text: Q:
  
  How can I make the dimensioning of a temperature monitoring circuit?
  
  I need to design a circuit to measure temperatures in the range 15-40 degrees C (this will later be the basis for a regulator of a peltier-element for a fridge).
  I tried a circuit using the bandgap-reference of 2 transistors, but it did not work. Now I would like to use a circuit with a transistor used as a diode. I could only come up with a transistor (with B and C connected together), in series with a resistor (for a supply voltage of 12 V I will choose a resistor of 11.3...12 k). But I think this circuit is too simple. 
  I also found this circuit, but I don't know how to make the dimensioning of the components.
  
  A:
  
  You'll want to drive the diode (or diode connected transistor) with a current source.  10uA is a nice value.  An easy(accurate) current source can be made with a voltage reference, opamp and resistor.  
  
  Edit.. I should add that there are a lot of power supply connections and bypass caps missing from the above schematic.  (for R1 try two 1 Meg's in parallel (1% or better).. with more time than money you can futz with the R's )
  
  