Kunz

flavicon
face
At 10:41 AM 3/30/99 -0800, you wrote:
>At 10:00 03/30/99 -0700, Gus Calabrese wrote:
>>should I use the CdS to charge up a cap and read that
>>with an A/D?  If so, what is the minimum cap size?
>>OR should I charge a cap until it crosses the digital threshhold
>>of a digital input and time that?  Same size cap?
>
>just to add this to what wagner said, this method was primarily invented to
>read analog values if you =don't= have an adc (you measure the time it
>needs to charge up to a certain threshold).

I use it to extend the precision of the internal ADC's.  Check the archives
for this sort of thing from 2-3 years ago posted by me, with assembly
source.  Just like Walter's schematic.

Andy

  \-----------------/
   \     /---\     /
    \    |   |    /          Andy Kunz
     \   /---\   /           Montana Design
/---------+   +---------\     http://www.montanadesign.com
| /  |----|___|----|  \ |
\/___|      *      |___\/     Go fast, turn right,
                              and keep the wet side down!

1999\03\30@154121 by Lawrence Lile

flavicon
face
Try microchip's app note AN512.  That is the basic method for pseudo-a/d
input to a PIC without using a real a/d input.  I've used this idea for many
years, recently began using it to generate 16 bit a/d without an expensive
a/d input.

Realistically, you won't get 16 bits of accuracy.  Your accuracy will
increase if you filter your input by various techniques.  I either take a
median or an average, with median being preferred (check my web page for
details on median filters) because of no math and no rounding errors.

Best Regards

Lawrence Lile
>> AutoCad Blocks for electronics
>>PIC stuff incl MEDIAN FILTER
>> PEN PLOTTER to PC board conversion
>> Amateurish pictures of me and my recumbent bicycles
HTTP://home1.gte.net/llile/





{Original Message removed}

1999\03\31@003525 by Tjaart van der Walt

flavicon
face
Gus Calabrese wrote:
>
> Having discussed bit and pieces of monitoring a CdS cell with PICsters
> , I now want to elicit final suggestions.
>
> The CdS cell varies from 9K to 500K ohms resistance.
> To take a sample of the cell resistance 3 times per second,
> should I use the CdS to charge up a cap and read that
> with an A/D?  If so, what is the minimum cap size?
> OR should I charge a cap until it crosses the digital threshhold
> of a digital input and time that?  Same size cap?
>
> I think I require a resolution of about 1 part in 25.
>
> Any and all responses are gratefully endured...
>

Gus, read the following page on Bytecraft's site :
(http://www.bytecraft.com/addaconv.html)

To interface with *any* A/D chip will take up much
more effort and code than to use this technique for A/D
conversion. If you only need a reading every 300ms, you
can get 8 bits of resolution by sampling a pin (digitally)
at around 853Hz. Nothing gets easier than that. By selecting
your two resistor values carefully, your total A/D code
will be under (and I am guessing now) 25 assembly lines.

--
Friendly Regards          /"\
                         \ /
Tjaart van der Walt        X  ASCII RIBBON CAMPAIGN
RemoveMEtjaartTakeThisOuTspamwasp.co.za  / \ AGAINST HTML MAIL
|--------------------------------------------------|
|                WASP International                |
|R&D Engineer : GSM peripheral services development|
|--------------------------------------------------|
| Mobile : spamBeGonetjaartspamBeGonespamsms.wasp.co.za  (160 text chars) |
|     http://www.wasp.co.za/~tjaart/index.html     |
|Voice: +27-(0)11-622-8686  Fax: +27-(0)11-622-8973|
|          WGS-84 : 26¡10.52'S 28¡06.19'E          |
|--------------------------------------------------|

1999\03\31@110836 by Wagner Lipnharski

picon face
Tjaart is correct.
Resolution bits in an A/D conversion is an exponential problem. To run 9
bits resolution instead only 8, you need to double the circuit accuracy
and precision. 16 bits ADC uses 256 more accuracy and stability than a
simple 8 bits. It is a lot.

In a 2V analog levels, 16 bits means 30.5uV of resolution. It is very
common to find values much bigger than that measured just in the ground
tracks of a PCB. So you can imagine the problems and care needed to work
with higher bit count.

Suppose you want to show the CdS cell readings in 4 digits like;
"9999", you should use a minimum of 15 bits (16384 max count), with the
last digit of accuracy, which is 0.01%. If you go to 16 bits (65536) you
would have an extra digit (not shown at the display), what gives you an
accuracy of 0.001% close to good instrumentation.

A common metal film resistor has 100ppm ("100 parts per million", means
0.01% resistance change per ¡C), it means that doesn't matter if you
spend $18 in a AD7714 22 bits ADC (resolution: 0.000028%) if you are
limiting the input accuracy using 0.01%/¡C resistors dividers / signal
conditioning...

For sure, 8 bits can be used in lots of applications, and it is. It
offers 0.39% (1/256) of resolution, so you need to swing the temperature
39¡C to the resistors reach the same percentage of error.

I just wanted to show what happens when you need a little bit better
accuracy.
Wagner.


Tjaart van der Walt wrote:
> To interface with *any* A/D chip will take up much
> more effort and code than to use this technique for A/D
> conversion. If you only need a reading every 300ms, you
> can get 8 bits of resolution by sampling a pin (digitally)
> at around 853Hz. Nothing gets easier than that. By selecting
> your two resistor values carefully, your total A/D code
> will be under (and I am guessing now) 25 assembly lines.

More... (looser matching)
- Last day of these posts
- In 1999 , 2000 only
- Today
- New search...