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I made low battery indicator with LM393. But circuit doesn't work as it should. If I connect 100k trim to IN- of 1st op amp which imitate "battery voltage", op amp turns red LED on when voltage from 100k trim is less than threshold voltage. But if I connect + pole from battery to IN- then circuit doesn't work. Why? Everything is connected on same ground potantial. I use 5V from step-up converter for LM393 VCC.

Schematic with battery

Schematic with 100k trim as battery

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    \$\begingroup\$ Where do you connect the minus side of the battery? \$\endgroup\$
    – zeta-band
    Apr 9, 2018 at 23:25
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    \$\begingroup\$ people that put ground connections upside down at top of schematic should be banned \$\endgroup\$
    – jsotola
    Apr 9, 2018 at 23:29
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    \$\begingroup\$ You should redraw this circuit including the battery and voltage regulators. \$\endgroup\$
    – Drew
    Apr 9, 2018 at 23:47
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    \$\begingroup\$ @SilvioCro Connect the minus terminal of the battery to ground. \$\endgroup\$
    – zeta-band
    Apr 9, 2018 at 23:56
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    \$\begingroup\$ You should also redraw the circuit showing the LM393 as two comparators, rather than as a mystery block. (And if you insist on leaving it as a mystery block, put pin numbers around the block!) \$\endgroup\$ Apr 9, 2018 at 23:58

1 Answer 1

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Put a pull-up resistor of 10 K from the pin 1 to VCC. LM393 has open collector output, cannot drive high by itself.

Later edit , the battery voltage is arround 4V which is out of the common mode range of LM393 with 5V supply (0..3V). Use a divider like in the schematic below or use the 100k pot that you used for tests to bring the voltage in the allowed range.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Nope, still doesn't work. \$\endgroup\$
    – Pararera
    Apr 13, 2018 at 10:05
  • \$\begingroup\$ @SilvioCro the battery voltage is under 3.5V? For 5V supply voltage this is the maximum common mode input \$\endgroup\$
    – Dorian
    Apr 13, 2018 at 11:04
  • \$\begingroup\$ Battery voltage is around 4V, I increased threshold voltage above 4V. \$\endgroup\$
    – Pararera
    Apr 13, 2018 at 11:13
  • \$\begingroup\$ Oh.. then is over the specifications , see the datasheet, page 7, for 5V it's even worse , it works only up to 3.0V . Use a divider to bring-it in the acceptable range, I will put a schematic in the answer \$\endgroup\$
    – Dorian
    Apr 13, 2018 at 11:25
  • \$\begingroup\$ Datasheet: ti.com/lit/ds/symlink/lm393-n.pdf \$\endgroup\$
    – Dorian
    Apr 13, 2018 at 11:34

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