Fail: Low battery indicator blinks and dims

[Danirov]

Hello! I would be very grateful if you can help me with that.
I have been building some circuits to turn on a LED when the battery is low (images), but there is a problem: The led dim and blink between the OFF and fully ON state for some minutes.

My circuit consumes 0.23A and is supplied by a 3.7v/1400mAh Li-Ion battery.
A variable resistor set the voltage
at which the led should light up, indicating "low battery".
For example, if I set 3.3v as low voltage, when I change abruptly from 4v to 3v the led turn on immediately, but if you leave the battery discharging, as would naturally be the case, there is a time lapse in which it start diming gradually and blinking until it lights up completely.

Is there any artifice I can implement to avoid detecting small variations in order to generate an immediate change from OFF to fully ON at the set voltage?

I would think of something like an averager or a rounding method, the problem here is that I do not use a microcontroller in the system to store values or program it to round values, everything is analog.

Thank you!

 

[Nigel Goodwin]

Is there any artifice I can implement to avoid detecting small variations in order to generate an immediate change from OFF to fully ON at the set voltage?

It's normally done using 'hysteresis', using a comparator with positive feedback, to ensure it 'snaps' ON ans stays ON.

Your existing circuits are rather current heavy, and even worse DECREASE current when the LED's light, so the voltage rises, and the LED goes back out - hence the dimming and blinking.

 

[Danirov]

Thanks Nigel. I added a debouncing switch and it works, but it fails when added to the full circuit (image 1).

Test1: Circuit without Debouncing/Load connected:
Voltage drops from 3.68v to 3.46v (image 2).
Low Battery Indicator: Blinks.

Test2: Circuit with Debouncing/Load connected:
1) Debouncing Circuit: The voltage drops to 2.19v at the output of the Smichtt Trigger, therefore it isn't possible to energize the Step Up and the rest of the circuit.
2) Low Battery Indicator: Blinks when switch is pressed.

Test3: Circuit with Debouncing/Load disconnected:
1) Debouncing Circuit: Works fine (image 3).
2) Low Battery Indicator: Doesn't blink when the switch is turned ON/OFF.

Is there something that I am forgetting to put in the circuit so the voltage is dropping?
I would suppose that the problem is at the DC/DC conversion of the Step Up, but everything is meeting its requirements (Vin:2v-24v/Imax: 2A).

My system details:
Power consumption: 1.702W
Low Bat Indicator: (0.037W, 3.7v/0.01A)
Load: 2 Fans (1.44W, 9v/0.16A), 3 Leds (0.059W, 2.92v/0.02A).
Debouncing Circuit based on a Schmitt Trigger 74HC14 (Vin:2-6v/Vout:2-VCC, VCC and GND connected to battery line).

 

[Nigel Goodwin]

Thanks Nigel. I added a debouncing switch and it works, but it fails when added to the full circuit (image 1).

I think we're at cross purposes? - the hysteresis needs to be round the voltage sensing circuit - which just as before is still drawing (and wasting) loads of battery power as well. 220 ohms across a Li-Ion is wasting 19mA all the time, not something you want to do with a battery.

There's a description of a similar battery circuit used in the Cybot robot on my website, in which I also mention the gradual turn ON of the LED. This uses a 2K7 resistor, so wastes a lot less power than yours.