Re: Looking for Temp and humidity sensors

[Marc F Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Sun, 8 Jan 2006 17:49:45 -0500, "Robert Green"
<ROBERT_GREEN1963@xxxxxxxxx> wrote in message
<cZednZkgjNcqBVzeRVn-gQ@xxxxxxx>:

>"Marc F Hult" <MFHult@xxxxxxxxxxxxxxxxxxxxx> wrote in
>
><stuff snipped>
>> Some very accurate temperature sensors (a silicon diode for example) cost
>> 1/1000th of what I'd speculate that a  HAI temperature module costs  And
>> calibrated thermistors, Dallas 1-wire devices, and LM34/35's cost on the
>> order of US$1 and so enjoy about a 40-fold cost advantage compared  to
HAI
>> (and Adicon and ELk and Aprilaire ) temperature _modules_.
>
>Why *IS* that?  I was dismayed by the price and the restricted function of
>the ADI "Bobcat" line of temperature and humidity sensors.  I see that
>Worthington has some very cheap sensors listed with their ADI, HAI and ELK
>offerings.  I would ideally line to monitor temperature and humidity info
>for perhaps 20 different points in the house, with a wide array of
>temperatures, too (chimney temps, hot water heater, fridge, etc).
>
>Monitoring temperature and humidity (as well as minute barometric pressure
>differences between rooms) from a lot of different locations *easily and
>cheaply* would be a very good way of making sure that everything in the
>house was functioning normally.  With Bobcats, that will run me about
$2500.
>:-(  We'll see how well Jeff Volp's analog I/O -solution works for me when
>I get around to ordering the equipment.  That depends on how well I
>predicted Christmas and travel related expenses.


I suggest that you close your pocketbook for now.

Let's back up a bit.

The Compleat Home Automation Controller needs to cause actions based on
environmental thresholds (aka "events", "triggers") *AND* be capable of 1)
measuring, 2) interpreting (performing calculations in order to define an
event) 3) recording (storing) and 4) reporting continuously variable
parameters such as temperature, pressure, illumination level and so on.

Most of the current crop of HA controllers and programs to a credible job at
event-based control, and some are adequate for many purposes at measurement
and using those measurements to define event thresholds.

But despite whatever comp.home.automation's well-intentioned "brand
advocates" might lead folks to believe, the devices known in these parts as
Home Automation Controllers (Adicon Ocelot, HAI, Elks etc) are mediocre or
useless for general-purpose monitoring purposes for a variety of reasons.

The answer to your question of why these sensors cost so much lies in large
part in that fact. If one needs only a few sensors, the add-on modules may
be cost effective and have adequate performance for many purposes. However,
as you have found, this approach becomes prohibitively expensive as you add
more modules.

If one compares the capabilities of today's HA controllers with even
20-year-old environmental data loggers one is immediately struck by:

1) Inadequate signal resolution (and in the case of some, precision) of the
analog-to-digital converter (Adicon/Ocelot, Elk)  or no analog input (HAI)

2) Inadequate data storage/retention.

There are excellent, inexpensive solutions.
Do check out www.phanderson.com for starters.

The key to good analog measurement is proper signal conditioning. I will
post the design for the conditioned four-sensor pcb where I posted the
16-channel analog input multiplexor a couple of weeks ago.

http://www.econtrol.org/analogsensors.htm

I use a previous version inside commercial motion detectors to provide
temperature, light level, humidity and motion detection in a single device.

With the addition of a Dallas 1-wire DS2450 Quad A/D converter
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2921
the data can be sent digitally or as analog signals. The DS2450 can provide
up to 16-bit resolution which makes use of high-precision, inexpensive
thermistors practical.

More later ... Marc
Marc_F_Hult
www.ECOntrol.org