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[on:]

I was curious if something like this will work!

Thermister:
+ shared to two arduinos
- to one arduino, and grounds connected to the arduinos together.

Then I can have a 10k resister on the + of the thermister at each side and have no differences in reading at each side?

i.e
10K Thermister has one side of Analog Input 1 from two arduinos connected (A1 from Arduino 1, A1 from Arduino 2).   The other side goes to ground on one of the arduinos, and both arduinos have the grounds connected together.

Would the measurement of temperature at each one then be identical?

What I want to do is similar, I'm just curious if something like that would work? The reason I ask is I gather a thermister connection requires readings from the mid point of a 10k circuit (i.e. thermister on one side, 10k resister on the other, temperature reading is at the mid point of that).

It was suggested to use UnityGain Amplifier - not sure how that works for it's input?
http://www.electronics-tutorials.ws/opamp/opamp_5.html
Whilst looking in to op amps, I came across this page, now will I be able to take the Positive side of the thermister and use some sort of following circuit to SHARE the input with both controllers?

[Reply #1 on:]

The analog input is just sensing a voltage - so long as the supplies and ground are the same you should get the same reading (+/- about one LSB).

[Reply #2 on:]

Well it senses resistance right?
That's what the problem will be I thought?

So, if I instead - use the resistance measurement circuit on one controller, and the other, I take that same midpoint and use a op amp in a unity follower setup -  with a common ground reference that would work I take it?

[Reply #3 on:]

If I take the resistance measurements directly, and output the measurements to a digital pot, would that work?

So, in the place of the current thermistor on the old circuit, I put the output of a DS1803 digital pot.

Will that work ? There's a 10k resister in the way though, so does that need to get removed? Does a DS1803 get directly connected to ground and the analog input or does it also require a 10k resister ?

[Reply #4 on:]

For this I plan on using a TL072 to follow the voltage.

I just want to be clear on how I'll connect - the TL072 + input is the same + from the thermistor (before the resistors ?), and then I input this to an analog input on the arduino (or do I also need to get the output of TL072 to the resistor first to get the same result as both controllers?)

I plan on doing some tests, but would like to be sure...

Does having a thermistor on all the time use a lot of current? Will the result of a thermistor be different if you only power it for a few microseconds before reading, when needed instead?

[Reply #5 on:]

Does having a thermistor on all the time use a lot of current?

Depends on the resistance of the thermistor and the voltage passing thru it.
You will have the thermistor in a voltage divider: http://en.wikipedia.org/wiki/Voltage_divider#Resistive_divider
So add the resistances in series and calculate power with P = V*V/R.

You should be able to share the thermistor but I'd use a cap to ground from the thermistor/divider output to reduce input impedance.

[Reply #6 on:]

That was the reason it was suggested to use TL072 - it has a very high impedance so as to not cause interference on the circuit.

With respect to the capacitor - I'm not sure where I should pick up the input from the thermistor?
I can get it from before the resistors on the other side, or after it - in fact I could go as far back as the MCU that's reading it - is that the ideal place to measure from?

What will happen is I'll get the voltage reading and build an array of the voltage to temperature values and that should give me close enough to a reference of the temperature being read by the other controller..

What I think will work is:
Thermistor -> Existing controller.
Then solder to the existing input (either at the thermistor, before the resistors, or after the resistors, at the MCU), cable to bring it out to TL072 Op Amp following the voltage reading.
The TL072 as a voltage follower will feed the same voltage reading into my Arduino and it will reference it's table of voltages to decide what temperature it is.

So, do I get the TL072 input from the + of the thermistor (i.e. the field side of the thermistor), or from the MCU side which will be after the resistor (i.e. the midpoint)?

[Reply #7 on:]

So, do I get the TL072 input from the + of the thermistor (i.e. the field side of the thermistor), or from the MCU side which will be after the resistor (i.e. the midpoint)?

You want to measure at the midpoint as that is the output of the divider.
If you measure without the divider, you will be measuring the supply voltage on the other side of the thermistor, not what you want...

Take a look at the picture at the top of this post:
http://n0m1.com/2012/01/28/adc-battery-voltage-divide-match-and-measure/
This is for measuring battery voltage, but in your case R1 or R2 will be a thermistor and the voltage will vary with temperature. 
The post explains the need for the capacitor and the math to calculate the output voltage as well...