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'[EE]: A cheap A/D interface'
2002\02\21@103813 by marco genovesi

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Hi all,
I have to interface my 16F84 Datalogger ( 4.5V supply) with sensors that
have a voltage output, but in my board I have only one free pin (RA4), so
I'm thinking to a voltage-frequency converter. I need 8 bit resolution.
For a lot of reasons, I would like a really cheap solution, preferably not
over 2-3$.
I have seen that a 4046 CMOS has a voltage-controlled oscillator, but I
haven't never used it: can it be an acceptable solution for my specs or I'm
going in a wrong way? Have anyone any suggestion about 4046 VCO use or other
low cost alternatives?
Thanks in advance

Marco

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2002\02\21@105245 by Lawrence Lile

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You could use a 12C671 PIC, which has an 8 bit A/D input.

--Lawrence

----- Original Message -----
From: "marco genovesi" <spam_OUTmarco.genovesiTakeThisOuTspamLIBERO.IT>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, February 21, 2002 9:36 AM
Subject: [EE]: A cheap A/D interface


> Hi all,
> I have to interface my 16F84 Datalogger ( 4.5V supply) with sensors that
> have a voltage output, but in my board I have only one free pin (RA4), so
> I'm thinking to a voltage-frequency converter. I need 8 bit resolution.
> For a lot of reasons, I would like a really cheap solution, preferably not
> over 2-3$.
> I have seen that a 4046 CMOS has a voltage-controlled oscillator, but I
> haven't never used it: can it be an acceptable solution for my specs or
I'm
> going in a wrong way? Have anyone any suggestion about 4046 VCO use or
other
{Quote hidden}

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2002\02\21@110124 by Martin Peach

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A CMOS 555 works well, temperature drift may be an issue. The input voltage
has to be kept in the 1/3 to 2/3 Vcc range. You can get very high resolution
on slowly changing signals such as temperature by counting for longer.
Accuracy depends on the linearity of the VCO transfer function, which can be
corrected for if necessary. For higher sampling rates measure the width of
single pulses.
/\/\/\/*=Martin

{Original Message removed}

2002\02\21@125940 by mark

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I would use a LM331, Voltage-to-Frequency converter.

On 21 Feb 2002 at 11:00, Martin Peach wrote:

> A CMOS 555 works well, temperature drift may be an issue. The input voltage has
> to be kept in the 1/3 to 2/3 Vcc range. You can get very high resolution on
> slowly changing signals such as temperature by counting for longer. Accuracy
> depends on the linearity of the VCO transfer function, which can be corrected
> for if necessary. For higher sampling rates measure the width of single pulses.
> /\/\/\/*=Martin
>
> {Original Message removed}

2002\02\22@030204 by Vasile Surducan

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on 16F84 there is a better way than u/f converter, take a look at:

http://www.dattalo.com/technical/software/software.html

there are also a few application note on Microchip on this theme.

best, Vasile



On Thu, 21 Feb 2002, marco genovesi wrote:

{Quote hidden}

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2002\02\22@054610 by marco genovesi

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Martin Peach wrote:

>A CMOS 555 works well, temperature drift may be an issue. The input voltage
>has to be kept in the 1/3 to 2/3 Vcc range. You can get very high
resolution
>on slowly changing signals such as temperature by counting for longer.
>Accuracy depends on the linearity of the VCO transfer function, which can
be
>corrected for if necessary. For higher sampling rates measure the width of
>single pulses.

I have seen 555 but I wasn't very convinced on his precision about
voltage/frequency conversion on this chip (applying a voltage to the CONTROL
VOLTAGE input, pin 5, ok?).
However, i will do some tests on it: i have only a pin free (RA4) and this
solution is compatible.

Marcelo wrote:
>>I would use a LM331, Voltage-to-Frequency converter.

It could be very good for my use, but the price is significantly over the 3$
(in Italy) and isn't very easy to find: however, it seems a bit more
accurate than 555 solution.

To Vasile: many thanks for your hint!  When I'm started in pics I have read
the Microchip App.Notes on this argument, but I supposed that  4 pin were
needed for this kind of measure. Also the code related was too "big" for my
program space.  My actual problem is that the '84 program memory is VERY
near to full and olnly RAa is free, because I use it also for other types of
frequency sensor (example: a 4093-based circuit for water conductivity
measurement).
I think that the code that you have suggested (very interesting) isn't
possible to merge in my current logger code: it is probably the right moment
to start in 16F87xx.

Many thanks to All for suggestions

Marco

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2002\02\22@093257 by Eoin Ross

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Would you be able to send me a copy of the conductivity side of the circuit?
Been looking on the net for something like that and not had much success.

>>> marco.genovesispamKILLspamLIBERO.IT 02/22/02 05:43AM >>>
<snip>

because I use it also for other types of frequency sensor (example: a 4093-based circuit for water conductivity measurement).

Many thanks to All for suggestions

Marco

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'[EE]: A cheap A/D interface (conductivity)'
2002\02\22@104132 by marco genovesi

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part 1 1405 bytes content-type:text/plain; (decoded 7bit)

Hi Eoin,
this is a simple circuit (I'm a newbie in electronic) so I don't know his
limits. I haven't necessity of an accurate measure and it is very essential
(but also cheap). Frequency output It is sensible to V+ variations, so a
regulator will be nice.
The C1 cap. value in the schematic may be very different due to water
hardness. You can start with a 0,1uF and then test other values. I think
this note is important:  to obtain a linear freq. variation versus water
conductivity, is mandatory that the electrodes of the probe will not "free"
but into a confined vane (with open sectors for water).
bye
Marco


{Original Message removed}
part 2 2674 bytes content-type:application/octet-stream; (decode)

part 3 144 bytes
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2002\02\23@053017 by Russell McMahon

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Look at Scott Datallo's "Sigma Delta Converter".
Can't get cheaper than that.
RA4 would not be the best pin for that though.

