Thanks you for your reply. Much apreciated! In the datasheet of the 555, “Control Voltage Level and Threshold Voltage” @ 15V is given with 9 min and 11 max. Given your answer, I think I am correct in my understanding that this is not a min / max for the pin but rather gives the accuracy at this pin if nothing is connected to CV (as this is exactly the 2/3VCC ± tolerance).
I tried approaching this from how the 555 functions. To my knowledge (which might be wrong, as my electronics knowledge is weak), comparators do not like negative voltages at their inputs. Also, if CV is negative, this would have effects on current flowing from ground through the other two 5k resistors to the CV in pin. Not sure how the comparators would react to that. I think I will do some more thinking around that.
Where I am currently stuck is: If I have a VCC of 12V (or 9V in the case of Sam’s circuit) and inject a voltage at CV, how do they add? I know its very basic electronics but I can’t wrap my head around it. Do the voltages simply add up or do I need to use superposition (and if whith wat internal resistive values).
Thanks for you help, it allows me brushing up my knowledge.