is it possible to connect Arduino to the thermistor, which is simultaneously connected to the thermometer?
I want to get something like:
I know it is wrong but I hope you get a point of what I would like to achieve.
The thermistor is probably a part of a voltage divider in the TMP-900. And the TMP-900 probably reads the voltage at the divided point. If Arduino reads the same point, it might affect the voltage, but probably not. The internal resistors are probably mega ohms, while the voltage divider is probably kilo ohms. With the right connection (not your) it could probably work. Just check that the max voltage in the TMP-900 is the same as in your Arduino. And that you connect GND on both things.
Lots of "probably" there, but you get the point.
If all my assumptions were right, it should be enough to locate the division point in the voltage divider in the TMP-900 and connect it to an analog input of the Arduino. And connect the GND between the two.
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Definitely not a way as on your picture.
You have a short circuit on your diagram - you connected GND directly to 3v3 pin
As about the question - typical thermistor scheme includes three wires - the power, the GND and the signal. But you have only two available outside the TMP-900. If one of the two wires is GND - the mission can be completed by measuring the signal voltage relatively to GND. If GND wire is not available, I think that is not possible without additional hardware.
I have only two wires from TMP-900 to the thermistor... one is 5V the other is ground I guess. so do you mean to connect the TMP-900 ground wire into analog Arduino?
Do you have details on the TMP-900? I'd be concerned about the 220V and a possible ground loop.
unfortunately, I do not have any details... on the internet, I only found some warnings not to exchange connection between L1 (220V) and N (zero) position otherwise it can go to the thermistor... so from this it can mean that somehow TMP-900 N-zero wire is connected with ground from thermistor? not sure if it could be.
thermistor can't be connected such way, one wire is a resistive divider point always. The other line can be either VCC (5v) or GND
It could be ... it is a case in many cheap impulse power suplies
Does your device show which are the 220V and N terminals?
I found some TPM-900 info at several places, and they don't seem to have indications showing which is L1 and which is N.
I also saw a 12V AC/DC version which seems safer:
I haven just found there are two different wirings on the internet on how to connect thermistor to Arduino using voltage divider, are both correct?
That's a good tip that it is not and isolated circuit. Connecting it wrong may expose you to lethal voltages.
I do not recommend trying to interface that device with an arduino
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How do you know that is not "N"? It looks like someone scribbled on a picture.
yes, I draw a red circle on the picture myself. I know it is L (220V) in this position because I have this device connected and working with my boiler and I measured the voltage on the input wire. Now I am looking for a way how to connect it to my Arduino simultaneously.
What is the pin in the left upper part of the image2 - in the green ellipse? - I can't read the label
And why did you connect it to the 3.3 volt pin?
this is AREF pin - it is used to set reference voltage for ADC see analogReference() - Arduino Reference
I found both examples (images 1 and 2) on the web when I was searching for how to connect a thermistor to Arduino. The point is that in image 2. the wiring is power supply->10K resistor -> split to Analog input and thermistor -> ground
while in image 1. the wiring is power supply->thermistor -> split to Analog Input and 10K resistor -> ground.
To me, it seems that image 1. is the correct wiring but some people are using image 2. instead...
I do not know which one I should use.
It might be L, the way you have it connected, but the other terminal N isn't "ground". There's a number of ways it could be built on the inside, and in some cases it could be dangerous, like 220V-into-your-USB-cable dangerous.
Why not use a separate thermistor? You'd need to calibrate your Arduino for a thermistor either way, and with a separate thermistor you could avoid some potentially deadly mistakes.
Any of these connections can be used, you just need to adjust the program code to correctly calculate the temperature depending on the selected connect option.
Added:
Of course, none of these options are suitable for "sharing" TMP-900 thermistor with Arduino.
you are right, it is not ground indeed but N (zero) my bad...
... not easy to use another thermistor as I cannot insert it into the boiler easily...
@riham88
If you still going to share TMP-900 thermistor - why did you asked about the connections of separate thermistor ? You can't use these schemes for TMP-900 connection.