Thank you tinkergnome. I'll have to look it over again as i was not able to find what i was looking for right away.
I have however looked at the diagrams again and i think i got it.
If you look at this link:
https://github.com/Ultimaker/Ultimaker3/blob/master/PCB files/2012-K Print head PCB/Schematic and layout pdf/Schematic Prints_printhead_2012-K_01.PDF
To the far right. right next to the "B" where it says "hotend 1". I think these are the 5 connections from the print head to the print core. wire 1 and 2 goes to the thermistor. Which data can be found on the far left right next to the "C". I assume this is linear. I tried plotting the resistance vs the degrees and it seemed linear.
Wire 1 (the output from the thermistor) is then connected to a analog to digital converter. So measuring the resistance from wire 1 to wire 2 gives me the degrees in celcius.
Wire 3 - where it says "1-wire_1" is connected to and IO that converts it to a i2c signal. I assume that this is the EEPROM that gives information about the print core - i dont really need this. I googled DS2482-100 to figure this out.
Wire 4 and 5 is heater 1 negative and positive. And from the far left top i assume this is 24 volts.
So what is wire 1 to 5 on this diagram?
Well the third wire is obviusly in the middle. And since the fourth from the right is connected to more than one thing i assume this is ground. Making the numbering from left to right being 5,4,3,2,1.
So what one needs to do to measure the degrees, hence the resistance over the thermistor, is to make some sort of cirquit that can measure the resistance and input it to an analog input in arduino or raspberry pi.
We then need a 24 volt power supply and probably a relay so this can be controlled from an arduino or raspberry pi. If the relay is fast enough i guess you can use a pwm to control the heater.
Some sort of PID controller in the arduino so you can set the degrees is probably a good idea.
I am pretty sure that's how it works. When i get the parcel with the spare print core i will test this theory. And hopefully i'll remember to update this thread. If i haven't posted anything in a week remind me.