If I understood you correctly, you want the LED to turn on for battery voltages lower than 3.3V (V_th) that means you calculate Low Battery Indicator Alarm Circuit Working Explanation A very useful circuit shown here can do this job very effectively by warning when your battery has reached a certain voltage point, for example when it is 90 percent empty, a 12V lead-acid battery shows 11.6. has settings to adjust low voltage alarm.
You can determine the value of the voltage divider by considering that the voltage at the negative input needs to be the same as the threshold voltage (at the positive input). A very good cheap device to monitor the health of lithium cells. There are also different types of comparator outputs, perhaps you need to drive the LED differently. 6: Low Battery Voltage Indicator using IC-8211. 5: Low Volt Battery LED Display using LM3909. 3: LED battery status indicator circuit using 4 Transistors. Make sure to calculate the values for your specific design. 1: Simple Battery Low Voltage Alarm Indicator. Simulate this circuit – Schematic created using CircuitLab If not, you need to build a small circuit that produces a voltage independent of the battery voltage: The easiest solution is to utilize a zener diode in reverse bias (and a current limiting resistor.) If that is the case, you can use this as the threshold voltage and simply scale down the battery voltage accordingly. A regulator is a good, simple idea especially if you already have a regulated voltage (buck converter or similar) present.
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