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I am using a 3V battery to light 2 LEDs that have a forward voltage of 2.7 V and a third led with a 2.4V forward voltage that acts as an indicator to tell the user the power is currently on.

All 3 LEDs are in parallel. The circuit is currently drawing 9ma and is only on for 5 seconds when used, which is maybe 3 times a day.

I am looking for a circuit diagram to make the indicator LED flash when the battery voltage drops below 2.7 volts, otherwise it stays on when the other unit it is turned on, so when a user turns it on and it is flashing the user knows to change the battery.

I understand that these low voltages are a problem to get working. Any and all ideas would be greatly appreciated.

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  • \$\begingroup\$ What is the amp-hour capacity of your battery, and how long do you need it to last? \$\endgroup\$ – Reinderien Oct 26 '20 at 18:37
  • \$\begingroup\$ And what is the minimum battery voltage for which you need this low-battery light to function? \$\endgroup\$ – Reinderien Oct 26 '20 at 18:38
  • \$\begingroup\$ Also, what chemistry is the battery? \$\endgroup\$ – Reinderien Oct 26 '20 at 18:39
  • \$\begingroup\$ The battery is a cr2032, so max amp hour is 190 mah ... Minimum battery voltage for which you need this low-battery light: 2.4 V \$\endgroup\$ – Semits Oct 27 '20 at 0:23
  • \$\begingroup\$ Wouldn't it be easier if your third LED just did not light at all if the power is low? This could be done by using a 2.7V LED for the 3rd LED, or by putting a schottky diode in series to bring its 2.4V drop up to about 2.7V. The blink requirement seems like unnecessary complexity and won't work at all if the battery is really low. \$\endgroup\$ – td127 Oct 27 '20 at 3:34
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Taking a wild stab at this, without knowing your battery longevity constraints:

  • Tie the battery output to an AP1603 to step up to 3.3V from as low as 1.1V
  • Use a voltage reference well below 2.7V such as the LM4041 1.225V that has quescient current below 100uA, between ground and the non-inverting input of an op-amp; with bias resistor tied between 3.3V and the same op-amp input
  • Make a resistive divider where the top of the divider goes to the battery, middle goes to the inverting input, bottom goes to ground. The values should be set such that, at 2.7V battery, the middle voltage is equal to the reference voltage. 4.7M and 3.9M should work.
  • The op-amp should have low quescient current, and doesn't need any bandwidth. It acts as a comparator. The MCP6231 is appropriate - it only consumes 20uA.
  • Tie the output of the op-amp to a simple oscillator, maybe a 4541B for which there are many options in 3V, also with low quescient current
  • Tie the output of the oscillator to an LED.
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