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### Topic: Feasibility of tapping into existing thermistor circuit (Read 644 times)previous topic - next topic

#### mmorcos

##### Jan 03, 2014, 09:05 pmLast Edit: Jan 03, 2014, 09:15 pm by mmorcos Reason: 1
Hi all,

First post here. I tried searching but couldn't find what I needed.

I'm trying to tap into my thermostat's thermistor voltage divider to get the same reading on my arduino as the thermostat's uC. The idea is to probe the thermistor voltage divider using the arduino.

I'm worried that by doing this that I will 'load' the circuit and the thermostat's temperature readings will be perturbed. Am I wrong? If so, are there any tricks to do this?

Thanks,

Marc

#### shooter

#1
##### Jan 03, 2014, 10:06 pm
yes probably the load is too high,but with a simple transistor and a resistor in the collector this is solvable
the collectorvoltage will follow the basevoltage.
an opamp will do this even better now you can even have a gain.
paul deelen
shooter@home.nl
making controls with codesys PLC and arduino

#### mrburnette

#2
##### Jan 04, 2014, 02:01 am
Quote
The idea is to probe the thermistor voltage divider using the arduino.

You need to know if the positive-side of the thermistor-resistor voltage divider is @ +5V.  If so, then you can simply connect the Arduino anaslog input to the junction through a 1K quarter Watt resistor.  The input impedance on an Analog pin is well above 100K, typically in the megohms at DC.

If the voltage is greater than +5 Volts, then the simplest way to do what you desire is not to do it... Seriously, simply use your own thermistor placed adjacent to the one in the thermostat.  Use a minimum of 10K with a 10K NTC to avoid self-heating issues!

Ray

#### mmorcos

#3
##### Jan 04, 2014, 04:15 am
Thank you for your quick replies.

Ray: What's the point of the 1k resistor if the analog input has an impedance of 100k?

#### mrburnette

#4
##### Jan 04, 2014, 04:28 am
Quote
What's the point of the 1k resistor if the analog input has an impedance of 100k?

In the event any of your Arduino toys pulls the line High or Low, the 1K will protect the expensive thermostat!  Protecting you from yourself

Ray

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