I'll have to try the relay idea when I can.

I tried to use the arduino to read the voltages, but I turned the arduino off to see what happens if the battery is flat, this caused the readings to jump to wrong values and therefore would not serve the purpose I need.

I should add - the purpose of this is to monitor the temperatures of a solar hot water collector and water tank, I mainly need to know if it's been above 60oC at least once every 3 days, but also, would like to have the data logged too.

I've thought of using Dallas sensors on the pipes, but this was inaccurate, measuring the voltage outcome from the sensor circuit isn't going to work.

Adding another thermistor isn't going to work as there's no room to.

So this leaves really, checking to see if the relay will work without causing the pump to kick in and out (I think that will be an issue).

Hack the PIC microcontroller on the controller unit to give the data to the arduino, then go wireless from there. I don't know how PIC is coded or pin outs...

Make my own unit to do exactly what it does (monitors the thermistors, if there's a 12 degree difference, kick the pump in, whilst the tank is under a set value). The problem with this is I'd have to dig up a lot of information to get it done right.

The unit by the way is the "SolaStat-ST" controller, it's not the "Plus" version that has RS485 (which would make this so much easier).

Thank you for your help CrossRoads.

Running a second thermistor crossed my mind, getting up on the roof, trying to fit another one in with the current one, it sounds like a second one isn't easily done.

http://www.littlebirdelectronics.com/products/12V-DPDT-5A-Mini-Relay.html
That sounds like what you are referring to.

The circuit in the controller measures resistance from the thermistor, so has resistors already.

I wonder how adding  a relay will change that resistance at all? Would it alter the readings? If not, sounds like the way to go.

Any ideas on a digital switch?

The easy solution for me is to just record the voltage outcomes, build a table and make conclusions of temperature based on the voltage outcome.

However, is there anyway of making the connection to the thermistor a switched one?

Is there anyway, I can have the thermistor cable go to two locations, even if only one could access it at any one time?
(and if this is the case, couldn't that be electrically switched some how)?

1 thermistor - 2 devices using it..

What would happen if I joined two wires at the device end of the thermistor and tried to get resistance measurements (i.e. the thermistor is still attached to the controller, that's powered on, and I've got some cable from the arduino to the controller's thermistor joint)?

Would that work, or would having the resistors there fry something?

Good idea, it works - now I just need to figure out how the value is determined?

I read them both, 1.36V and 1.63V for them both.

I know 1.36V = 52oC
I know 1.63V = 48oC

The voltages go up with the temperature drop, I'll have to figure out how that is calculated and work from there!

I have 2 thermistors, these cable into a controller unit with some sort of logic programmed into it to switch on and off a pump.

The problem for me is I want that data wirelessly, but I can't figure where I'd get the data out from the circuit design they have.

I'm not sure if I could just piggy back on the same cabling, I think this would cause feedback back into the controller unit (or vice versa), so I would prefer to have the ability to switch between them.

Any ideas on how I can have a digital switch between the thermistor and the arduino?

How would you connect to the thermistors?

Oh?

Code:

void setup() {
 myName = "2";
}
void loop()
{
  while (Serial.available())
  {
    delay(10);
    char c = Serial.read();
    readString += c;
  }
  if (readString.length() >0)
  {
    //Commenting this out, you don't need to echo it back to everyone.
    //Serial.println(readString); //prints string to serial port out
   if (readString.substring(1, 2) == myName) { // this if statement gets the unit to check if it's for it.
    if (readString.substring(0) == "START")
    {
      Serial.print("!");
        Serial.print(myName);
        Serial.println("-START-OK");
      readString=""; //clears variable for new input
      Flash();
    } elseif (readString.substring(0) == "RESET")
    {
        Serial.print("!");
        Serial.print(myName);
      Serial.println("RESET-OK");
      readString=""; //clears variable for new input
      Reset();
    } else {
       Serial.print("!");
       Serial.print(myName);
       Serial.println("COMMAND-UNKNOWN");
    readString=""; //clears variable for new input
  } //end if checking for valid command match
} else {
   //do nothing - or something if you need to?
} //end if - command was not for me
}

Let me know if this is what you are aiming for..

In your code, I can't see how you address the devices?

How does each client know the request is for them?

As to your requirement, that seems easy enough:

Code:

   if (readString.substring(0) == "START")
    {
      Serial.println("START detected");
      readString=""; //clears variable for new input
      Flash();
    } elseif (readString.substring(0) == "RESET")
    {
      Serial.println("RESET detected");
      readString=""; //clears variable for new input
      Reset();
    } else {
          Serial.println("Not understood!");
          readString="";
          Reset();
    }

Though if you send Not understood all devices would receive it - so they all need to make sure they ignore requests that aren't for them.

let's assume you want to reset device 5 of 6.

BaseStation sends out #5RESET
Your code should check for and remove #5, and then move to Reset.
If it isn't #5 it should ignore it.

In it's reply, you could do !5RESET-OK

This would let your BaseStation know #5 reset was OK.

That raises questions

I want to read the voltage across a thermistor from another device.
The other device has resistors and the like providing power across the thermistor, so I just want to read that voltage and make my own conclusions from it using the arduino.

So, the 'signal' ground of the sensor, does that need ground in my op amp circuit (the purpose of the buffer circuit, I take it, is to not affect the resistance of the circuit being checked).

If so, should I take the ground and attach it to the circuit ground? Will this impact on the resistance of the circuit being monitored (ideally this will not even know it's being monitored).

Hmm, I didn't need the resistor or the capacitor with the external 9V supply.

Pin 9 PWM = 2.49V from the op amp output

First thing to fix is adding a wire from the 9 volt battery's negitive terminal to a Arduino ground pin. Your external circuitry and the arduino must share the same common reference ground.

That did it!
I'll have to look up why !

Also about that meter? does it really measure something using just one lead?  

Nah (I wish), it's negative is to the breadboard's ground

Please forgive my poor drawing.

I also removed 3.3V (leaving input open), and that gives 7.24V on output to multimeter.

But why do I get 7V when it's 3.3V from arduino (no PWM), and 9V batteries to the op amp circuit?

I'm trying to setup a buffer using the LM358 dual op amp.
I'm supplying it 5V from arduino, connected ground to arduino ground.

+ Input = Pin9 on arduino, analogWrite = 127
- Input = Connected to output
Output - connected to + of multimeter.

Then I have ground from the multimeter to ground of the circuit, which as above goes to the ground on arduino.

Basic Math - 5V is what is put out on the pins at analogWrite 255, so if I use 127, I expect to find ~2.5V.

At the arduino this is the case, I put the multimeter to it and got 2.46V.
I expect to find 2.46V at the output of the LM358.

I find 1.83V.

Why is that?  :-?

Interesting also, I put 3.3V into the opamp (from arduino 3V3 pin) and the multimeter = 3.30V

So it has to be something related to PWM :S

Hooked up 9V batteries to the circuit, 3.3V from arduino registers as 7.2V through op amp.

A relay is what you want then