Reading a photodiode output into a window comparator.

Discussion in 'Analog & Mixed-Signal Design' started by twosacrowd, Aug 26, 2016.

  1. twosacrowd

    Thread Starter New Member

    Jul 3, 2009
    5
    0
    002.jpg

    Hi, I wonder if anyone could help me with this. I am wishing to indicate three different voltage levels using the attached circuit. I am looking for LED's to indicate a Low, Medium and High condition, so I only ever want one LED on at a time. I thought this circuit would do this for me, but I have conditions where I can have a Low and Medium LED on, or a Medium and High LED on. Am I doing something wrong or have I made a mistake in the design? If anyone can suggest improvements or other designs I would be very grateful thanks.
    John
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    I have taken your image rotated it (I do not want to turn my monitor to see it), changed white balance (the image was hardly visible) and scaled it down:

    twosacrowd_002_corrected.jpg

    Bertus
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,809
    1,105
    The LEDs should have sereies resistors to limit their current.
    Try connecting pin 11 to 2.5V (using a voltage divider between the power rails) instead of to the pot wiper.
     
  4. twosacrowd

    Thread Starter New Member

    Jul 3, 2009
    5
    0
    Thanks for your reply. And appreciate your editing of the document. I am using the 33 ohm as my LED current limit. I need the option of varying the thresholds and that is why I am tying pin 11 to the wiper. I need a variable window to illuminate the centre LED; but I get the condition of a certain times middle and upper or middle and lower LED's are on. I must be doing something wrong but don't see what it is. Any further assistance greatly appreciated ?
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,809
    1,105
    You need only the two adjustable threshold voltages you already have in order to define the variable window: the low and the high thresholds established with the two pots. That is why I suggested 2.5V as the reference for the third opamp. Try it, and see if it cures the problem.

    Edit: You could also try adding a resistor (~100k) from pin 10 to ground, to provide a fixed path for the opamp bias current. 33 Ohm seems rather low for the limit resistor. What is the LED current rating?
     
    Last edited: Sep 29, 2016
  6. schmitt trigger

    Active Member

    Jul 12, 2010
    37
    7
    33 ohm is way too low for LEDs supplied from 5 volt.
    With such a low value, the opamp will attemp to sink 90 mA. which of course it can't. The end result is that it will current-limit and the output voltage will not drop enough (close to zero) to allow your diode-OR to operate correctly.

    Use 330 ohm.
     
  7. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,968
    387
    Yep, agree, 330Ω for the LEDs and 2.5V on pin 11.
     
Loading...