I'm trying to make a simple low-battery indicator(3.6V 18650 LiIon) that will turn on red LED. I already tried something(schematic is below) but it doesn't work. I already have zener diodes(1/2W, 2.7V), and op. amps(UA741CN DIP-8 UA741 LM741 ST), and of course resistors. Threshold voltage is 3.6V(+/- 0.1V).