Hi everyone,
I've a strange problem with a LM311N voltage comparator, I've put on the inverter pin a reference voltege, 2.5V and in non-inverter pin i've put the output pin of my pir motion sensor, now when i move in front of the sensor the output of the sensor go almost at 4V so the output of the LM311N should go high but it doesn't, so any ideas why this happen? Thanks to everyone for the attention

 

The lm311 has an open collector output, so it needs a pull up resistor to the positive supply.

 

thanks dodgydave it works, but now i've another problem, i don't know why but now the pir sensor give in output always an high voltage, also if i'm no in front of it, do you know why?