LM339 comparators?

Question

i build a non-inverting voltage comparator and set the reference voltage to 4.15 volt. but some how the output wont output high until the input reach 5 volt. does anyone knows why? please help

Answer 1

You probably have it hooked up wrong. Check the hookup on the bottom of page 7:

http://www.national.com/ds/LM/LM339.pdf

The LM339 is designed to interface to TTL circuits, and has an open collector output. So, you need a pull up resistor. Do you have one?

http://en.wikipedia.org/wiki/Comparator