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IR LED not working What gives?

  1. May 12, 2009 #1
    Hello all, quick question because this circuit doesn't seem to be working at all...

    I bought a few IR LEDs to attempt a few of the projects located http://johnnylee.net/projects/wii/" and for some obscure reason none of them are working. I hooked a AAA cell (multimeter rates it at 1.2 V) to a simple circuit featuring my IR LED and a switch. The description that came with the LED rates the forward voltage at 1.2 V, so I shouldn't be frying it, but the stupid thing won't light up.

    Before you ask, I tried it both ways, and my method of making sure it worked was aiming my camcorder at it. Tested to work on both my TV remotes and the Wii sensor bar.

    Any ideas? I don't claim to know much about electronics, but according to my experience this should be working..
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. May 12, 2009 #2

    vk6kro

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    Wikipedia says the forward voltage on IR LEDs is about 1.9 volts.

    So, maybe you don't have enough voltage to turn it on.
    I'd look for about 5 volts and put in a series resistor of about 200 ohms.

    An easy source of 5 volts is the red and black wires on the plug that goes into a hard drive in a computer. Look for a spare power plug.
     
  4. May 12, 2009 #3
    Also, be sure that you actually have the polarity of the LED correct. What is the peak wavelength of the LED?
     
  5. May 12, 2009 #4
    You cannot see whether and IR LED is ON or OFF. How are you detecting it? A photoswitch should be ok, but not a TV remote sensing input, unless you are transmitting a legitimate code.
     
  6. May 12, 2009 #5

    mgb_phys

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    OP said they was using a camcorder and checked the camcorder with a TV remote.
    It is possible that the IR led they are using is too red for the camcorder eg a 905nm while the TV remote is 780nm
     
  7. May 12, 2009 #6

    dlgoff

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    Did you have a current limiting resistor in series with the diode? If not, you might have blown the junction.
    http://en.wikipedia.org/wiki/AAA_battery" [Broken]
     
    Last edited by a moderator: May 4, 2017
  8. May 13, 2009 #7
    Ok, some stats I pulled off the info sheet of the LED.

    @vk6kro Forward Voltage: 1.2 typical, 2.0 maximum. Tested at 20mA

    @Topher925: 940 nm

    Don't know if this helps, but the Continuous Forward Current is rated at 50mA

    Also, @dlgoff, yours is probably to be the most likely option -_- Especially considering I never used a resistor in the beginning. Although it was my understanding that the circuit only used what it needed of the available power, it seems I was wrong.

    Any particular resistor I should be using? The battery claims it produces 850mA.
     
  9. May 13, 2009 #8
    I think thats the problem. If you didn't put a resistor in series with the LED its probably blown. If your only powering it with 1.5 volts then a ~51ohm resistor should work fine and keep the current on the safe side.
     
  10. May 13, 2009 #9

    mgb_phys

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    That's very red to see with a camcorder, Si CCD QE is way down by 940nm
    The remotes you tested are more likely to be 780-820nm
     
  11. May 13, 2009 #10

    berkeman

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    He could use another IR LED as a detector. Just put the LEDs nose-to-nose, and measure the output voltage of the pickup LED. It won't be very big, but he should see a small voltage if the TX LED is working.
     
  12. May 14, 2009 #11

    vk6kro

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    Fortunately, he probably hasn't blown up the LEDs. Not on 1.2 volts.
    It is possible to check them anyway.

    Check their resistance with a miultimeter. If they are short circuited in both directions, they are blown up.

    If they are not short circuited, wire them up to 5 volts via a 200 ohm resistor.
    The + of the 5 volts should go to the resistor. The other side of the resistor goes to the long lead of the LED and the other lead of the LED goes to the -ve side of the 5 volt supply.

    Now, with a multimeter on a 10 volt range, measure the voltage across the LED.
    If it is about 1.9 volts the LED is OK.

    You should be able to check this LED by viewing the LED with a camcorder or a digital camera, as you have been doing. It will light up in the camera viewfinder screen when you apply power.

    If the voltage is not about 1.9 volts and is the same as the supply voltage, try reversing the LED. If that doesn't fix it, the LED is probably open circuit.
     
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