How to keep an LED ON even after the voltage drops to zero?

[Ejaz]

I need to use an LED in such a way that the LED will lit when the voltage across the LED is sufficient to lit it but the LED will remain lit even after the voltage across it drops to zero. Is there any simplest way to do that?

 

[230842]

How long? If not too much, let's say one second, a simple capacitor could be enough. Julian

 

[melone]

Look at the LM555 timer IC. Please look at the application notes and you should be able to perform the required function.

However, if you are trying to latch on the LED (once the voltage is present once, then you want the LED to stay on until you manually reset it) then you might want to look at the 74LS74. This is a D flip-flop. Try poking around in those app notes and see if there is something that will work for you there.

LM555 -

http://www.national.com

74ls74 -

http://www.onsemi.com/home

Good luck and keep us posted!!!

 

[buzzp]

There is no way for the LED to be lit if the voltage across it is zero. I believe you are asking how to keep it on when the main power source is removed. In this case a cap will work but will depend on how long you want it to stay on after main power is removed. If your looking for times above, say, 20-30 seconds then a capacitor is probably not the way to go(more time:larger cap, the brightness requirement of the LED will affect times and cap values as well). If you want the LED to stay on (and you still have a power source)by virtue of a 'momentary' push button or something then the D flip flop would be one way to achieve this. Good luck

 

[MMAshwin]

Hi,

How long is the voltage zero for ?. If its for a long time
then this simple circuit might be suitable.

The diode is to stop the battery powering the circuit when
there is no power.
The first resistor limits the charging current to the battery.
The second to the LED (when its powered by the battery)
Both resistors together are the current limiting for the LED with power applied.
You'll have to do the calcs depending on your choice of components.

Any help ?, yes no let me know.

Regards

 

[230842]

To Laffalot: what application allows to insert graphs on a post?
Thanks in advance Julian

 

[buzzp]

Probably copy and paste.

 

[MMAshwin]

Hi 230842 (is that your birthday ?)

In answer to your question about posting graphics.

You are not able to store graphics on the server that hosts eng-tips, but you can put in references to graphics on another server.

I set up a small web site just to hold the graphics at

http://www.laffalot.freeservers.com/

Then in your post you refer to the images on your site/directory like this

*img

http://www.laffalot.freeservers.com/workb.jpg*

but replace the * with open & close square brackets [].

I had to put stars otherwise all you would have seen is the image like this

which is were I do my electronics stuff.

Any help ?, yes no let me know.

Regards

 

[230842]

Hi Laffalot. Confidentially: Yes, It is my birthday. I think that I was born too early, under an electronic/informatic viewpoint of course, and these ar matters to be learn during youth or even childhood (as many others, like languages). Moreover, I am an electrical teacher and have not too much time; curiosity and helping wishes.
Anyway, I thank you very much for your kindness.
Regards. Julian