I'm working on a handheld project that runs off a single cell 3.7V Li-ion cell. I would like to trigger an LED to turn on when its voltage drops down to 3.3V. I have seen other answers with a TL431 IC that uses the a voltage divider to set the threshold (if I'm understanding the circuit correctly), but I have access to a regulated 3.3V line to reference as the threshold.

Can someone point me to a circuit/IC example that uses that regulated 3.3 V line to "turn on" the LED when the Li-ion cell drops down to that threshold? Or would the circuit with the TL431 IC suffice in my application?

Thank you!

 

A Li-Po Cell can be ruined by running it's Voltage too low.

You need a "Window-Comparitor" Circuit to prevent damage to the Battery
when the "Low-Voltage" Warning LED pulls the Battery-Voltage below a safe level,
and also removes power from the Comparitors.

So, not only is this project not particularly "simple",
it is also not all that practical either.

This is why BMS-Boards were developed,
they disconnect the Cell from the Circuit when the Voltage drops dangerously low,
as well as several other useful functions that may be available from a given particular design.
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Can someone point me to a circuit/IC example that uses that regulated 3.3 V line to "turn on" the LED when the Li-ion cell drops down to that threshold? Or would the circuit with the TL431 IC suffice in my application?

Comparator circuit using a LM393.

EDIT: Changed values for R3-R5.

 

Keep in mind that the above Circuit is placing a Load on the Cell at all times.
If You forget about it for a couple of days, your Cell may be destroyed.
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Keep in mind that the above Circuit is placing a Load on the Cell at all times.
If You forget about it for a couple of days, your Cell may be destroyed.
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Would that be mitigated if the LiPo is disconnected by a switch?

 

Changing R3, R4 to 100K and R5 to 1M will draw less than 50ua when the battery is fully charged at 4.2 volts and appx 35ua at 3.3 volts. I wouldn't worry about destroying the battery.

 

I suppose it depends on how many Charge-Discharge-Cycles You expect to get from the Cell.
If you're planning to replace the Cell every ~2 to ~3 months,
then running it completely stone-dead ~10 or ~15 times may be completely inconsequential.

But I don't like replacing Batteries,
so I try to follow the general recommendations as closely as possible.
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I'm working on a handheld project that runs off a single cell 3.7V Li-ion cell. I would like to trigger an LED to turn on when its voltage drops down to 3.3V. I have seen other answers with a TL431 IC that uses the a voltage divider to set the threshold (if I'm understanding the circuit correctly), but I have access to a regulated 3.3V line to reference as the threshold.

Can someone point me to a circuit/IC example that uses that regulated 3.3 V line to "turn on" the LED when the Li-ion cell drops down to that threshold? Or would the circuit with the TL431 IC suffice in my application?

Thank you!

How can we possibly tell you whether "the circuit with the TL431 IC suffice in my application" when all we know about it is that you "have seen other answers with a TL431 IC that uses the a voltage divider to set the threshold"?

Where is this regulated 3.3 V line coming from? Is it being derived from the Li-ion cell that you are monitoring? If so, does it really stay regulated at 3.3 V even if the battery voltage has dropped to 3.3 V?

How is your LED and sensing circuit being powered? From the Li-ion cell? If so, you have other issues you need to contend with.