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[steven r]

Anyone have an idea how I would detect when one zone (outside) in cooler than another zone (indoors)?

I can handle the X10 end of it if I could figure out a way to know when it is few degrees cooler outside. The application I'm considering is for a window fan.

[Oldtimer]

Anyone have an idea how I would detect when one zone (outside) in cooler than another zone (indoors)?

I can handle the X10 end of it if I could figure out a way to know when it is few degrees cooler outside. The application I'm considering is for a window fan.

Try a Google search on: "differential thermostat". 

This do it yourself kit was on the first search page: JC-Solarhomes: Differential Thermostat Kit @ $29

I didn't go any further.  Please let us know what you finally decide on so we can all benefit.  It's a common question.


[TTA Edit: Added description to LINK.]

[steven r]

I thought about google after I posted but don't recall seeing the link you found. I had searched on "differential temperature switch" so that might be the difference. I guess my brain was in the "have any of the experts here tried it mode." i.e. Has anyone here made or purchased a solution they like.
If I decide to pursue it, your link seems reasonable.
Thanks.

[Oldtimer]

Years ago I looked into differential thermostats several time for various home automation applications but the price always stopped me from proceeding since the cost benefit simply wasn't there in my cases.  It looks like that may have changed lately but I don't have an application at the moment.

[TakeTheActive]

Anyone have an idea how I would detect when one zone (outside) in cooler than another zone (indoors)?

...figure out a way to know when it is few degrees cooler outside. The application I'm considering is for a window fan.

"Thinking-out-load Mode":

• Is the EXACT temperature difference needed?
   - If not, depending on the contents of your JunkBox, 2 'old-fashioned' (leftover?) mercury heating thermostats connected to PowerFlash Modules could serve as flags:
   - - OUTSIDE Temp below 70 (switch closed, Module ON); INSIDE Temp above 80 (switch open, Module OFF); DO FAN ON
  
  
• I have a fuzzy recollection of something called a 1-wire temperature probe; possibly related to an add-on module for a StarGate or Ocelot Controller. You could SEARCH over at their forums for ideas...
  
  
• Oldtimer's LINK looks pretty good *IF* you have to spend money anyway and want it to be adjustable and EXACT. It's been SO MANY YEARS since I studied IC circuits / theory / etc..., all that comes to mind now is '741 Op Amp', right-pointing triangle with + input on top, - input on bottom and output on right, which is what I believe JC-Solarhomes is doing.
  

[steven r]

...Is the EXACT temperature difference needed?...

No. I just don't want to blow hotter outside air into the house. Just enough difference that the fan wouldn't cycle every few sec. At this time of year in GA, its already warm during the day but it does cool down enough at night to not need AC.

[gil shultz]

Differential Temperature Sensor

It is not to hard to do if all you want to know which side is hotter.  I do not know your level of expertise so I am assuming you have some electronic knowledge in analog circuits. None of the values are critical so try what you have. 

You need two thermistors (same value) probably less then a dollar each.  Two resistors same value about the same resistance as the thermistors, a resistor about 125X the value of the other resistors, Resistors less then $0.10 each, an op-amp (LM-358 dual op-amp) probably less then $1.00, decoupling capacitors (0.01 uf,  22 uf) and a power source. 

Operating DC voltage of the electronics is not critical as the unit operates in differential mode, pick what is convenient for you. NOTE it must be low voltage DC less then 30 VDC is fine.

Put appropriate length of leads on each of the thermistors.  Connect one wire from each to the power -, connect the other wire from each to the resistors.  Connect the other end of each resistor to power +.  Connect the junction of each thermistor and resistor to one of the inputs of the op-amp.  Connect the last resistor from the output of the op-amp to the + input.  Connect the + input of the second op-amp to the output of the first.  Connect the – input of the second op-amp to it’s output.  The output of the op-amp will go either to the + voltage or – voltage depending on which thermistor is warmer.  The feed back and the difference between on and off is controlled by the third resistor.  If it is too wide of a gap increase the resistance.  If the delta is to narrow then just decrease the resistance.  The output is low current and supplies only a few Ma.  Connecting from output of the op-amp to your X-10 is in your hands as you requested.

If there is a demand for the schematic I can generate one if I catch the posting.

I saw that a 741 was mentioned, it would work OK but needs more voltage and preferred a + and – supply.  The LM358 is about 30 years old and very stable and a common part.  The second op-amp helps isolate the sensing from the load.

