Hello,
My knowledge of electronics has hit a brick wall, and I cannot get the desired results from what I know.

I have been trying to devise a simple circuit that holds an LED in the on position when my battery is above 3.65v and flashes the same LED when the voltage of the battery drops below 3.65v. The battery is running a 5V booster board, and I prefer not to use the LEDs on the board but an LED inside an "after-market" momentary switch that turns the 5V booster board on. Preferably the circuit would be powered when the 5V board is on.

Any help would be much appreciated.

 

Welcome to AAC!
If you can program an MCU, that would keep the component count to a minimum.
As an alternative you could use a quad opamp/comparator plus an accurate voltage reference. One opamp/comparator would compare the battery voltage with the reference voltage and control an LED-driving oscillator comprising the other opamps/comparators.

 

What hysteresis are you allowing?

 

Use a 555-timer circuit or a UJT circuit to flash the LED.
Use a comparator circuit so that when the detected voltage is above 3.65V it resets the flash circuit and leaves the LED in the ON state.

 

I have tried using an LM324 with the power and +in coming off the battery (checked w/ battery over and under the 3.65 reference from the 5v resistor divider) and both times the LED has stayed on. My LED has an unknown internal resistance. I have also tried an LM393 and TLC272, and none of these IC have given me the desired results. I am new to using comparators and opamps, and know that there are different types, but I do not know what IC will work for me. This is the circuit I have been trying to find the proper IC for.
I ask you please read my note before dissecting/responding to my circuit Disclaimer: I realize that the transistor on the output of the comparator circuit does not have a base connection, and this is the way the circuit is setup from what I have seen both in schematic and application/test (video -

). The 100ohm resistor connected to the LED is a place holder for the unknown resistance in the LED I plan on using.

I don't believe that my circuit is wrong, though it could be, I believe my problem with the circuit is the ICs that I have been trying. Also if I have to connect the ground leads from the battery and 5v ground, please let me know, though I have tried that in the past with inconsistent results.

 

If you don't have hysteresis (see post #3), when the battery voltage is low the current drawn by the LED will pull the voltage down further and may result in unwanted oscillation about the comparator trip point.

 

If you don't have hysteresis (see post #3), when the battery voltage is low the current drawn by the LED will pull the voltage down further and may result in unwanted oscillation about the comparator trip point.

How would I achieve that? Like I said, I'm new to using these components.

 

How would I achieve that? Like I said, I'm new to using these components.

You would use google to find how to apply hysteresis to an opamp/comparator circuit. You could also google "Schmitt Trigger", which is a circuit that uses hysteresis. You might find this for example:

http://www.ti.com/lit/ug/tidu020a/tidu020a.pdf

or maybe this:

https://en.wikipedia.org/wiki/Schmitt_trigger

 

LOW VOLTAGE FLASHER
This circuit flashes when the voltage drops to 4v.
The voltage "set-point" can be adjusted by
changing the 150k on the base of the first transistor.

 

Final post on this thread!
I have designed the circuit that achieves the desired result.
Thanks for the help.

Cheers