design for photo-plethysmography circuit at IR-LED wavelength of 1550nm

Discussion in 'Digital Circuit Design' started by Haripriya R, Jun 22, 2017.

  1. Haripriya R

    Thread Starter New Member

    Jun 14, 2017
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    I'm designing a device for blood glucose measurement. The wavelength of the IR led is chosen as 1550nm as it is only at this frequency, the glucose molecules are absorbed.
    i've attached the circuit below.
    this circuit gives the output only for 950nm but i require a circuit that can detect a wavelength of 1550nm.
    kindly suggest the changes to be made in the circuit for proper output.
    upload_2017-6-22_16-34-2.png
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I'm thinking all you need to change is the LED transmitter/emitter pair. If those require different circuitry, you may need to adjust this.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    I don't see anything in the circuit that differentiates between 950nm and 1550nm.
    Change the IR emitter and detector to match the desire wavelength.

    For your application, I would use two identical circuits, one at the desired wavelength paired with another at a wavelength just outside of your desired wavelength. Take the difference between the two readings. This provides a reference or control to cancel out all other effects (e.g. temperature) not related to the parameter of interest.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Is the 39K @ pin 18 of PIC a little large ? Base drive of around 65 uA; might be better @ 3 mA ?. R of 1K ?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    And by the way I would use a dedicated pair that employ internal filtering of the carrier, which you might want to be higher than just 1kHz. There's no reason to reinvent the wheel and make your own filter. Very cheap components can do an excellent job of this for you.
     
  6. Haripriya R

    Thread Starter New Member

    Jun 14, 2017
    2
    0
    Thanks Bernard..
    I changed the resistance to 1K..
    but the problem is that the output comes properly with just the transmitter and detector and no significant output voltage occurs when a finger is placed between them.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    It could be that the amplification is too high and the output is saturating in both cases.
     
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