Newbie - IR LED array with red LED status light?

Discussion in 'The Projects Forum' started by lavalamp, May 6, 2010.

  1. lavalamp

    Thread Starter New Member

    May 6, 2010
    Hi all,

    I am a bit unsure about the correct and easiest way to add a power indicator LED to a array of infrared LEDs.

    I am using a 9V battery + 7 IR LEDs in an array. The IR LEDs require:
    Forward Voltage: 1.2 V
    Forward Current: 100 mA

    So I am wiring the circuit in a series with a 6.2 Ohm resistor.

    Here is where my challenge comes in. Since the IR LEDs cannot be scene by the naked eye, I need to add a "status light" to indicate the array is powered on.

    So, could anyone make a suggestion as to how I can add this feature along with the series array?

    The Red LED requires 1.7V and a forwarding current of 20 mA.

    I am a bit confused about how to stick this one into the equation. Any help would be GREATLY appreciated!

    Thanks so much :)
    Last edited: May 6, 2010
  2. SgtWookie


    Jul 17, 2007
    Did you get the IR LEDs from Radio Shack, by any chance?
    If so, if you try to put more than about 30mA continuous current through them, you will fry them. A sample I tested burned up at 43mA.

    The package is marked for Vf @ 100mA, I know. However, the bags were labelled by monkeys, because 100mA is the peak current that can be applied for about 1mS at a 10% duty cycle (10mS off).

    If you are operating them from a 9v battery, it will not last long at all with a 100mA drain. Alkaline 9v batteries are rated for around 500mAh; that's being discharged at a rate of 25mA over 20 hours.

    9v "transistor" batteries have a high internal resistance compared to other batteries. The heavier the load, the more power will be dissipated inside the battery itself. If you want your LEDs to light for a much longer period of time, then use AA batteries, either NiCD or alkaline.

    However, you'll need to give more information about how you want to use these first.
  3. Bychon


    Mar 12, 2010
    If you connect a resistor of size from 390 ohms to 750 ohms to the +9 volt supply, then to the red LED, and the other end of the red LED to the - terminal of the battery and get the LED installed in the right direction, it will light up when the voltage is turned on.
    lavalamp likes this.
  4. lavalamp

    Thread Starter New Member

    May 6, 2010
    Wow thank you for all of the information! Yes these IR LEDs are from Radio Shack. I certainly found it odd that the rating was 100mA, since I could not find any IR LEDs rated that high anywhere upon searching the web.

    The IR array will be used for night vision with a infrared sensitive camera. It will be running for about 2 or 3 hours at a time. So where do you recommend I go from here? I could do two "AAA" batteries instead. I need to keep the completed project small and light in weight.

  5. retched

    AAC Fanatic!

    Dec 5, 2009
    You can use 2 AAAs, You will have a little less time, but It will work.

    Consider using a battery pack on your belt. then run 2 wires to the circuit.
  6. SgtWookie


    Jul 17, 2007
    Have I gotta circuit for you!

    Have a look at the attached .pdf document.

    Scroll to the bottom of the 3rd page.

    A single AAA battery won't make the LEDs very bright. Two AAA batteries will make them REALLY bright.