Hi,

I wanted to design a simple circuit to monitor a charging voltage. Ideally, I'd like to have an input voltage between 0 and 12V, whereas at about 7V (tunable) an LED should switch on, below it turns off again. First I thought I can solve this simply with a Zener diode, however, when reaching the Zener voltage I still have a not-sharp slope in the current, so my LED turns on "slowly" (it should happen within 0.1V to full brightness). Also, when increasing the voltage to 12V, it simply blows the LED, and using a resistor for once delays the turn-on of the Zener and then reduces the brightness.
I played around with op amps and Schmitt triggers, which kinda led to the expected result, but both usually require an external amp source, which I don't want - I want to use the supplied voltage on the line, which is enough to power an LED. Furthermore, the trigger does not reset when simply turning of the input. Does anyone have a solution for me?

/Andre

 

http://www.alldatasheet.com/datasheet-pdf/pdf/112282/NSC/LM431.html

 

The circuit you are looking for is called an analog comparator.

Besides specifying the increasing threshold voltage +Vth, you also need to specify the decreasing threshold voltage -Vth.
Note that +Vth cannot be the same as -Vth.
Furthermore, +Vth will be greater than -Vth. The difference (+Vth - -Vth) is called the hysteresis voltage.

There are many IC comparators available. LM393 is a popular dual comparator (i.e. two comparators in a single 8-pin package).

A resistor is placed in series with every LED in order to limit the current. If your LED is blowing then the value of the resistance is too low. If the LED is not bright enough then the value of the resistance is too high.

 

The circuit you are looking for is called an analog comparator.

Besides specifying the increasing threshold voltage +Vth, you also need to specify the decreasing threshold voltage -Vth.
Note that +Vth cannot be the same as -Vth.
Furthermore, +Vth will be greater than -Vth. The difference (+Vth - -Vth) is called the hysteresis voltage.

There are many IC comparators available. LM393 is a popular dual comparator (i.e. two comparators in a single 8-pin package).

A resistor is placed in series with every LED in order to limit the current. If your LED is blowing then the value of the resistance is too low. If the LED is not bright enough then the value of the resistance is too high.

The analogue comparator is the basic building block, the TS wants sharp threshold - so a smidge of positive feedback might be needed.

 

Here's the LTspice simulation of a circuit using an LM/TL431 adjustable reference, as tcmtech suggested, configured to act as a comparator with a built-in reference voltage.
It shows the LED current versus the battery voltage for several U2 pot settings.
R6 adds around 100mV of hysteresis so it doesn't oscillate at the trip point.