I have my LM311 comparator connected with the output transitor grounding the base of a PNP transistor to turn on misc things. I use a voltage divider to make a reference halfway between Vcc and ground at IN- and an optocoupler providing a simple Vcc or nothing to IN+. My question is: why when I have my 13V at IN- does the output change just by touching my test probe to IN+?? More importantly how am I supposed to figure how to get it working if I can't even see what the voltages are at the inputs without changing them?