LED needs say 5mA at 2.2 V, so 4.5V - 2.2 = 3.1V /5mA = 620 Ohms.

Any value from 470 ohms to 1K will do.

Ahh, thank you very much

 

Hi.

I have a circuit with 3 voltage levels: +40V, +8V and +5V. I want to detect if these voltage levels are present using 3 LED's.
My idea is that when the voltage levels are below 10% of the required levels, so +36V, +7.2V and +4.5V respectively, I want the LED's to turn on. When the voltage levels are above, they turn off.

Any idea how to do this, maybe using comparators?

how much detection accuracy to you need? For example, what if the 7.2 led turns on at 7.1?

what is the supply voltage for the detector circuits?

 

what is the supply voltage for the detector circuits?

The TL431 circuits are powered from the supplies being monitored.

 

I think this will work (based on DodgyDave suggestion).
The LEDs turn on when the voltage is low and eventually turn off if the battery continues to drain.
The LEDs are set for 5mA.

 

I think this will work (based on DodgyDave suggestion).
The LEDs turn on when the voltage is low and eventually turn off if the battery continues to drain.
The LEDs are set for 5mA.

Thank you so much, eetech00, your curcuits suits my needs perfectly.
One last question though: How do you determine the resistor values, i.e. what are the formulas?

 

How do you determine the resistor values

The TL431 changes state from conducting, when the sense voltage is above 2.5V, to non-conducting, when it is below 2.5V, so you just select the divider resistors to generate that sense voltage at the desired battery detect voltage.

The TL431 requires a minimum cathode-anode current of 1mA when conducting so those resistors are selected accordingly.