Redundant secondary LED for burn out

Discussion in 'The Projects Forum' started by leonhart88, Aug 27, 2011.

  1. leonhart88

    Thread Starter AAC Fanatic!

    Feb 23, 2007
    Hey guys,

    I'm designing an IR transmitter using an IR LED. I'd like to have a secondary LED in my circuit in case the first one burns out after prolonged use. Any suggestions on how to do this? Ideally, the secondary LED would not draw any current or ever be in use until the main one shorts.

  2. kubeek


    Sep 20, 2005
    The problem is the failure mode of the LED. IIRC leds don´t short or open when they die, they just stop emitting light and maybe the Vf changes. Look here for starters.

    Small changes in current of Vf could be pretty hard to sense and also its hard to set the point when the diode is considered failed, so you could change your setup to allow direct sensing of the IR light produced.
    strantor likes this.
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    Yes, perhaps using another LED to sense that the IR LED is functioning. If so OK. If the sensing LED does not detect IR from the IR LED then switch to backup. This test would occur each time you transmit with the device. But then who is to say that the sensing LED has not burned out first?

    Would you also need to sense that the batteries are not strong enough to power the IR LED as this would also be an unchecked failure.

    An option would be to leave the IR LED fairly accessible and use some sort of plug-in socket for the IR LED. When it does fail you can unplug and replace easily.
    Last edited: Aug 28, 2011
  4. wayneh


    Sep 9, 2010
    You could just use two (or more) LEDs in series. No change in current, but you'll need more voltage and thus more power. It would be unlikely they'd all fail together. And you'll get more distance in the meanwhile.

    Would it help to just simply KNOW whether it's out or not? Just look at it with your cellphone camera - most of them will detect and display IR and you can quickly tell whether it's on or not.
  5. leonhart88

    Thread Starter AAC Fanatic!

    Feb 23, 2007
    Thanks for the input guys. I'm not really keen on making a circuit to detect if the LED is burnt out. Partly because I'm trying to fit the circuit into a small box so I don't want to add extra electronics for space reasons. Also, the point that iONic brought up about the sensing LED failing is another reason.

    I was going to make the entire circuit surface mount and have it potted into a very small transmitting box. However, I like the idea of the plug-in LED that makes it easy to change. I will think about this.

    Just FYI, the transmitter is going to be powered by a wall wart, so no concerns about battery power.

    wayneh: Now that I think about it, I'd like to keep a constant voltage as opposed to current. I think extra current is acceptable. This means I could put the two LEDs in parallel. However, even in series or in parallel, if one burns out, it probably means the other will soon burn out as well doesn't it? Of course, this is assuming the LEDs burn out due to reaching their max lifetime as opposed to some other unexpected burn out.

    I guess I could have two in parallel to account for an unexpected burn out, and have the IR LEDs pluggable to the circuit so I can replace them easily if needed. What do you guys think?
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    If you put 2 in parallel, and each has it's own series resistor, then if one burns out the other will still work although light output fromt he device is halved. That's probably much better than no light!

    Also with 2 in parallel you can run less current per LED and significantly increase the life of both LEDs.

    Adding a small cap across each LED can also give you increased life as it gives a gentler turn on and some protection from HF spikes.