BS2 small voltages

Archiver

Hi there.

I'd like some suggested methodologies for reading voltage ranges in the 0
- 200 MV range on the BS2. Ultimately I either need to be able to read a
voltage range or set up an intelilgent "trigger" point of about .1 volt
that triggers a high line ; as it is, the voltage is too small to send the
line high.

Thanks.

Sean T. Lamont, CTO / Chief NetNerd, Abstract Software, Inc. (ServNet)
Seattle - Bellingham - Vancouver - Portland - Everett - Tacoma - Bremerton
email: lamont@a... WWW: http://www.serv.net
"...There's no moral, it's just a lot of stuff that happens". - H. Simpson

Archiver

Hi Sean,

An opamp with a gain of 25 would provide 0 to 5 volts. If you only need the
signal to trigger a logic level, just use a comparator with its reference
set at the desired trigger voltage.

Ray McArthur

---

Original Message

---

From: Sean T. Lamont .lost. <lamont@a...>
To: Basic Stamps <basicstamps@egroups.com>
Sent: Tuesday, January 09, 2001 8:14 PM
Subject: [noparse][[/noparse]basicstamps] BS2 small voltages


>
> Hi there.
>
> I'd like some suggested methodologies for reading voltage ranges in the 0
> - 200 MV range on the BS2. Ultimately I either need to be able to read a
> voltage range or set up an intelilgent "trigger" point of about .1 volt
> that triggers a high line ; as it is, the voltage is too small to send the
> line high.
>
> Thanks.
>
> Sean T. Lamont, CTO / Chief NetNerd, Abstract Software, Inc. (ServNet)
> Seattle - Bellingham - Vancouver - Portland - Everett - Tacoma - Bremerton
> email: lamont@a... WWW: http://www.serv.net
> "...There's no moral, it's just a lot of stuff that happens". - H. Simpson
>
>
>
>

Archiver

On Tue, 9 Jan 2001, Ray McArthur wrote:

> Hi Sean,
>
> An opamp with a gain of 25 would provide 0 to 5 volts. If you only need the
> signal to trigger a logic level, just use a comparator with its reference
> set at the desired trigger voltage.

I know nothing about comparators - could you please briefly explain or
give me a URL reference?

Thanks.

Sean T. Lamont, CTO / Chief NetNerd, Abstract Software, Inc. (ServNet)
Seattle - Bellingham - Vancouver - Portland - Everett - Tacoma - Bremerton
email: lamont@a... WWW: http://www.serv.net
"...There's no moral, it's just a lot of stuff that happens". - H. Simpson

Archiver

> > An opamp with a gain of 25 would provide 0 to 5 volts. If you
>only need the
>> signal to trigger a logic level, just use a comparator with its reference
>> set at the desired trigger voltage.
>
>I know nothing about comparators - could you please briefly explain or
>give me a URL reference?



Hi Sean,

100k 2000
+5 ---/\/\--o--/\/\---common
|0.1
|volt
| |\ 1/2 LM358
'--|-\
| >---P0 stamp
signal -/\/\---|+/
1000 |/

output to Stamp P0 is high when the signal is >0.1 volt, low when
signal <0.1 volt. Voltage level set by resistors or potentiometer.
The 1000 ohm resistor protects the input. Also needs power
connections, pin 8 to +5 volts, pin 4 to common.

You can use an op-amp, such as the cheap LM358, which gives you two
comparators in an 8-pin package for 30 cents. Or you can use a
dedicated comparator chip, such as the LM393, also a dual package for
30 cents. The LM393 would require a pullup resistor on the output.
The difference between an op-amp and comparator is, not much. A
comparator is optimized for speed of operation in switching, while
the op-amp is optimized for stability as an amplifier. For use on
the stamp, there isn't going to be a noticeable difference.

Positive feedback can be added to make the switching snappier:

100k 2000
+5 ---/\/\--o--/\/\---common
|0.1
|volt
| |\ 1/2 LM358
'--|-\
| >----o--P0 stamp
signal -/\/\-o-|+/ |
1000 | |/ |
'----/\/\-'
100k

Switches high at 0.101 volt, low at about 0.0.099 volt. Do you need
to be able to set different thresholds, other than 0.1 volt?

