Want to read temperature sensors in heatingsystem,but they have common positiv.

My solar collector controller has three NTC 10K temperature sensor and one T1000 type.

With my multimeter I can measure about 4V and 1V (depending on the temperature) DC between the two wires for the NTC sensors.

(The T1000 measures between 60 and 50 mV DC depending on temperature, a later issue)

Now I want to read this sensor in parallel to my heating system, without disturbing it too much with my Arduino. Accuracy to about 1°C (about 25 Ohms/K, non-linear) is fine.

I was thinking of just analogread the sensors but I measure that the NTC sensors have common positive.
Is there hope and can go into the world of optamps or does this mean I will end up with both common ground and common positive which sounds really bad.
Please help me out here.

/O

Please give us a circuit of the system so we can understand what you have.

Weedpharma

Differential measurement?

It a SR868C8 solar controller.
I have no schematics for it.

I’ll have to learn more about differential mesurements if you think this is a way to go!?

T2 - T4 are connected to NTC 10K temperature sensors
and the T1 has a T1000 type.

Thanks for your input!

"I was thinking of just analogread the sensors but I measure that the NTC sensors have common positive. Is there hope and can go into the world of optamps or does this mean I will end up with both common ground and common positive which sounds really bad."

Are you saying that with the -Ve probe of your meter on the gnd terminal, one terminal of each of the sensors is the same positive voltage? And the other terminal has a variable voltage with respect to gnd?

Weedpharma

This is what I did:

Measuring one sensor at a time at its two terminals with my voltmeter probes gives a stable voltage that I can effect by changing sensor temperature.

So I thought good. I’ll just check with my ohm meter so they have a common ground.

But no. The probe terminals have no direct connection with the ground (as seen in the #3 picture - output ports).

The "minus" terminal (black voltmeter probe) as no direct connection with the other "minus" sensor terminals.

However the "positive terminal" (red probe when doing voltmeter reading) has a direct connection with the other sensors "positive" terminal.

Hope I made myself clear and hey, thanks for your time! /O

What is the common positive voltage with respect to gnd?

Is there the same R from the sensor pin to gnd. IE from the not positive side.

Weedpharma

So is it safe to read the voltage from "sensor pin" (blue line) to A0 and ground the arduino with gnd (black line)?

/O

Olberg:

So is it safe to read the voltage from "sensor pin" (blue line) to A0 and ground the arduino with gnd (black line)?

/O

I did not say that. I asked questions in order to find out if it is safe.

Weedpharma

I understand and appreciate it very much!

Let me rephrase: Do you see any direct problems with reading the voltage as stated above?

Olberg: I understand and appreciate it very much!

Let me rephrase: Do you see any direct problems with reading the voltage as stated above?

Without knowing the amount of the positive voltage with respect to gnd I cannot say.

Weedpharma

I see. Let's say it is (as measured) 2.35V (as in #8 @21 degrees) and that I've calculated that it won't go over 5V.

Do you see any other direct problems?

For example if I also wish to voltage-read from T3 (to A1)and T4 (to A3) in a similar manner?

You have previously told us that there is a COMMON positive voltage, IE, common to all sensors. So one terminal of each of these sensors has the same voltage as the same terminal of the other sensors. This should not vary if they are COMMON. What is this voltage with respect to the Gnd terminal?

Without knowing the voltages, I cannot say if it is safe.

What I am assuming is that there is a common positive voltage. There is then a connection to the sensor and the return side goes to a resistor to gnd.

I suspect that it MAY be wired as you would using an analogue input to the arduino with the input pin tied to gnd. Without knowing the COMMON positive voltage, I cannot suggest any connection to the Arduino.

Does the voltage with respect to the gnd terminal (on the non positive common terminal), go up or down with increasing temperature?

Weedpharma

You have previously told us that there is a COMMON positive voltage, IE, common to all sensors. So one terminal of each of these sensors has the same voltage as the same terminal of the other sensors. This should not vary if they are COMMON. What is this voltage with respect to the Gnd terminal?

As in picture #8. It is 5.05V

And I´ve checked. It does not vary with the temperature.

The voltage is there even when I disconnect the temp sensor.

What I am assuming is that there is a common positive voltage. There is then a connection to the sensor and the return side goes to a resistor to gnd.

Yes I think you are right. Impressive!

Does the voltage with respect to the gnd terminal (on the non positive common terminal), go up or down with increasing temperature?

Voltage goes up with increasing temperature.

All good but will I run into problems if I also wish to voltage-read from T3 (to A1)and T4 (to A3) in a similar manner?

There is 10.7K ohm between the non-positive terminals.

/O

From this I would suspect that what you have is as I suggested earlier.

When the sensor heats up, the R reduces so a greater voltage appears on the 10k R that is connected to earth.

Your Arduino input should be ok reading the voltage across the R to earth. I would not advise going across the sensor as you would have to common the 5v supply when conventionally it is the gnd that is common.

The other sensor may be similarly wired but you will need to check this.

I would be interested in others commenting on the arrangement in case I have missed something.

Weedpharma

Think you for your help. I'll try this out after work.

5.05V eh?

Strongly suggests the machine is using 5V logic and analog circuits.

If its internals suggest this - 7805 or similar regulator and HCMOS logic chips, then you should have not trouble at all with the voltages and indeed, could probably even power your Arduino from that 5.05V. :D

It works!
This is for anybody else traveling down this road:

It is a 7805 inside which can power the arduino, thank you Paul__B.

Here is the code with coefficients for the thermistor A01 used for the solar collector controller SR868C6.

#include <math.h>

double Thermistor(int RawADC) {
 double Temp;
 Temp = log(10000.0*((1024.0/RawADC-1))); 
//         =log(10000.0/(1024.0/RawADC-1)) // for pull-up configuration
 Temp = 1 / (0.00101650271305106 + (0.0002554500515440731 + (0.00000001181580396970194 * Temp * Temp ))* Temp );
 Temp = Temp - 272.15;            // Convert Kelvin to Celcius
 //Temp = (Temp * 9.0)/ 5.0 + 32.0; // Convert Celcius to Fahrenheit
 return Temp;
}

void setup() {
 Serial.begin(9600);
}

void loop() {
 Serial.println(int(Thermistor(analogRead(0))));  // display Celcius
 delay(100);
}

Program to calculate Steinhart-Hart coefficients:
http://thermistor.sourceforge.net/

/O