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IR Illuminator help — Parallax Forums

IR Illuminator help

NWUpgradesNWUpgrades Posts: 292
edited 2012-06-12 18:10 in General Discussion
OK, So I bought these IR LED's on EBay: http://www.ebay.com/itm/290554033785?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

I want to make an IR Illuminator as I have an LPR camera that only works in daylight. I would like to run on 12V DC. Could someone tell me what I would need for resistors using all 100 of the LED's? I would like to use as few resistors as possible so as not to take up a lot of space on the board I am using. Thanks for any help you can provide.

Comments

  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-06-12 13:29
    Will this be regulated 12V or a 12V battery?
    If a battery, white type?

    Duane J
  • Mike GreenMike Green Posts: 23,101
    edited 2012-06-12 13:40
    The number of resistors depends on how many LEDs you can put in series. Each series string has to have a resistor. It would be nice to know the forward voltage of the LEDs (Vf). For IR LEDs, Vf is less than 1.63V, but could be as low as 1.4V. Let's say you put 7 in series, that would be a total voltage drop of between 9.8V and 11.4V. You'd need a series resistor that would limit the current to somewhere on the order of 15mA for a typical LED. The exact resistor value would depend on the actual voltage drop and the maximum continuous current that the LED is designed for. It would also depend on the actual power supply voltage which may not be exactly 12V. For a start, measure the power supply voltage. Put 7 LEDs in series and try a 470 Ohm resistor in series, then measure the voltage across the LEDs and the current through them.
  • NWUpgradesNWUpgrades Posts: 292
    edited 2012-06-12 14:03
    The power supply will be a 12V DC wall wart. The forward voltage stated on the web site is 1.3~1.7. I plan to run 10 rows of 10 LED's in series and then run the series to parallel, if that makes sense.
  • NWUpgradesNWUpgrades Posts: 292
    edited 2012-06-12 14:05
    Mike, I will do the test of 7 LED's once they arrive. Thanks for the help.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-06-12 14:07
    NWUpgrades wrote:
    I plan to run 10 rows of 10 LED's in series and then run the series to parallel, if that makes sense.
    It would only make sense if your PS puts out well over the stated 12VDC -- which it may at low currents. Even at the lowest end of the forward voltage spectrum, you would need at least 13V to light a string of 10 LEDs in series.

    -Phil
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-06-12 14:25
    NWUpgrades wrote: »
    The forward voltage stated on the web site is 1.3~1.7. I plan to run 10 rows of 10 LED's in series and then run the series to parallel, if that makes sense.

    No, not really. 10 in series would require 13V to 17V to power them.

    I think you want enough voltage across the resistor that small voltage difference don't cause dramatic current changes. So you might want to only use five or six LEDs in series so the current remains relatively stable with small changes in voltages.
  • schillschill Posts: 741
    edited 2012-06-12 14:44
    Also, the more you have in series, the more light you lose when one of them fails. If one fails "open", then you will lose all of the LEDs in that series until you replace the failed one. If one fails "closed" (a short), then you will be increasing the current through the others possible reducing their lives as well.

    This was (maybe still is) a problem with a lot of cheap imported flashlights and illuminators. They would put 20 or more LEDs (more for most illuminators) and over drive them to begin with to get more light out of them. Once one LED failed, others would quickly follow.
  • NWUpgradesNWUpgrades Posts: 292
    edited 2012-06-12 14:51
    I was going to do 10x10 because after researching online I found that a square will produce more distance for the light. Thanks to all for the tips.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-06-12 15:03
    NWUpgrades wrote: »
    I was going to do 10x10 because after researching online I found that a square will produce more distance for the light. Thanks to all for the tips.

    You can still arrange them in a square, you'll just want to use seven or less (probably less) LEDs per resistor.

    A resistor every 5 LEDs might be easiest. You could have the ground wire running across the center of the square. The 12V line could run across the top and buttom of the square with a resistor between the 12V line and the first LED of the 5 LED strand Or it might be better to switch the 12V to the center with grounds on the top and bottom. Either way, I think a 10 x 10 array broken into stands of 5 LEDs would make your task a bit easier.

    Are you going to use perf board to mount them?

    Here's a 10x12 array I made.

    attachment.php?attachmentid=80519&d=1303582009

    I'd think it should be possible to mount the LEDs closer together than this by staggering the rows. The board I used here was 8cm by 10cm (3.15x3.94") so I think a 10x10 array should be able to fit within a 3" square.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-06-12 15:03
    Here's a circuit that will provide constant illumination over a range of supply voltages and ameliorate some of the issues that schill brought up:

    attachment.php?attachmentid=93474&d=1339538590

    The LM317 regulates the overall current of the four strings of LEDs to 83 mA, or about 20 mA per string. You will need five such circuit blocks for 100 LEDs.

    -Phil
    20_leds.gif
    214 x 493 - 4K
  • xanaduxanadu Posts: 3,347
    edited 2012-06-12 15:30
    100 LEDs and resistors for $5... Wow.

    Is 850nm good for IR illumination? Most of the CCTV ones I've seen emit enough visible red light I would think they were closer to ~750nm. I guess it really depends on the camera. I'd love to see your results with it :) IR illumination is expensive off the shelf...

    Nevermind I looked it up most are 850nm.
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-06-12 15:45
    850nm is pretty standard for infrared LEDs.

    Basically these are strait silicon LEDs.
    The same material as in Photo Voltaic solar cells.

    Duane J
  • jim N8RHQjim N8RHQ Posts: 85
    edited 2012-06-12 16:17
    I would go ahead and arrange them 10 x 10, but wire them in groups of 5.

    I bought some led panels, that had two parallel groups of 56 leds in series. Out of a total of five, only one, with a weak power supply, survived. It was a very bad design, no current limiting resistors, nothing. Most of the leds failed closed, so naturally the rest got gradually brighter until they all went dark.

    Keeping the groups smaller means less to replace if(when) something goes wrong in a group, like when you start increasing the current because you want it brighter...
  • NWUpgradesNWUpgrades Posts: 292
    edited 2012-06-12 18:10
    Thanks for all the great advice. I think I will go with Phil's advie and see how that works out. I will post after the LED's arrive and i get this completed.
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