Convert blinking LED to continous

Discussion in 'The Projects Forum' started by tunafish24, Oct 21, 2011.

  1. tunafish24

    Thread Starter New Member

    Oct 21, 2011
    12
    0
    I couldn't come up with a better title...but here's the explanation. :D

    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!
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    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.
     
  3. tunafish24

    Thread Starter New Member

    Oct 21, 2011
    12
    0
    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?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    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.
     
  5. tunafish24

    Thread Starter New Member

    Oct 21, 2011
    12
    0
    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?
     
  6. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
  7. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    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.
     
  8. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    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
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    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.
     
  10. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    i attached a circuit diagram..
     
  11. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    That's pretty much what's going on in a 555. :)
     
  12. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    @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.
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    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).
     
  14. tunafish24

    Thread Starter New Member

    Oct 21, 2011
    12
    0
    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?
     
  15. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    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!
     
  16. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    That is an aesthetic argument, and totally up to you. ;)

    [​IMG]

    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.
     
  17. tunafish24

    Thread Starter New Member

    Oct 21, 2011
    12
    0
    Bill, yea I just figured it'll look a bit cooler :p 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.
     
Loading...