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I have an existing circuit powered with a 3.7V LiPo battery. To indicate when the battery is under 3V, I wanted to add an LED which shows me to charge the battery.

The circuit has to be very small.

I searched for circuits on the internet but they mostly had rather complicated circuits and not suited for my system. I need it to show me only when the battery is under 3V. It doesn't need to be adjustable. What would the circuit look like? I saw many circuits with ICs and transistors. I didn't get how to choose the right components for my task.

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    \$\begingroup\$ The LED will add a load to the rapidly depleting battery and accelerate it towards full discharge so wouldn't a flashing led (50:1 duty cycle) be better i.e. on for 0.1 seconds and off for 5 seconds? \$\endgroup\$ – Andy aka Nov 27 '20 at 9:57
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    \$\begingroup\$ Far more foolproof to have the LED flash to show the battery is ok like they do with smoke alarms. \$\endgroup\$ – Finbarr Nov 27 '20 at 10:30
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Here is a basic circuit that you can adjust with R1 when you want your LED to light up when the voltage decrease.

  • D1 is a Zener diode, you can use something between 1-2.5V with low leakage.
  • R1 is a trim-pot that you can adjust.
  • OA1 needs to be chosen to fit your supply range and has enough current output to supply the led.
  • Use a low current LED like 1-2mA.
  • You can add an NPN transistor after OA1 if you need more current for the LED.
  • R2 has to be big enough not to deplete the battery, but small enough so that the leakage current of D1 is not significant.

schematic

simulate this circuit – Schematic created using CircuitLab

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As already mentioned in the comments, in order to save power you could make use of a flashing LED which is activated if the battery voltage drops below 3V.

Here is small simulation:

circuit

Basically it consists of a 2.5V reference voltage (TL431) and a small logic for turning the astable multivibrator circuit on and off.

How it works:

If the supply voltage is above 3V

  • The cathode end of the TL431 will pull the gate of \$M_2\$ to ground
  • Fet \$M_1\$ and \$M_2\$ will be turned off
  • Astable multivibrator is disconnected from the supply
  • Quiescent current is very small

If the supply voltage is below 3V

  • The cathode end of the TL431 acts as a open collector
  • Fet \$M_1\$ and \$M_2\$ will be turned on
  • Astable multivibrator is connected to the supply
  • The frequency at which the LED flashes can be adjusted by capacitors \$C_1\$ and \$C_2\$ and resistors \$R_9\$ and \$R_8\$.

If you really want to keep the LED on all the time, you could replace the multivibrator with a joule thief circuit, but it causes the battery to be depleted rather quickly:

circuit_2

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