4046 may be marginal on linearity to achieve 8 bits without quite a lot of
work.
Still be cheapish though.

You can do a fair dual slope VCO with an opamp (even a cheap LM324) plus
perhaps 1 transistor
NatSemi do an app note for this in the LM324 data sheet AFAIR.


     Russell McMahon


> I have to interface my 16F84 Datalogger ( 4.5V supply) with sensors that
> have a voltage output, but in my board I have only one free pin (RA4), so
> I'm thinking to a voltage-frequency converter. I need 8 bit resolution.
> For a lot of reasons, I would like a really cheap solution, preferably not
> over 2-3$.
> I have seen that a 4046 CMOS has a voltage-controlled oscillator, but I
> haven't never used it: can it be an acceptable solution for my specs or
I'm
> going in a wrong way? Have anyone any suggestion about 4046 VCO use or
other
> low cost alternatives?

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2002\02\24@200549 by Russell McMahon

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Look at Scott Datallo's "Sigma Delta Converter".
Can't get cheaper than that.
RA4 would not be the best pin for that though.

4046 may be marginal on linearity to achieve 8 bits without quite a lot of
work.
Still be cheapish though.

You can do a fair dual slope VCO with an opamp (even a cheap LM324) plus
perhaps 1 transistor
NatSemi do an app note for this in the LM324 data sheet AFAIR.


     Russell McMahon


> I have to interface my 16F84 Datalogger ( 4.5V supply) with sensors that
> have a voltage output, but in my board I have only one free pin (RA4), so
> I'm thinking to a voltage-frequency converter. I need 8 bit resolution.
> For a lot of reasons, I would like a really cheap solution, preferably not
> over 2-3$.
> I have seen that a 4046 CMOS has a voltage-controlled oscillator, but I
> haven't never used it: can it be an acceptable solution for my specs or
I'm
> going in a wrong way? Have anyone any suggestion about 4046 VCO use or
other
> low cost alternatives?

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2002\02\25@062901 by marco genovesi

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Ok Russel,  I like opamp solution. I have found in LM358 AppNote a VCO
example with few low cost components. The only problem is a bigger current
consumption than 4046 VCO, if I'm not wrong (my logger must operate for 4-6
months with 3 AA cells), so I will try to build and compare the two types of
circuits.
Many thanks for support

Marco



{Original Message removed}

2002\02\25@072325 by Jinx

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> Ok Russel,  I like opamp solution. I have found in LM358
> AppNote a VCO example with few low cost components.
> The only problem is a bigger current consumption than 4046
> VCO, if I'm not wrong (my logger must operate for 4-6 months
> with 3 AA cells), so I will try to build and compare the two types of
> circuits.
> Many thanks for support

If you change the F84 for an F628 you can get 3 more i/o lines.
The two OSC pins and Mclr. The advantages are

F628 is cheaper than the F84, and is same pin-out

You can use those 3 lines + the one you have spare now to
operate a serial ADC like the TLC549. Use one for its Vcc,
one for CS, one for Clock and one for Data. The TLC549 is
a cheap, fast and accurate 8 pin 8 bit ADC

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2002\02\25@111658 by Thomas McGahee

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You can build a simple voltage to pulse-width converter to do the job.
You can even have multiple "channels" quite easily. The basic idea is
to create a triangle wave that goes from 0 to +5 volts. This is fed
into the (-) input of a comparator or op-amp. The voltage you want to
measure
goes into the (+) input. 1 volt will generate a 1/5=20% duty cycle,
2 volts will generate 2/5=40% duty cycle, etc..

The one triangle wave can feed into any number of comparators, so
you can easily generate multiple channels.

******

The triangle wave method above is self-repeating and so requires only
one PIC i/o line. If you have two i/o lines available, then you
can make a single-slope converter instead.

A single slope converter consists of  a capacitor driven by a constant
current source, and a comparator. The capacitor is discharged, then
allowed to charge. The ramp is connected to the (-) input of a comparator,
and the voltage to be measured is connected to the (+) input of the
comparator. When the output of the comparator goes LOW, the time is
proportional to the voltage. It is useful to have the constant current
source adjustable, so the scaling can be adjusted via a pc pot rather
than having to do it via software.

Note that if the compliance voltage of the constant current source is
less than +5 volts, then you may have to use a simple voltage divider
at the (+) input of the comparator to reduce the max input voltage
to something less than the compliance voltage.

Multiple comparators can be driven by a single ramp circuit, so
each additional analog input requires one i/o pin and one comparator.

Fr. Thomas McGahee

>
>
> > I have to interface my 16F84 Datalogger ( 4.5V supply) with sensors that
> > have a voltage output, but in my board I have only one free pin (RA4),
so
> > I'm thinking to a voltage-frequency converter. I need 8 bit resolution.
> > For a lot of reasons, I would like a really cheap solution, preferably
not
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2002\02\26@110054 by marco genovesi

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Thomas, thank you very much.
I'm a newbie in PIC & electronics, these suggestions are very nice!
mmm.., at the moment I have only RA4 free on 16F84, but i'm starting to port
the logger code on 16F628 with a new design, so i think that it will be
possible free two or three pins.

Marco


{Original Message removed}

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