Humidity sensor

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Has anyone used the GE HS12P or HS15P humidity sensors? If so, what drive
circuitry did you use with it in order to interface to a uC?

Alternatively, does anyone have any other favourite humidity sensors instead
of this one that they could point me at?

TIA.

Rog.



Re: Humidity sensor

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Well, Honeywell make some that can be connected easily (i.e. directly) to a
microcontroller A/D input.

<http://content.honeywell.com/sensing/prodinfo/humiditymoisture/

You can get them from Farnell.


--

John Devereux

Re: Humidity sensor
On Thu, 10 Jun 2004 11:07:07 +0100, "Roger"

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I don't know about these specific sensors, but if they are like other
bare sensors that I have seen, they can be easily damaged by running
too much current through them for too long - especially DC.  Since you
are interfacing with a uC, take advantage of that fact and provide
intelligent drive to the sensor.  Leave it totally unpowered most of
the time.  Then, when you need to take a measurement, switch on the
power just for the duration of the measurement.  I think they require
AC drive because of the chemical reaction that takes place at the
terminals.  If you use DC, then electrolytic deposits build up in the
sensor, and it degrades.  I think this is the same for pH sensors.  So
keep the average current as close to zero as possible, and limit the
instantaneous current too.  Perhaps you should use a low-leakage .33
uF cap in series with the sensor just to ensure that the average DC
current is zero.



-Robert Scott
 Ypsilanti, Michigan
(Reply through this forum, not by direct e-mail to me, as automatic reply
address is fake.)

Re: Humidity sensor
no-one@dont-mail-me.com (Robert Scott) writes:

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True for resisitive (polymer) humidity sensors. Not so for pH
sensors, which are DC voltage sources (with awfully high impedance).

One possible configuration for a resistive sensor is to use it
as a part of an oscillator. The output is then in frequency,
which makes the uC interfacing easy. In many cases a simple
relaxation oscillator will do.

However, do not expect vary accurate results from a resistive
pH sensor. They tend to have very high temperature coefficients,
and the unit-to-unit variation is significant. If the application
requires only the knowledge whether there is too much water around,
resistive sensors are an inexpensive and adequate solution.


- Ville

--
Ville Voipio, Dr.Tech., M.Sc. (EE)

Re: Humidity sensor
says...
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address is fake.)

The HS12P and HS15P are not bare sensors; they consist of a small PCB
with a polymer sensor and a conditioning circuit.  They want regulated
5VDC, and output 1-3V proportional to RH.

--Gene

Re: Humidity sensor

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Sensirion SHT11. Simple to drive (I2Cish interface),
surprisingly accurate. IIRC, Farnell has them.

- Ville


--
Ville Voipio, Dr.Tech., M.Sc. (EE)

Re: Humidity sensor

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                                      ^^^
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"ish" is right!  Fortuitously, it will work on an I2C bus (IIC if you
prefer) with other devices implementing the "real" I2C protocol as it
is specified.

I have always been curious (rhetorical question) -- why the I2Cish
interface instead of a "standard" implementation?  Licensing?  It
didn't make a lot of sense to me.  Oh well, it works and it talks, so
I used it and moved on.

--
Dan Henry

Re: Humidity sensor
snipped-for-privacy@sprynet.com says...
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I2C is the intellectual property of Philips.  Any device sold as I2C
must be registered with Philips (who will grant you a device identifier)
and a royalty paid.  Thus, many "pseudo" I2C implementations abound.

--Gene

Re: Humidity sensor

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Those "pseudo" I2C implementations appear to use the standard signal
sequences but avoid calling it "I2C".  Sensirion, on the other hand,
not only avoids "I2C", but also makes use of unconventional signal
sequences (e.g., their so-called "Transmisstion Start" sequence).  I
guess they're being extra careful.

--
Dan Henry

Re: Humidity sensor

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We asked this question (non-rhetorically) from the Sensirion
people. The answer was that they did not want to pay royalties
to Philips.

I am not quite sure if they would have had to pay anything
even if they had realized a full I2C. AFAIK, making something
that is interoperable with I2C does not require any licence
-- as long as that something is not called I2C. TWI is a commonly
used acronym for "this is I2C but we do not want to call it I2C
in order to avoid Philips".

I do agree that the almost-but-not-quite I2C on the Sensirion
sensors is annoying. Especially because the sensors are otherwise
so good.

- Ville

--
Ville Voipio, Dr.Tech., M.Sc. (EE)

Re: Humidity sensor

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So there we have it.  Thanks for clearing that up.

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Yes, I've run across the "TWI" term with other devices.

Sensirion sure would have made life easier for all of us and probably
any support staff they may have if they had simply gone down the "TWI"
path to avoid paying royalties and had not taken it to the next level
of messing with the signalling.

--
Dan Henry

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