How to light a blue LED?

Discussion in 'General Electronics Chat' started by joeyd999, Aug 31, 2016.

  1. joeyd999

    Thread Starter AAC Fanatic!

    Jun 6, 2011
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    Of course I know how to light a blue LED. The question was click bait.

    I'm working on a new design -- a PIC based instrument powered by 2 'AA' batteries. Let's call the low battery condition 1.8V.

    The PIC will be powered by a 3.0V high efficiency switcher. I need to drive a single 3.3 Vf (10 - 20mA If) blue LED from the MCU.

    I have my own way of doing this. I'm looking for new and different ideas. Guidelines, in order of importance:

    1. cheap
    2. efficient
    3. number of parts

    All parts must be small (relatively speaking) and surface mount.

    Edit: assume a full compliment of PIC hardware/peripherals are at your disposal.

    Floor's open. Anyone?
     
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  2. OBW0549

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    Maybe drive it from a capacitor/diode charge pump driven by a PIC PWM output? Or, if you use any of the dsPIC33EP series, you could drive it from the Reference Oscillator output if the PWMs are all spoken for.
     
  3. RichardO

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    What voltage is the PIC running at?
    My first thought is to charge an inductor through a small MOS-FET. When the FET is turned off the charge is dumped into the LED. Choose the inductor and drive frequency to set the current in the LED.
     
  4. AlbertHall

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    Depending which PIC, and whether you are using the internal clock source, you can output the clock or a reference clock, which could drive a diode pump.
     
  5. joeyd999

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    One more preference: I'd like to power the led circuit directly from the batteries, not through the switcher. And LED brightness should be consistent from full to low batteries. You may assume that I know the exact battery voltage.
     
  6. SLK001

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    Instead of adding circuitry to light a blue LED that you found, why not get a blue LED that you can light with less than 3V?
     
  7. joeyd999

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    PIC 18F, btw.
     
  8. joeyd999

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    I'm open to suggestions. Links?
     
  9. joeyd999

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    3V from the switcher.
     
  10. OBW0549

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    In that case I'd vote for RichardO's suggestion of an inductive pump powered off the batteries, with the MOSFET driven by a PWM output from the PIC. You can then adjust the LED current and brightness by varying the PWM duty cycle according to the measured battery voltage.
     
  11. SLK001

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  12. joeyd999

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    Naturally, I've already done a parametric search at Digikey.

    I did mention surface mount above, but I didn't mention it had to have a domed top. There is only one part that fills those requirements:

    http://www.digikey.com/product-detail/en/kingbright/APTD1608LVBC-D/754-1948-2-ND/5177449

    It runs between 2.2 and 3V at 2mA -- not sure I'll get enough brightness out of it on the high end (due to current limiting resistor) or at all since it is such low power. Also, the brightness will change considerable wrt temperature.

    And sole source...big no-no...
     
  13. SLK001

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  14. wayneh

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    Sep 9, 2010
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    How is your need different than a solar garden light, which runs an LED off a single cell? Since they give these things away, they must at least be cheap. Never saw one that was surface mount but surely there is one.
     
  15. Dodgydave

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    Get a garden solar light, they give out 3.6v from a 1.2v nicad @ 100mA.
     
  16. joeyd999

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    Anything built by the millions in China is always cheap. This ain't it.

    Sorry, but I am not going to install a garden light into my product simply to light a blue status LED.

    I am looking for clever ideas that are cheap, small, and efficient. I'll probably just use a switched cap, but it's not all that efficient.
     
  17. wayneh

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    What I was thinking was that you could use the same technique they use with an IC and a single inductor, with your PIC performing the same role as the IC. It's just a Joule thief with an enable switch.
     
  18. BobTPH

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    Just run it directly from the 3V switcher. I expect it will be plenty bright and constant. I win on parts count, anyway.

    Bob
     
  19. RichardO

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    Does it have to be 3 volts? Most low voltage logic will run fine up to about 3.6 volts. This includes your PIC, I expect. ;)
     
  20. joeyd999

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    3.3V is max. Still considering my options.
     
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