I just need a single LED to light up when the voltage drops below a threshold like say 3.3 volt ( sharpness is not that important ). I know basic electronics like transistors,logic gates etc. On searching a bit, I think a comparator is what I need (all about which I know is that it gives a logic signal based on which input voltage is higher). 
Can I use just the comparator for the 3.4 volt LED and achieve what I need, from the link given below ?
<http://www.instructables.com/id/Li-Ion-battery-level-indicator/>