this is a dual channel opamp (lm358) can the second channel make the led be lit for the whole duration of the battery being 4.7V or below?

 

Well your battery has to have enough voltage to power the opamp. What is the value of the zenner diode?

 

Config OpAmp as comparator with hysterisis -

https://www.analog.com/-/media/imag...-with-hysteresis/comparators-fig-04.gif?la=en

Single supply example -

Note you do not need output transistor for simple indicator LED. You would mod this
circuit to suit your needs. Maybe add some hysterisis as well.

https://daycounter.com/Calculators/Comparator-Hystereses-Calculator.phtml

http://sim.okawa-denshi.jp/en/compkeisan.htm



Regards, Dana.

 

Config OpAmp as comparator with hysterisis -

https://www.analog.com/-/media/imag...-with-hysteresis/comparators-fig-04.gif?la=en

Single supply example -

Note you do not need output transistor for simple indicator LED. You would mod this
circuit to suit your needs. Maybe add some hysterisis as well.

https://daycounter.com/Calculators/Comparator-Hystereses-Calculator.phtml

http://sim.okawa-denshi.jp/en/compkeisan.htm



Regards, Dana.

I'd go for a 2mA high efficiency LED, use an open collector output comparator and ditch the transistor. there is a battery level indicator in the TL431 application notes - but it probably wastes more current while the battery isn't doing much else.

 

Below is a circuit that keeps an LED ON above the set voltage, using a TLV431 programmable reference, a low-power version of the TL431.
Pot U2 allows adjustment of the set voltage.
The circuit uses very little current when the battery is below the 4.7V set point (basically just the current through U2 and R2).

 

Not sure why this hysteresis opamp doesent work.

You need a resistor in series with the transistor base.

 

Simulation shows Its very inefficient compared to opamp.

Not sure why this hysteresis opamp doesent work.

Probably because op-amps don't do all that great as comparators.

 

Probably because op-amps don't do all that great as comparators.

Other than being slower than comparators, why is that?

 

Probably because op-amps don't do all that great as comparators.

Comparators are usually the bare minimum of internal circuit elements - op-amps contain all manner of internal doohickery to reduce the possibility of non linearity. That's what makes them slower.

That delay often causes phase problems - sometimes you end up with loads of neutralising components trying to stop it bursting into song...…….

 

Here's the simulation of the LTV431 circuit, showing the low value of supply current (red trace) when the LED goes off (about 0.34mA).

 

op-amps contain all manner of internal doohickery

Now let's not get too technical.

That delay often causes phase problems - sometimes you end up with loads of neutralising components trying to stop it bursting into song...

Comparators can also have a problem with oscillations ("bursting into song"), but that is usually simply taken care of for op amps used as comparators, as well as comparators, with a small amount of positive feedback to generate some hysteresis.
Actually op amps, with their lower frequency response, are generally less likely to oscillate than comparators.

 

Can you share the TLV431 sym and sub?

The files are attached.

It says it has a much lower working current it might use less power than opamps.

The TLV431 uses essentially no power in this application as it's either fully on or fully off, and it's ON current flows through the LED.
It's rather like using a MOSFET with an accurate gate threshold to switch the LED.
So all the circuit current is from the LED, and U2 and R2.