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).

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    \$\begingroup\$ How are you supplying the 741 rails? \$\endgroup\$ – jonk Apr 22 '17 at 19:59
  • \$\begingroup\$ @jonk With 5V(from step-up booster). Op. amp has VCC at pin 7, and GND at pin 4. \$\endgroup\$ – SilvioCro Apr 22 '17 at 20:02
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    \$\begingroup\$ I think you might need to use a different op-amp. The basic idea appears sound. Maybe you just need to use a single-supply op-amp, or a comparator instead of an op-amp. The 741 doesn't work correctly when powered from a 5V supply. You may also want to add some hysteresis (large resistor from output back to non-inverting input, plus add a series resistor between D2 and non-inverting input). \$\endgroup\$ – mkeith Apr 22 '17 at 20:37
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    \$\begingroup\$ Given that the 741 will only swing within 2V of the rails with that load (on a good day), it's not surprising your circuit doesn't do what you'd expect. 5V - 2V - 2V = 1V. Use a comparator or a rail to rail op amp. \$\endgroup\$ – uint128_t Apr 22 '17 at 20:44
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    \$\begingroup\$ It might work. Ideally, you would run the 741 on a bipolar supply. The easiest solution to your problem is a comparator. Get yourself an LM339, they're dirt cheap. \$\endgroup\$ – uint128_t Apr 22 '17 at 20:57

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