I want a "low battery" indicator LED, but since LEDs can consume 20mA running constantly, I want to avoid draining the 3.7v battery too fast while I visually alert to the fact that the battery is low. This is something that will be checked every few days, so when it's in power saving mode it needs to last.
Sparkfun describes a TL431 voltage shunt circuit that activates an LED when the battery voltage drops below a certain level. You set the trigger voltage based on resistors attached to the shunt. It requires a 2.5v drop LED, or else the LED stays constantly lit.
Could I combine the LED with a TS555 timer circuit so the LED blinks instead of being on constantly?
The TL431 would on low voltage activate the TS555 flasher circuit. Normally the TL431 uses a 2.5v forward voltage LED, what do I need to put between the TL431 and the timer to maintain this requirement?
Ignore the resistor values on the 555, I just realized I had copied this over from another circuit where I was trying to have the 555 control an NPN transistor, so it was "on" most of the time.
CrossRoads:
I don't know about your circuit to allow astable operation, but I think you'd want to use one of the low power variants of 555 for long battery life:
I suggest to use a simple µpower comparator (e.g. Comparators product selection | TI.com)
Circuitry that way below (R2 and R3 you have to recalculate to your needs)
fungus:
This is a perfect job for something like a Tiny85. It could replace all that circuitry with a single chip.
You know, I never considered that. I had assumed the Tiny85 would consume too much power, or that I'd need a bunch of extra gear like external crystals and stuff. But apparently not.
