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LED torch adaption

Discussion in 'Electronic Basics' started by James Luff, Jan 6, 2008.

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  1. James Luff

    James Luff Guest

    Hi guys,

    I need to adapt a little LED torch from a standard LED light to an
    Infra-red LED. It's all very simple, but I don't know what sort of IR
    LED to get. The current LED just has one pin touching the batteries and
    the other bent back to make contact with the switch when it's on. The
    batteries are 2 x CR2032 3V Lithium batteries. Can anyone tell me what
    sort of rating IR LED I would need to get as a replacement I can't see
    any writing on the current LEDs to get a clue?
     
  2. Ecnerwal

    Ecnerwal Guest

    There generally isn't any writing to be found.

    If the driver is controlling current, just get the polarity of the new
    LED correct, and pretty much any IR LED should work - if scavenging, the
    optical encoders in mechanical computer mice provide two good IR LEDs -
    or just buy any suitable one. With a current driver, ANY LED should be
    fine.

    If it's controlling voltage, and the present LED is white, you could be
    in for trouble - but voltage control is a lousy way to drive LEDs
    anyway, as they are not well-specified in voltage, but very well
    specified in current.
     
  3. James Luff

    James Luff Guest

    Thank, but I'm new to all this so:

    <------ your explanation

    /"""""""\
    | o o |
    (| " |) <- My head
    | --- |
    \_____/

    What's a driver? How do I get the polarity right? If what's controlling
    voltage?

    There's just a single sliding switch on the torch and two pins from the
    LED, one to - on the battery and the other to the switch.
     
  4. Randy Day

    Randy Day Guest

    James Luff wrote:

    [snip]
    If you're absolutely certain there are no
    other components in the circuit, the 'driver'
    he's referring to may be built into the LED
    itself.

    If that's the case, you may have to experiment
    with an IR LED and some resistors to find
    something that will work in your torch...

    HTH
     
  5. James Luff

    James Luff Guest

    Yep, the circuit couldn't be simpler, so the driver (still don't know
    what a driver actually is) may be built into the LED I guess. Is there a
    way to check? Think I'll just pop down to Maplins, buy an IR LED and
    just see what happens, the worst I can do is bugger up the LED which
    will cost me next to nothing anyway.
     
  6. JeffM

    JeffM Guest

  7. Ecnerwal

    Ecnerwal Guest

    A circuit to control the current or voltage being fed from the battery
    to the LED.
    ...and what, if anything, is on the other side of the switch? If it's
    wired directly to the other side of the battery, then there is no driver
    as such (or it's remotely possible that there's something built into the
    base of the LED - but then the LED would look a bit different than the
    standard LED.) That would be a simple, failure-prone, but cheap
    (consumer goods like cheap, especially if the maker is well-insulated
    from customers) method of voltage control, dependent on the voltage of
    the batteries being low enough that the LED won't draw too much current.

    If the switch is connected to the other side of the battery by a
    resistor, that's a simple, though inefficient, form of current limiting,
    still somewhat dependent on the operating voltage of the LED.

    http://en.wikipedia.org/wiki/Resistor

    A White LED tends to operate at a relatively high voltage - in either of
    these cases you might burn out a IR led swapped in - but as you say,
    they are cheap, so give it a try. Might try it with only one battery at
    first, which would cut the voltage in half.

    The longer lead of a new LED should be the lead that is connected to the
    positive side of the battery.

    http://en.wikipedia.org/wiki/LED

    If there's some sort of slightly more complex looking lump (often
    enclosed in epoxy or plastic) there might be a more sophisticated driver
    controlling the current - or not. Never can tell with consumer goods
    until you dissect them.
     
  8. I think go for infrared LEDs based on output and having a beam width or
    beam angle that appears good (or is about the same as that of the visible
    LEDs if any secondary optics exist), and having overall dimensions about
    the same as those of the existing visible LEDs. I give a fair to high
    chance that if you pop out the visible LEDs and pop in the infrared ones,
    things work out at least largely A-OK.

    Battery life will probably decrease, since IR LEDs generally have a
    lower voltage drop than visible ones and if current is unregulated they
    will draw more current. They can also take more current, due to lower
    voltage drop (and also accordingly thicker bonding wires since a given
    wattage requires more current due to lower voltage drop).

    Sorry, warranty is limited to refund of what I got paid by unsatisfied
    readers (showing a receipt or cancelled check) to post this! :) :)

    - Don Klipstein ()
     
  9. redbelly

    redbelly Guest

    Yes, that seems to be how these el-cheapo 1-to-1.5" LED flashlights
    are being made. I remember measuring a few i-v points on one of these
    a year or so ago, it resembled a bare LED with no current limiting.
    The internal resistance of the batteries is the only current limit
    here.


    Yes, I'd say definitely start with just one battery and see what
    happens.

    James, do you have any way of telling if an IR LED would be, in fact,
    producing a "sufficient" IR beam in your setup? If not, you could
    experiment with a visible red LED, and if things work out then try the
    IR LED.

    Regards,

    Mark
     
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  12. James Luff

    James Luff Guest

    The problem is the little keylight style torch case has no space for a
    resistor. Let alone changing the batteries as another poster suggested.
     
  13. Then you can do without the resistor.
    The worst that can happen is you blow your IR LED, which is probably
    unlikely anyway.
    Or take out one battery, you only need one for an IR LED, there is no
    need for the 6V provided by the two batteries.

    Dave.
     
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