Bristol Digest, Vol 539, Issue 3

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Today's Topics:

1. Opamp - somewhat off topic (Andrew)
2. Re: Opamp - somewhat off topic (Andrew)
3. Re: Opamp - somewhat off topic (Dave SMITH)
4. Re: Opamp - somewhat off topic (Dave SMITH)
5. Re: Opamp - somewhat off topic (Andrew)
6. Re: Opamp - somewhat off topic (Dave SMITH)
7. Re: Opamp - somewhat off topic (Andrew)


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Message: 1
Date: Mon, 24 Feb 2014 12:44:48 +0000
From: Andrew <andrewsoltau@gmail.com>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: [bristol] Opamp - somewhat off topic
Message-ID: <530B3EC0.7040209@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

Greetings luggers

Still progressing slowly on the project to get a raspberry pi working to
fill ink cartridges.

We are using a load cell to measure the amount of ink. It gives an
output of about 5 mV. As stated by a helper "The input range on the
Custard Pi 3 is from 0 to 2.5V. So to get the best accuracy from this
load cell, we need an amplification of around 400 using an opamp."

Does anyone know if I can get this opamp off the shelf, or whether it
would have to be custom made.

Thanks

Andrew



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Message: 2
Date: Mon, 24 Feb 2014 12:46:18 +0000
From: Andrew <andrewsoltau@gmail.com>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B3F1A.8050604@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

On 24/02/14 12:44, Andrew wrote:
> Greetings luggers
>
> Still progressing slowly on the project to get a raspberry pi working
> to fill ink cartridges.
>
> We are using a load cell to measure the amount of ink. It gives an
> output of about 5 mV. As stated by a helper "The input range on the
> Custard Pi 3 is from 0 to 2.5V. So to get the best accuracy from this
> load cell, we need an amplification of around 400 using an opamp."
>
> Does anyone know if I can get this opamp off the shelf, or whether it
> would have to be custom made.
>
> Thanks
>
> Andrew
>
ps I forgot to mention that I have done a load of googling but I am not
coming up with anything I understand.
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Message: 3
Date: Mon, 24 Feb 2014 13:56:05 +0100
From: Dave SMITH <Dave.Smith@st.com>
To: <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B4165.1020102@st.com>
Content-Type: text/plain; charset="ISO-8859-1"

On 24/02/2014 13:46, Andrew wrote:
> On 24/02/14 12:44, Andrew wrote:
>> Still progressing slowly on the project to get a raspberry pi working
>> to fill ink cartridges.
>>
>> We are using a load cell to measure the amount of ink. It gives an
>> output of about 5 mV. As stated by a helper "The input range on the
>> Custard Pi 3 is from 0 to 2.5V. So to get the best accuracy from this
>> load cell, we need an amplification of around 400 using an opamp."
>>
>> Does anyone know if I can get this opamp off the shelf, or whether it
>> would have to be custom made.

You would use a standard OpAmp that has a very large gain, and add a few
resistors around it to set the gain at x400 in non-inverting mode.




------------------------------

Message: 4
Date: Mon, 24 Feb 2014 14:13:36 +0100
From: Dave SMITH <Dave.Smith@st.com>
To: <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B4580.1090305@st.com>
Content-Type: text/plain; charset="ISO-8859-1"

On 24/02/2014 13:46, Andrew wrote:
> ps I forgot to mention that I have done a load of googling but I am not
> coming up with anything I understand.

Have a look at the "closed loop, non-inverting" section of the OpAmp
page on Wikipedia. There's a circuit diagram showing how you connect up
an OpAmp with two resistors to define the gain you want.



------------------------------

Message: 5
Date: Mon, 24 Feb 2014 13:25:59 +0000
From: Andrew <andrewsoltau@gmail.com>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B4867.2040304@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

On 24/02/14 13:13, Dave SMITH wrote:
> On 24/02/2014 13:46, Andrew wrote:
>> ps I forgot to mention that I have done a load of googling but I am not
>> coming up with anything I understand.
> Have a look at the "closed loop, non-inverting" section of the OpAmp
> page on Wikipedia. There's a circuit diagram showing how you connect up
> an OpAmp with two resistors to define the gain you want.
>
Thanks Dave. That looks really simple.

How do I define what I am looking for as the opamp itself, on ebay or
elsewhere. This is the bit I was getting really frustrated with - it
seems like a whole language defining these kinds of things.

Andrew
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Message: 6
Date: Mon, 24 Feb 2014 14:49:34 +0100
From: Dave SMITH <Dave.Smith@st.com>
To: <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B4DEE.20406@st.com>
Content-Type: text/plain; charset="ISO-8859-1"

On 24/02/2014 14:25, Andrew wrote:
> How do I define what I am looking for as the opamp itself, on ebay or
> elsewhere. This is the bit I was getting really frustrated with - it
> seems like a whole language defining these kinds of things.

You just want a general-purpose OpAmp. I don't have any datasheets to
hand so I'm not sure what to recommend I'm afraid. Many years ago, the
standard would've been the 741, but that's been well and truly
superceeded nowadays. When I've got a bit of time (I'm currently at
work in a meeting), I'll do a bit more digging around to give you some
suggestions.

In reality, however, virtually any OpAmp you can buy will probably do
what you want, as your requirements are not particularly taxing.




------------------------------

Message: 7
Date: Mon, 24 Feb 2014 13:58:36 +0000
From: Andrew <andrewsoltau@gmail.com>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Opamp - somewhat off topic
Message-ID: <530B500C.50508@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

On 24/02/14 13:49, Dave SMITH wrote:
> On 24/02/2014 14:25, Andrew wrote:
>> How do I define what I am looking for as the opamp itself, on ebay or
>> elsewhere. This is the bit I was getting really frustrated with - it
>> seems like a whole language defining these kinds of things.
> You just want a general-purpose OpAmp. I don't have any datasheets to
> hand so I'm not sure what to recommend I'm afraid. Many years ago, the
> standard would've been the 741, but that's been well and truly
> superceeded nowadays. When I've got a bit of time (I'm currently at
> work in a meeting), I'll do a bit more digging around to give you some
> suggestions.
>
> In reality, however, virtually any OpAmp you can buy will probably do
> what you want, as your requirements are not particularly taxing.
>
>
Great. Thanks. My colleague is about to go off and commission the
building of purpose specific circuit boards, but I was sure the things
we want must be available off the shelf. I would be very grateful if you
could point me to a specific source. It would be sad if I wind up buying
the one thing which looks as if it would do, but doesn't!

Andrew

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