How to design a circuit in which an LED turns on when there is a voltage drop

[brainfunction]

I want the LED to be off when there is a certain input voltage let's say 5v (the exact value doesn't matter), but when that voltage drops I want the LED to turn on. I would also like the LED to remain on even when the voltage goes back up to 5v.

I was thinking of using some sort of logic gate combination to keep the LED on even when the voltage goes back up to 5v, but I'm not sure what to do to keep the LED off for a high voltage value and on for a lower value. Would I need some sort of transistor?

Any help would be great!

 

[DragonPetter]

I want the LED to be off when there is a certain input voltage let's say 5v (the exact value doesn't matter), but when that voltage drops I want the LED to turn on. I would also like the LED to remain on even when the voltage goes back up to 5v.

I was thinking of using some sort of logic gate combination to keep the LED on even when the voltage goes back up to 5v, but I'm not sure what to keep the LED off for a high voltage value and on for a lower value. Would I need some sort of transistor?

You could use a comparator with some hysteresis or another option would be to just charge a capacitor when the signal goes low that drives a FET which turns on the LED and then to reset, you'd need to discharge the cap. Another way to keep the LED on after the voltage comes back would be to use a latch that you would also need to be able to reset.

As far as turning on an LED when the signal goes low, there are multiple ways to do that with transistors and other devices. You could also sink the current directly through the LED with the signal if the signal driver can handle this, but that alone doesn't satisfy the need to keep the LED on when the signal goes back to high.

 

[pantaz]

I was recently looking for something similar. I can't yet recommend a specific circuit, but here's what I searched:
http://www.google.com/search?q=low+battery+indicator+circuit