Light Detection circuit Improvements

Discussion in 'The Projects Forum' started by JDR04, Jul 10, 2012.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Could the members of this forum please make any improvement suggestions to my circuit. They are always appreciated.

    I am really interested in finding out how one would determine what specifications the Phototransistor should be.


    Thanks a lot-JDR04
     
    Last edited: Jul 10, 2012
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    I am missing the ground connection of the LM393.
    Pin 4 should be tied to ground.
    Also the photo transistor will burn when the potentiometer is a 0 Ohms.
    Put a resistor of 100 Ohms in series with the potentiometer to protect the transistor.

    Bertus
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    Why use a LM393 dual comparator when you only need a single comparator such as LM311?
     
  4. mcasale

    Member

    Jul 18, 2011
    210
    12
    To prevent oscillations at the switching point, add some positive hysteresis around the comparator. Just a few millivolts is all you need.

    The observations that "bertus" makes are good ones.
     
  5. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks Bertus, I spotted the "no ground" after posting. Appreciate your other tip.
     
  6. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks MrChips, that's why I come to this forum all the time!
     
  7. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks mcasale, could you explain how I would go about adding positive hysteresis please. I'd appreciate that -JDR04
     
  8. mcasale

    Member

    Jul 18, 2011
    210
    12
    Here are a couple of basic references. Essentially, you add positive feedback from the output to the "+" input so that it shifts a bit after the comparator switches. It looks like an op amp amplifier that is connected wrong, but it's not. It really works!

    One note: if the comparator is an open-collector output, like the LM311, you need to include a pullup resistor to Vcc.

    http://www.analogzone.com/acqt0327.pdf
    http://home.cogeco.ca/~rpaisley4/Comparators.html
     
  9. mcasale

    Member

    Jul 18, 2011
    210
    12
  10. wayneh

    Expert

    Sep 9, 2010
    12,115
    3,039
    D1 is backwards - it'll never light as shown.
     
  11. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks mcsale, I'll have something to read in the bed tonight and better still to tinker with the day after. Really appreciate all the info. JDR04
     
  12. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks wayneh, have always appreciated your input. I'll fiddle around with the circuit and let you guys know how it goes. Thanks again -JDR04
     
  13. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,790
    If you do use a multi opamp/comparator chip and don't use all of them, don't just tie the inputs together and leave it go at that. What should the output be? It wouldn't take much noise to cause that unused amp to oscillate (and, once it got going, there might be enough parasitic feedback to keep it going). Drive the output into a known state. This can be down by tying one input to each rail (but make sure that the data sheet shows that the rails are within the allowed input voltage range). Another way is to configure it as a voltage follower and then ground the input. Yet another way is to put three resistors in series between the rails and have the amplifier see the voltage across the middle resistor. The two end ones can usually be large (100kΩ or more) and the middle one can be about a tenth that size (10kΩ).

    For amps packaged like the 393, you can use the amp on the top (high-numbered pins) as your active amplifier and then just tie Pin 1 to Pin 2 (making the other amp a unity-gain voltage follower) and tie Pin 3 to Pin 4, making the voltage that is being followed the negative rail, which is within the input voltage range.
     
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