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Author Topic: Feasibility of tapping into existing thermistor circuit  (Read 146 times)
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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  
« Last Edit: January 03, 2014, 03:15:13 pm by mmorcos » Logged

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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.
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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
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Thank you for your quick replies.

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

Ray
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