I couldn't come up with a better title...but here's the explanation.

Basically, I need to create a small circuit for my PC to attach to the power LED. The way my motherboard works is that if the system is powered-on then the LED is ON. But when the system goes to standby the LED starts blinking.

I want to do two things:

1. Have two separate color LEDs for each state i.e. powered-on(green) vs standby (amber).
2. I don't want the standby LED to blink.

I figured that I can make the blinking signal continous by attaching a capacitor and a diode (to prevent current going back to the motherboard). That will take care of standby light not blinking.

However, I can't figure out how to switch LEDs when the current goes from continous to intermittent. So when the system goes to standby the LED switches from green to amber. Any ideas? Btw, I don't want a expensive or complex circuit. I need something that will Not require any outside power-source and will work just from the current coming for the power-LED.

PS. I'm a Computer Science guy and don't have much understanding of analog Electronics - so be nice to me.

Thanks!

 

There is a 555 circuit that would do this, it is basically a monostable that will time out and go dark if it doesn't get another pulse.

The 555 Projects

555 Monostable

Be careful loading an unknown output, it is easy to blow something like a motherboard.

 

There is a 555 circuit that would do this, it is basically a monostable that will time out and go dark if it doesn't get another pulse.

The 555 Projects

555 Monostable

Be careful loading an unknown output, it is easy to blow something like a motherboard.

I'm kind of a beginner...so can you please elaborate a bit more. I'm not sure if I'm getting how would 555 keep the green light off and amber on?

 

Every time the flash turns the monostable on, it times for a set amount of time. When the blinking goes away the monostable would turn off, and the light would go out.

So the question is, do you know what a monostable is? It is a flip flop, also called a timer, that turns on for a set amount of time when triggered.

 

Thanks for your response Bill! Yes, I did some research on 555 monostable mode after your first post. I can see how the 555 monostable can be used to convert blinking signal into an always-on standby LED.

Now the final piece of the puzzle. How do we take care of the power LED (green) - which should turn off while the standby(amber) LED is on and vice-versa. Any suggestions for that?

 

Try a variation of the 555 missing pulse detector. In this demonstration the component values are only approximate. Click on the switch to change the source from blinking to fixed.

http://www.falstad.com/circuit/#%24+1+0.0020+1.9265835257097934+51+5.0+50%0A165+352+160+464+160+0+5.0%0Aw+352+192+352+288+0%0Aw+352+192+320+192+0%0Aw+320+192+320+288+0%0Ac+320+288+320+352+0+1.0E-6+0.6557154714049549%0Ag+320+352+320+368+0%0Ar+320+192+320+128+0+270000.0%0Aw+320+128+416+128+0%0AR+320+128+320+80+0+0+40.0+5.0+0.0+0.0+0.5%0At+224+320+272+320+0+-1+5.0+4.3442845286000455+100.0%0Aw+272+304+272+288+0%0Ar+272+288+320+288+0+100.0%0Aw+272+336+272+352+0%0Aw+272+352+320+352+0%0Aw+224+256+224+320+0%0AR+96+256+96+304+0+2+2.0+2.5+2.5+0.0+0.5%0Aw+224+256+352+256+0%0AS+144+240+96+240+0+0+false+0%0Ac+416+320+416+352+0+1.0E-8+3.333333333333333%0Aw+320+352+416+352+0%0Ap+144+240+144+224+0%0Aw+144+272+144+352+0%0Aw+96+128+96+224+0%0A162+512+192+512+224+1+2.1024259+0.0+1.0+0.0%0A162+512+224+512+256+1+2.1024259+1.0+1.0+0.0%0Ar+512+192+512+128+0+220.0%0Ar+512+256+512+352+0+220.0%0Aw+416+352+512+352+0%0Aw+416+128+512+128+0%0Aw+480+224+512+224+0%0A152+144+256+224+256+0+2+5.0%0Aw+144+352+272+352+0%0Aw+96+128+320+128+0%0Ap+512+224+544+224+0%0Ax+553+230+578+233+0+12+OUT%0Ax+139+214+151+217+0+12+IN%0Ax+248+250+302+253+0+12+TRIGGER%0Ao+20+16+0+34+5.0+9.765625E-5+0+-1%0Ao+16+16+0+35+5.0+9.765625E-5+0+-1%0Ao+33+16+0+34+5.0+9.765625E-5+0+-1%0A

 

Another approach:

When the power is blinking on and off, it has less DC content. Through an RC network (or a rectifier even better) you could get that DC voltage on a capacitor.