I recall there is something about comparators in Al William's FAQ.

I hope this clarifies it a bit,


-- Tracy Allen
http://www.emesystems,com

Archiver

To add to Tracy's excellent response:

> You can use an op-amp, such as the cheap LM358, which gives you two
> comparators in an 8-pin package for 30 cents. Or you can use a
> dedicated comparator chip, such as the LM393, also a dual package for
> 30 cents. The LM393 would require a pullup resistor on the output.
> The difference between an op-amp and comparator is, not much. A
> comparator is optimized for speed of operation in switching, while
> the op-amp is optimized for stability as an amplifier. For use on
> the stamp, there isn't going to be a noticeable difference.

I have not used the LM358 opamp, but one time we had a design to debug using
many opamps as comparators. The opamps would sometimes latch high or low
when driven hard to one rail, (don't remember which rail), and sometimes
even reverse gain polarity. We fixed the circuit by using genuine
comparators. Don't remember what the opamps were, but since that time we
like to use comparators for that function.

Regards,
Ray McArthur

Archiver

Thanks Tracy,

That was a fine explanation and sketch.

Appreciate it.

Been wondering about comparators for a while now.

Mark

---

Original Message

---

From: Tracy Allen <tracy@e...>
To: basicstamps@egroups.com <basicstamps@egroups.com>
Date: Wednesday, January 10, 2001 11:56 AM
Subject: Re: [noparse][[/noparse]basicstamps] BS2 small voltages


>> > An opamp with a gain of 25 would provide 0 to 5 volts. If you
>>only need the
>>> signal to trigger a logic level, just use a comparator with its
reference
>>> set at the desired trigger voltage.
>>
>>I know nothing about comparators - could you please briefly explain or
>>give me a URL reference?
>
>
>
>Hi Sean,
>
> 100k 2000
> +5 ---/\/\--o--/\/\---common
> |0.1
> |volt
> | |\ 1/2 LM358
> '--|-\
> | >---P0 stamp
> signal -/\/\---|+/
> 1000 |/
>
>output to Stamp P0 is high when the signal is >0.1 volt, low when
>signal <0.1 volt. Voltage level set by resistors or potentiometer.
>The 1000 ohm resistor protects the input. Also needs power
>connections, pin 8 to +5 volts, pin 4 to common.
>
>You can use an op-amp, such as the cheap LM358, which gives you two
>comparators in an 8-pin package for 30 cents. Or you can use a
>dedicated comparator chip, such as the LM393, also a dual package for
>30 cents. The LM393 would require a pullup resistor on the output.
>The difference between an op-amp and comparator is, not much. A
>comparator is optimized for speed of operation in switching, while
>the op-amp is optimized for stability as an amplifier. For use on
>the stamp, there isn't going to be a noticeable difference.
>
>Positive feedback can be added to make the switching snappier:
>
> 100k 2000
> +5 ---/\/\--o--/\/\---common
> |0.1
> |volt
> | |\ 1/2 LM358
> '--|-\
> | >----o--P0 stamp
> signal -/\/\-o-|+/ |
> 1000 | |/ |
> '----/\/\-'
> 100k
>
>Switches high at 0.101 volt, low at about 0.0.099 volt. Do you need
>to be able to set different thresholds, other than 0.1 volt?
>
>I recall there is something about comparators in Al William's FAQ.
>
>I hope this clarifies it a bit,
>
>
> -- Tracy Allen
> http://www.emesystems,com
>
>
>
>
>
>
>

Archiver

Ray McArthur wrote:

>I have not used the LM358 opamp, but one time we had a design to debug using
>many opamps as comparators. The opamps would sometimes latch high or low
>when driven hard to one rail, (don't remember which rail), and sometimes
>even reverse gain polarity. We fixed the circuit by using genuine
>comparators. Don't remember what the opamps were, but since that time we
>like to use comparators for that function.


Good point, and one fine point among many. Both the op-amp and the
comparator will have a "common mode input range". Operating the
inputs outside of that range may give bad results and particularly if
the input goes beyond the power supply rails. The common mode input
range for either the LM358 or the LM393 on a 5 volt supply goes from
-0.3 volt up to about +3.5 volts.

-- Tracy