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So, is it safe to assume ground is pin #2 ?

Don't know about "safe", but I'd assume pin 2 was ground, yes.
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ok, so I'll be connection pin #2 to ground on the arduino and pin #1 to one of the analog pin.

I'm kind of uneasy about bringing this voltage into the arduino.  This comes from reading about how thermistor are usually connected to the arduino, using the 5v supply, ground and a analog pin to obtain reading.  In those case the thermistor is using the arduino power supply.

In my case, the power (variable voltage) is coming from the heat pump controller.  Is it ok to bring this into the arduino ?  

Like I already mentionned, I did make this connection over the weekend and I got the reading in the serial monitor.  Just want to make sure I'm not doing something that'll eventually cause problems.

Hopes this makes sense....

Also, I wanted to mention I did calculate a 3rd order polynomial formula based on the table from the manufacturer.  I did it in excel using the chart fonction and trendline.  I actually gives good result.  I don't have the R2 value with me but it was around 0.996 I think.  I also did a 4th order.  Thanks for the help on this.
« Last Edit: July 05, 2011, 09:19:09 pm by nicxsi » Logged

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Hopes this makes sense....


 It would make better sense to me if everywhere you said "digital pin", you say analog input pin instead?

Lefty
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thanks.  changes done.
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Perhaps a lot of people here already figured this out but I just realized (with some help from my brother...) that the heat pump controller is sending a constant current to the thermistor.  That's how I'm we're able to get variable voltage readings from the 2 header pin.

This is different than many of the exemple I had read about using thermistor with the arduino using constant voltage and a resistor.

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This is different than many of the exemple I had read about using thermistor with the arduino using constant voltage and a resistor.

Either method of driving a thermistor can be used, constant voltage or constant current. While constant current drive is usually a little more complex, it allows one to set a constant lower current thus decreasing a possible calibration error cause by 'self-heating' of the thermistor. The voltage developed across the resistor is equal to E= I X R, so the voltage detected will be proportional to the change of resistance of the thermistor only.

Lefty


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I don't have the R2 value with me but it was around 0.996 I think.

Well you've done well then. The best I could get was 0.9971, and that was leaving out the suspicious 180degF reading.

Amanda
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Eventough I haven't yet completely finalized the thermistor reading part of my setup (someone else want yo chime in ?), I'm now looking at the heat pump control portion.

Like I mention earlier, the heat pump controller has a remote switch already built in.  Two headers are present on the board for this. With a jumper cut, the heat pump is now controlled through these header. You just need to put this circuit in either the open or close position to control the system.  

I've checked these headers with my multimeter and got a 5v reading and 1mA.

My initial idea is to use a 5v relay to open/close this circuit. Is this the only way to go ?  Considering the low voltage and current involved, is there a way to achieve thus more simply ?
« Last Edit: July 06, 2011, 07:18:16 pm by nicxsi » Logged

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Considering the low voltage and current involved, is there a way to achieve thus more simply ?

If that is indeed the measured control voltage when open (5v) and current flow (1 ma) when closed, then a any small npn transistor should work, emitter to negative control terminal AND to arduino ground pin, collector to the positive control pin, base to a 1k series resistor to an arduno output pin.

Lefty

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I've read a bit on why I would need a resistor between the arduino and transistor but can you explain how you got to a 1k value ?
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I've read a bit on why I would need a resistor between the arduino and transistor but can you explain how you got to a 1k value ?

It's a very uncritical value for this application, anything from 200 to 20,000 ohms would probably work. A transistor is a current amplifying device, so base current X device gain (beta value) dictates maximum collector current possible. As you only require 1 ma of collector current, the 4 ma of base current would certainly work. It's not critical to value so much as not having a resistor would cause 'short circuit' current through the base and possibly burn out the output pin. It would be like running a led without a resistor.

Lefty
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I went to an electronic store last night. They had a lot of transistor but unfortunately, nobody was available to help choose an appropriate one.

Could someone give me an indication on what I need exactly in terms of transistor ?
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I went to an electronic store last night. They had a lot of transistor but unfortunately, nobody was available to help choose an appropriate one.

Could someone give me an indication on what I need exactly in terms of transistor ?

 Nearly any small signal NPN transistor would work. Popular stocked types might be 2N2222A, 2N3904.
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Using info found on this forum, I was able to connect my heat pump existing thermistor to the arduino board on A05 and GND.  I also kept the connections to the heat pump.  I got the reading to be availble from the web using the ethernet shield and a simple webserver sketch.  Two things I want to validate :

1- When the arduino is not powered up but the wires connected, the heat pump controller results from the thermistor are all wrong.  I guess this is normal but can someone explain ?  I've connected the thermistor wires to a split which sends the signal both to the heat pump controller and to the Arduino. To the arduno the wire are 20awg and run about 50'.  When is use my multimeter on the wires' end, the reading are OK (volt).  Using the multimeter on the cable connected to the arduino pins, I can see the voltage is lower.

2- how can I stabilize the arduino analor readings ?  When using the multimeter on the wires' end, the reading I get is stable.  But in the arduino, it varies a lot.  Not in a significants matter but still.  I know that the ethernet shield can introduce variance but I'm not getting stable reading from the Uno analog pin as well even with the shield disconnected.

The cable lenght I'm using a the moment Is temporary. I just used an old speaker wire I had lying around.

thx
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Anyone ?

Let me know if my description is unclear.

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