Then you can compare that DC voltage with the full-on voltage with a comparator. Depending on the comparison result, you can power either one LED on another.

 

brief thought, can you not, using a transistor, a capacitor and a power signal if you have one...
split the led signal in, branching it, putting one through a capacitor, and the other driving the constantly on green.. the capacitor would block the dc of the green, amber only flashing at startup, then use the tranny to drive the amber led from the pulses the capacitor lets through?
the benefit of this to a 555 is the cost, though you could use the pulses to drive a monostable or pop a capacitor on it and run directly.. (See attached)
maybe im barking up the wrong tree, but it seems the much easier idea

 

Georacer, chrisw1990,

The OP is a newbie and will most likely need a circuit drawn out for him so that he can better understand his options. Personally I am liking KJ6EAD's circuit. It's slick, but does use an extra IC. Seeing a side-by-side comparison might convince me otherwise.

 

i attached a circuit diagram..

 

Another approach:

When the power is blinking on and off, it has less DC content. Through an RC network (or a rectifier even better) you could get that DC voltage on a capacitor.

Then you can compare that DC voltage with the full-on voltage with a comparator. Depending on the comparison result, you can power either one LED or another.

That's pretty much what's going on in a 555.

 

@OP

I 'm afraid moderating doesn't let me experiment with circuits in a simulator right now. Heck, to be honest, I got my hands full, and I don't plan to dedicate more hours in AAC than I currently do.

If it is needed, I 'll try to come up with a schematic, but I choose not to for now.

Thank you for your understanding.

 

A simple, straightforward monostable would work for this design. A transistor may be added to increase the pulse height.

To the OP, sometimes with electronics there are no easy answers. There are answers, and you might not like them, but if you want it to work you have to bite the bullet and implement the solution.

LEDs are very simple components, give them current and they will light. Give them less current and they will light less brightly. You might be able to smooth out the light level, but the odds are there will be flicker, no matter what. How much flicker will require some experimentation. One of the things I really like about electronics is the results are very reproducible, you try something, and you can pass on how well or not it works.

The 555 solution will not flicker. There will be a short delay between when the 555 receives its last pulse and it turns off the LED, but it will be clean. It is why I recommended it. You have indicated you're willing to build circuitry, it is a matter how complex you want to make it.

I'll be back with a schematic. The family is wanting to go out to eat, so it will take a little longer than usual, but I will draw something that will use +5VDC. I will plan for red (different colors drop different voltages, it has to be designed for).

 

Hey guys...sorry I was out with family all day so couldn't reply yesterday. Actually, I have done a bit of electronics work in the past, even etching pcbs and soldering components etc so I'm comfortable with that. However, since my background is Not in electrical eng., I don't know much about "RC network" and such terminologies, but I do know about capacitors, resistors etc and what they do. Basically, I need help with the schematic (even a rough diagram) so I can put some components together and see if it works or not.

Also, I was thinking since the major issue is keeping the standby LED always on, would it be easier if it were pulsating (fading between almost-fully-on to half-way-on)? I think it may actually look cooler because one of the reasons I don't want a fully on/off (blinking) LED is that it makes it unbearable to sleep at night. A slightly pulsating LED at partial of its total light output will actually be better. In my (naive) mind, it may actually make the circuit simpler. What do you guys think?

 

use my little capacitor network, if you choose the second hold up capacitor right, you can get it to throb.
this also requires minimal components and not much cost!

 

That is an aesthetic argument, and totally up to you.

I found this in my library, it comes very close to what I was talking about. Figure around 1KΩ for the input resistor, and 10KΩ for the base ground resistor.

 

Bill, yea I just figured it'll look a bit cooler Btw, is it possible to make the standby LED pulsate/throb with your design? What do you think about chrisw1990's schematic (on page 1) would that achieve the effect I want?

chrisw1990, I'm looking at your diagram right now. I really like the simplicity of it. I'm just waiting to hear back from the motherboard manufacturer regarding the voltage and max. current draw on the power led so we can select appropriate components. For now, just from google, it seems like a circuit designed around +5V and 20mA would be a safe bet.