Wire Question
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I am using an LM34 temp sensor in conjunction with my BS2SX to take
some temp readings. The problem is that what I am measuring is 6-8
feet away from the stamp, and when I use wires in between the LM34
and the stamp, every so often I get a reading that is way off. But
when the LM34 is in the breadboard it works fine. I am using solid
22 guage wire, should I be using something else?
some temp readings. The problem is that what I am measuring is 6-8
feet away from the stamp, and when I use wires in between the LM34
and the stamp, every so often I get a reading that is way off. But
when the LM34 is in the breadboard it works fine. I am using solid
22 guage wire, should I be using something else?
Comments
the stamp end (0 volts for the stamp supply) and the screen left
disconnected at the sensor end.
Chris
Original Message
From: <pstrittmatter@y...>
To: <basicstamps@yahoogroups.com>
Sent: Monday, September 24, 2001 11:38 AM
Subject: [noparse][[/noparse]basicstamps] Wire Question
> I am using an LM34 temp sensor in conjunction with my BS2SX to take
> some temp readings. The problem is that what I am measuring is 6-8
> feet away from the stamp, and when I use wires in between the LM34
> and the stamp, every so often I get a reading that is way off. But
> when the LM34 is in the breadboard it works fine. I am using solid
> 22 guage wire, should I be using something else?
>
>
>
>
>
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>
> > some temp readings. The problem is that what I am measuring is 6-8
> > feet away from the stamp, and when I use wires in between the LM34
> > and the stamp, every so often I get a reading that is way off. But
> > when the LM34 is in the breadboard it works fine. I am using solid
> > 22 guage wire, should I be using something else?
Depending on the cable capacitance, the LM34 output can go into wild
oscillations that show up as bad readings. If you have a 'scope you
can see the instability. The LM34 data sheet proposes a couple of
fixes. The easiest is to put a 3kohm resistor in series with the
output of the sensor, maybe also ~0.1uf across the cable. The
shielding Chris suggested is also a good idea if there is noise
pickup, however, the shielding can aggravate the oscillation problem,
so you will still probably need the resistor.
-- regards,
Tracy Allen
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com
or solid wire?
--- In basicstamps@y..., Tracy Allen <tracy@e...> wrote:
> > > I am using an LM34 temp sensor in conjunction with my BS2SX to
take
> > > some temp readings. The problem is that what I am measuring is
6-8
> > > feet away from the stamp, and when I use wires in between the
LM34
> > > and the stamp, every so often I get a reading that is way off.
But
> > > when the LM34 is in the breadboard it works fine. I am using
solid
> > > 22 guage wire, should I be using something else?
>
> Depending on the cable capacitance, the LM34 output can go into
wild
> oscillations that show up as bad readings. If you have a 'scope you
> can see the instability. The LM34 data sheet proposes a couple of
> fixes. The easiest is to put a 3kohm resistor in series with the
> output of the sensor, maybe also ~0.1uf across the cable. The
> shielding Chris suggested is also a good idea if there is noise
> pickup, however, the shielding can aggravate the oscillation
problem,
> so you will still probably need the resistor.
>
> -- regards,
> Tracy Allen
> electronically monitored ecosystems
> mailto:tracy@e...
> http://www.emesystems.com
>or solid wire?
>
>--- In basicstamps@y..., Tracy Allen <tracy@e...> wrote:
> > > > I am using an LM34 temp sensor in conjunction with my BS2SX to
>take
> > > > some temp readings. The problem is that what I am measuring is
>6-8
> > > > feet away from the stamp
No big difference electrically. Whatever you need for mechanical
considerations. Twisting the wires together (~3 turns per inch)
helps with cancelling magnetic fields. Shielding helps reject
capacitively coupled noise from electric fields. Worst from the
noise standpoint is two independent wires dangling in midair!
However, by bringing the wires close together, twisting and shielding
them, you are increasing their mutual capacitance, which leads to the
oscillation problem in the LM34 output. That then calls for the
resistor between the sensor output and the cable.
-- Tracy