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help with IR LED?

Discussion in 'LEDs and Optoelectronics' started by rebeltaz, Mar 2, 2015.

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  1. rebeltaz

    rebeltaz

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    Nov 22, 2012
    I am trying to design an IR-based automatic light for a hall. I have the flip-flop/control portion of the circuit working fine. My problem is the IR detector pair. I tried using a generic IR LED and photodiode I had in my stock and I couldn't get reliable distance using those. So I purchased a couple of OPTEK OPB100Z pairs from DigiKey. The pair consists of an OPB100-EZ LED and an OPB100-SZ phototransistor and is supposed to offer a range of up to 36". The datasheet, though, only talks about a range of 12".

    I wrote the company and they told me to get 36", I need to pulse the LED at 300mA for 1us. Looking at the LED's datasheet, though (OP298), it shows that maximum peak current is 2A for 25us.

    My problem, though, is that in experimenting with cheaper LEDs right now (so I don't burn out this somewhat expensive (more than a standard LED)) I can't get it working right. I have built a 555 timer circuit (attached). I am testing with blue LEDs rated at 2v 24mA - peak 75mA powered by 12v. Using 147 ohms for R1 (for 70mA), the LED is at least half as dim as when I power an identical LED directly from 12v through a 470 ohm resistor (for 20mA). Also, a current meter in series with the 147 ohm resistor only shows 28mA, not 70mA. I figure that MIGHT be because the voltage is pulsed, but that doesn't explain why the LED is DIMMER. I know I am running the 555 at about 57% duty cycle, but I would think anything higher would risk burning out the LED.

    I am open to any advice at this point. Thanks.
     

    Attached Files:

  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    It's because the LED is only on for approx. 1/2 the cycle time. This is how light dimming works for LEDs using PWM. Over a certain period of time between pulses if you increase the on time of the LED it will appear brighter. Think about it like this, the longer the on time the closer it is to being on all the time, i.e constant D.C and not varying D.C. It's how our eyes perceive the brightness, but for a sensor it only see's the irradiance of light it doesn't see this so called dimness we do.
    Cheers
    Adam
     
  3. rebeltaz

    rebeltaz

    32
    0
    Nov 22, 2012
    Yeah... that was what I figured. So the sensor will "see" the LED at the full (in this case) 70mA brilliance? And I can ignore the 28mA reading, right? Thanks :)
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    The peak irradiance is directly linked to the current supplied to the LED so if the peak current is 70 mA then the receiver will see an irradiance of what ever mW/sr the LED produces at that current with the distance, surface area of the detector and the receiving angle of the detector (lambert's cosine law) taken into account.
    Thanks
    Adam
     
  5. rebeltaz

    rebeltaz

    32
    0
    Nov 22, 2012
    I understand that. I just want to be sure that the meter reading I am getting is reading AVERAGE current and not peak. Thank you for your help.
     
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    If it a multi meter then yes.
    Adam
     
  7. rebeltaz

    rebeltaz

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    Nov 22, 2012
    Awesome. That was what I thought. Just wanted to double check. Thank you!
     
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