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'How to measure 0 to 0.24 VDC with a PIC16F877A ?'
2003\02\25@021211 by Augusto Yipmantin - OA4CVT

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Hi Group,

May you guide me on how to measure a source from 0 to 0.24 VDC with a
PIC16F877A ?
The 0 to 0.24 VDC is a source from a VU meter (0.24 VDC means full scale)
What I need is to measure the voltage then when it is over 0.17 VDC need to
set on a pin to drive a relay.

Any idea is wellcome.

Regards,

Augusto, OA4CVT

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2003\02\25@034211 by Tan Chun Chiek

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I think a simple non-inverting amplifer, a comparator + a few discretes is
all that's needed to achieve your purpose.

Chun Chiek
{Original Message removed}

2003\02\25@054816 by Werner Soekoe

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I have some ideas, using a comparator built into the 16F877.

Get a 1.2V voltage reference unit
(http://www.national.com/pf/LM/LM385-1.2.html will do) , and divide the
voltage using resistors in serial. You'll have to use 1% tolerance resistors
for this to work well. Since you want to switch the relay when the voltage
exceeds 0.17V, I'll work on that value. the 0.17V will be 14.2% of the value
of the voltage reference. Use, lets say a 12k and a 82k resistor, the 12k
connected to ground and the 82k to your voltage reference. The centre tap
and common point for the two resistors will then be 0.153V. Connect this to
the voltage refence of the PIC, and the VU meter's signal to the positive
input of the comparator. In code then program it to trigger the relay when
the comparator triggers the Vin > Vref state. This should work.

I've never used it myself before, but the operation is rather straight
forward. You should also compensate in code for fluctuations, as I assume
you'll using the sound source for this purpose, and that is obviously a
variable signal which goes poth positive and negative in respect to ground.
But never the less, I hope this helps somewhat.

Cheers
Werner

{Original Message removed}

2003\02\25@074001 by Thomas C. Sefranek

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The Analog to Digital converter will give you 19.6 mV resolution steps
unmodified.
So you could use it to find .17647 volts or higher.
If you need finer resolution you would have to add an op-amp (Gain of 20)
and then read the result with the A-D.  That would get you .98 mV
resolution.

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> {Original Message removed}

2003\02\25@100526 by Ned Konz

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On Monday 24 February 2003 11:09 pm, Augusto Yipmantin - OA4CVT wrote:

> May you guide me on how to measure a source from 0 to 0.24 VDC with
> a PIC16F877A ?
> The 0 to 0.24 VDC is a source from a VU meter (0.24 VDC means full
> scale) What I need is to measure the voltage then when it is over
> 0.17 VDC need to set on a pin to drive a relay.

If you don't need too much resolution, you could just connect it to an
A/D input and read it with the A/D converter.

If you use a 5V reference (i.e. not adding any extra components), at
0.24V input you'll have 1024*0.24/5V full scale, or a reading of
about 49.

You can read this periodically, perhaps adding some averaging (how
well smoothed is this voltage?).

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2003\02\25@103820 by Augusto Yipmantin - OA4CVT

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Hi Ned,

I4m trying to "read" the voltage from a Audio Decibel meter, when the needle
of the VU meter is at middle of scale ( 0.17 V ) I need to check it for
about 30 seconds and then set any other pin to high to drive a relay for
about 5 minutes.  If the time is less than 30 seconds over the  ( 0.17 V )
no action over the relay will be taken.  Accuracy of 90% arround the 0.17 V
will be enough.
I need to read this at least one time per second from the VU meter in fast
mode (not in average mode).

Augusto


{Original Message removed}

2003\02\25@104651 by Jai Dhar

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Quoting Augusto Yipmantin - OA4CVT <.....oa4cvtKILLspamspam.....EC-RED.COM>:

> Hi Ned,
>
> I4m trying to "read" the voltage from a Audio Decibel meter, when the needle
> of the VU meter is at middle of scale ( 0.17 V ) I need to check it for
> about 30 seconds and then set any other pin to high to drive a relay for
> about 5 minutes.  If the time is less than 30 seconds over the  ( 0.17 V )
> no action over the relay will be taken.  Accuracy of 90% arround the 0.17 V
> will be enough.
> I need to read this at least one time per second from the VU meter in fast
> mode (not in average mode).

Using the full scale from 0 - 5V would be sufficient for your case then. Since
there are 1024 steps, you will basically have a resolution of 5/1025, or
~0.005V, which is more than you required. Considering you would only be using
values from 0 - 49, you wouldn't even have to consider using the full 10-bits
of the ADC (which makes working with the registers easier since you only need
one register, the LSB - ADRESL). If you need some sample code of using an ADC
with an f877, let me know.

>
> Augusto
>
>
> {Original Message removed}

2003\02\25@110524 by Augusto Yipmantin - OA4CVT

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I will appreciate very much a little example code on how to handle the data
with the ADC of the F877

Many thank4s in advance.

Augusto


{Original Message removed}

2003\02\25@145446 by jim barchuk

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Hi Augusto!

On Tue, 25 Feb 2003, Augusto Yipmantin - OA4CVT wrote:

> I will appreciate very much a little example code on how to handle the data
> with the ADC of the F877
>
> Many thank4s in advance.
>
> Augusto

--> http://www.microchip.com
----> Application Notes
------> By Function

See the articles on DAC and also Display/LED which includes 'Four Channel
Digital Voltmeter with Display and Keyboard.' That's a full fetured app;
all you're looking for is the initialisation plus about a dozen lines of
code.

There're tons of HOWTOs out there if you learn how to find them. Google
for it too.

Have a :) day!

jb

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2003\02\25@152857 by Peter L. Peres

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Use a low offset opamp that includes ground in its input voltage range and
connect it as DC amplifier with 18k 1% + 1k 1% as feedback and 1k 1% to
ground. This should make 4.8V out of your 2.4V.

Peter

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2003\02\25@211732 by Jai Dhar

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That's the basic code you need. Make sure your banking is right (I extracted
this out of a lot more code). This should be all you need tho. Result will be
in ADRESH and ADRESL.

ADStart

movlw 0xC1           ;Select A/D on, Channel 0 (RA0) and RC Clock
movwf ADCON0


bsf ADCON0, GO
Loop1

btfsc ADCON0, GO
goto Loop1



retlw 0x00

Quoting Augusto Yipmantin - OA4CVT <KILLspamoa4cvtKILLspamspamEC-RED.COM>:

> I will appreciate very much a little example code on how to handle the data
> with the ADC of the F877
>
> Many thank4s in advance.
>
> Augusto
>
>
> {Original Message removed}

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