Scaling voltage measurment cct.

Archiver

Two series resistors. Voltage divider.
ACJacques

moparjody gravada:

> Dunno what happened to my last post?
>
> Anyways, what I want to do is measure a voltage using a similar
> circuit as is in
>
> http://www.parallaxinc.com/downloads/Resources/Stampworks/StampWorks_E
> xperiment_27.PDF
>
> This uses a national semi. Serial ADC to read in a variable voltage.
> This is for 5 volts, but the problem is, I want to read 12VDC.
>
> What is a quick and dirty way to scale this before putting it into
> the ADC?
>
> Thanks
> Jody
>
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Archiver

Dunno what happened to my last post?

Anyways, what I want to do is measure a voltage using a similar
circuit as is in

http://www.parallaxinc.com/downloads/Resources/Stampworks/StampWorks_E
xperiment_27.PDF

This uses a national semi. Serial ADC to read in a variable voltage.
This is for 5 volts, but the problem is, I want to read 12VDC.

What is a quick and dirty way to scale this before putting it into
the ADC?

Thanks
Jody

Archiver

For Jody

Connect a pot across your 12VDC input and ground. Take the wiper of the pot
to the ADC input, and set the ADC input at 5VDC with a full 12VDC across the
pot. At full scale - 12VDC the ADC will think it is seeing 12V.

In the processing section of your ADC read routine, the output of the ADC
will appear as "result" or something similar. For instance

AD0 = result

Now then you have to increase AD0 until it equals 12 VDC.

AD0 = AD0 * 2.4

The Stamp can't handle a decimal so you have to trick it a little by writing

AD0 = 48*/2 ' This will give you the equivalent of AD0 * 24.

At this point your ADC output will be something like 12000 if you are using a
12-bit ADC. To get this to a usable value, write:

serout 16, N9600, [noparse][[/noparse]"Voltage = ", dec AD0/1000, ".", dec3 AD0]

This is the general approach. It you are using an 8-bit or 10-bit ADC then
the numbers will have to be adjusted accordingly. If I can be of further
help let me know.

Sid

Archiver

For Jody

Where I wrote 48*/2. change it to
AD0 = 240*/100

That will make it a bit more manageable.

Sid

Archiver

In a message dated 2/11/02 10:00:31 AM Central Standard Time,
jody.gallant@s... writes:


> What is a quick and dirty way to scale this before putting it into
> the ADC?
>

Create a voltage divider with a precision (1%) resistors.

-- Jon Williams
-- Parallax (Author of "StampWorks")


[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

What size POT?

Jody

--- In basicstamps@y..., Newzed@a... wrote:
> For Jody
>
> Connect a pot across your 12VDC input and ground. Take the wiper
of the pot
> to the ADC input, and set the ADC input at 5VDC with a full 12VDC
across the
> pot. At full scale - 12VDC the ADC will think it is seeing 12V.
>
> In the processing section of your ADC read routine, the output of
the ADC
> will appear as "result" or something similar. For instance
>
> AD0 = result
>
> Now then you have to increase AD0 until it equals 12 VDC.
>
> AD0 = AD0 * 2.4
>
> The Stamp can't handle a decimal so you have to trick it a little
by writing
>
> AD0 = 48*/2 ' This will give you the equivalent of AD0 * 24.
>
> At this point your ADC output will be something like 12000 if you
are using a
> 12-bit ADC. To get this to a usable value, write:
>
> serout 16, N9600, [noparse][[/noparse]"Voltage = ", dec AD0/1000, ".", dec3 AD0]
>
> This is the general approach. It you are using an 8-bit or 10-bit
ADC then
> the numbers will have to be adjusted accordingly. If I can be of
further
> help let me know.
>
> Sid

Archiver

Jody, I use a 100K Bourns Trimpot in my application.

Sid

Archiver

2 choices, either a voltage divider which will give you 0 to 12v range. Or
use a zener diode connected to the positive point and a resistor to ground,
the connection point between the 2 will read the input voltage minus the
zener voltage this method gives higher resolution. For example a 1N4741 and
a 4.7K resistor to ground will give a range of about 10 to 15 volts.

KF4HAZ - Lonnie

---

Original Message

---

From: "moparjody" <jody.gallant@
> Anyways, what I want to do is measure a voltage using a similar
> circuit as is in
>
> http://www.parallaxinc.com/downloads/Resources/Stampworks/StampWorks_E
> xperiment_27.PDF
>
> This uses a national semi. Serial ADC to read in a variable voltage.
> This is for 5 volts, but the problem is, I want to read 12VDC.
>
> What is a quick and dirty way to scale this before putting it into
> the ADC?
>
> Thanks
> Jody