Gil Shultz

[steven r]

Differential Temperature Sensor...
...If there is a demand for the schematic I can generate one if I catch the posting....

A schematic would help me get a clearer picture of how to wire everything.
Thanks for your post.

[JeffVolp]

Anyone have an idea how I would detect when one zone (outside) in cooler than another zone (indoors)?

If you have a higher-end automation controller, such as the Ocelot, it is easy to do.  Units like that provide the ability to measure real-world parameters, and act on them.  We have several temperature monitors (cheap thermistors) that monitor outdoor and indoor temperatures in several locations to control ventilation fans exactly like you want to do.  Monitoring the outdoor temperature also allows me to adjust the irrigation cycle depending on the amount of heat stress on the plants.

Jeff

[steven r]

Reminded of my original post here (thanks Oldtimer), I want to update my need and get some clarification on the original suggestions. With warmer weather on the way, I'm interested in pulling the project off the back burner.

I have a window fan that I'd like to come on if the room gets over say 70^o but only if the outside temperature is say less than 65^o. Ideally I'd like the temperatures to adjustable but would accept fixed settings if it simplified things. My first draft idea is to use 2 cheap thermostats, as TakeTheActive suggested above, one on the outside and one on the inside of the house wired in series and connected to a DS10A sensor. This should work but would be a kludgey approach. Also I expect there would be mounting problems as well as weather proofing problems for the outside thermostat.

My circuit knowledge is limited but I can can handle a basic soldering project. I tend to need a bit of hand holding and clear instructions, however.

Oldtimer: Is your suggestion basically a kit version of gil shultz suggestions?
$49 ($29 + two $10 temperature probes) is a bit more than I was thinking the project would cost but I'm ok if that is the best and most reliable solution. Also I guess I'll need to spend a few bucks on 12V power supply. Am I correct that the kit would close the circuit for a DS10A while remaining power isolated. (Did I say that right?)

[Oldtimer]

Oldtimer: Is your suggestion basically a kit version of gil shultz suggestions?
$49 ($29 + two $10 temperature probes) is a bit more than I was thinking the project would cost but I'm ok if that is the best and most reliable solution. Also I guess I'll need to spend a few bucks on 12V power supply. Am I correct that the kit would close the circuit for a DS10A while remaining power isolated. (Did I say that right?)

steven r I've researched this again and still think this kit is the best deal around for what you need.  I also believe it is a kit version of what gil shultz was describing.  Finally, the relay in the kit should isoloate the wiring to the DS10A.  From the pictures it would appear to be a very simple soldering job, just don't use too big an iron or you'll damage components.  20 Watts or so is probably OK as long as you're fast.  The instructions should cover that point.

I've rethought the application since my last post and still don't see any other way out than using a differential thermostat.

[dave w]

OK since this old thread might be resurected I am in the "thinking out loud mode",  tinkering with a more expensive but somewhat simpler idea:

I was thinking of using two cheap home heating thermostats and two "Power Flash" modules. I was going to mount the "outside" thermostat on a protected porch or in a weather proof box. I use Homeseer for automation control, but Homeseer, AHP, etc would turn on an attic fan based on "inside" being hotter than "outside".

FWIW  

[Oldtimer]

OK since this old thread might be resurected I am in the "thinking out loud mode",  tinkering with a more expensive but somewhat simpler idea:

I was thinking of using two cheap home heating thermostats and two "Power Flash" modules. I was going to mount the "outside" thermostat on a protected porch or in a weather proof box. I use Homeseer for automation control, but Homeseer, AHP, etc would turn on an attic fan based on "inside" being hotter than "outside".

FWIW  

The problem is the simple thermostats only work at one temperature whereas a differential thermostat works over a wide range of temperatures.

Check out this thread again for simple thermostat control of an attic fan.  It's practical because the enclosed attic, particularly with the fan off, will always be hotter then the outside air.

http://www.x10community.com/forums/index.php?topic=13275.msg71811#msg71811

[EL34]

I am doing it via my Dallas one wire network and the X10 SDK.
I have a bunch of sensors all over the place that are on a dallas one wire network.
I recently integrated my one wire network into X10 via a x10 SDK and using visual basic

I can read any of the sensors on my one wire network, make decisions via software and then turn on and off X10 modules based on those decisions.

I can read temp sensors, humidty, rain gauges, etc.

I can compare two temp sensors and turn on one or several X10 devices.

A bit technical, but it can be done if you use the X10 SDK and do some programming.