What circuit? For LED

[sk614]

Hi I don't know what to search for to create the following circuit. 
I have a single shot active-low input.  I want to turn on an LED and stays on when that input signal is activated. I've been reading on latches but it looked complicated for just turning on a single led. But then again I don't know much to judge.   

[uncle_bob]

Hi

Does activated = you get a negative going edge?

-- or --

activated = while the signal is low?

-- and / or --

for some time after the above event?

The circuit you use is dependent on what you want it to respond to.

Is your signal a relay closure, or a 1.2 V logic level, or ECL or .... Does it have noise on it?

The circuit you use is dependent on what the signal looks like.

Is the LED a 5 ma 1.6V SMD red blinker or is it a stack of 32 Cree white LED's running off of 72 volts.

The circuit you use is dependent on what sort of load you have.

What kind of background do you have in electronics? Is an MCU something you are already using? Is this for a one off or for production? How fast do the "one shots" come in? Do you already have an inventory of parts (7400 TTL) you would like to use?

Lots of questions.

Bob

[sk614]

Does activated = you get a negative going edge?

YES.

This is a TPS61020.  It have a "Low Battery Indicator" (LBI) input, which I set to 2V.  When LBI goes below 2V, LBO (Low battery output) triggers (see waveform).  What I want to do is use LBO to turn on an LED using Vout as the source. 

[uncle_bob]

Does activated = you get a negative going edge?

YES.

This is a TPS61020.  It have a "Low Battery Indicator" (LBI) input, which I set to 2V.  When LBI goes below 2V, LBO (Low battery output) triggers (see waveform).  What I want to do is use LBO to turn on an LED using Vout as the source.

Hi

Ok, so you only want to activate on an edge. That means that it must be high before and then go low. It does not activate if it starts out low.

How long do you want the LED to stay on after the edge?

--- or ----

Do you want a circuit that simply detects the low state and turns on the LED no matter what?

Bob

[sk614]

I want to have the LED ON when LBI goes low.  I plan to have VOUT power the LED...so LED will turn off when VOUT falls out of regulation or not enough to power Vled.

[uncle_bob]

I want to have the LED ON when LBI goes low.  I plan to have VOUT power the LED...so LED will turn off when VOUT falls out of regulation or not enough to power Vled.

Hi

Ok, so you don't want an edge triggered circuit. You want a circuit that turns on the LED based on a voltage level.

The conventional way to do this is with a comparator. One one input is (say) a forward biased diode. The other is a scaled version of the voltage level. Everything runs off the same supply that will power the LED.

Make sense so far?

Bob

[Signal32]

I want to have the LED ON when LBI goes low.  I plan to have VOUT power the LED...so LED will turn off when VOUT falls out of regulation or not enough to power Vled.

Sounds like you'dd want the LED on as long as LBO is low. Why not just put the LED directly from ViN to LBO (with appropriate current limiting) ?

[sk614]

Sounds like you'dd want the LED on as long as LBO is low. Why not just put the LED directly from ViN to LBO (with appropriate current limiting) ?

I can't. If you look at the graph, LBO on goes low momentarily. 

[uncle_bob]

Sounds like you'dd want the LED on as long as LBO is low. Why not just put the LED directly from ViN to LBO (with appropriate current limiting) ?

I can't. If you look at the graph, LBO on goes low momentarily.

Hi

In your graphs, Vout promptly goes to zero (likely in a few mili seconds) after Vin drops out. What is the LED actually running on?

Bob

[sk614]

In your graphs, Vout promptly goes to zero (likely in a few mili seconds) after Vin drops out. What is the LED actually running on?

The LED can only be power by VOUT.  So if Vout drops out so will the LED and that's what I want.

I think I am getting close to.  If you can critique my circuit.  I added a D-flipflop so to latch the Vout to the LED so that LED don't follow LBO.

My problem is now LED current (ILED) comes on on the rising edge of (LOW BATTERY OUTPUT) LBO.  I don't know how to make it on the first falling edge of LBO. 
 

[uncle_bob]

In your graphs, Vout promptly goes to zero (likely in a few mili seconds) after Vin drops out. What is the LED actually running on?

The LED can only be power by VOUT.  So if Vout drops out so will the LED and that's what I want.

I think I am getting close to.  If you can critique my circuit.  I added a D-flipflop so to latch the Vout to the LED so that LED don't follow LBO.

My problem is now LED current (ILED) comes on on the rising edge of (LOW BATTERY OUTPUT) LBO.  I don't know how to make it on the first falling edge of LBO.

Hi

What do you power the flip flop off of?

If you power it off of Vout, you have the same problem. Once the Vout voltage goes low, you loose whatever "memory" it had.

Bob