Phototransistors not going completely LOW

Discussion in 'The Projects Forum' started by beenthere, Mar 30, 2010.

  1. beenthere

    Thread Starter Retired Moderator

    Apr 20, 2004
    I see a couple of potential problems. Your emitters are not being driven very hard. The mocrocontroller's outputs are not meant to push current. Using a Darlington array (one of the ULN series, like the ULN2803) will let you operate those IRLED's at closer to 50 ma so they are a lot brighter.

    Second, your phototransistors turn on when lit and pull the level down. Using a larger resistor in series, like 27K, will let the signal drop closer to ground. At any rate, going to within .1 volt is well within the LOW logic level.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    The sensor output needs to apply a high to the input of the PIC when the phototransisor is illuminated. Conversely, the sensor output need to apply a low to the input of the PIC when the phototransistor is not illuminated. One possible explanation would be there is a portion of ambient light source illuminating the phototransistor. You can determine if this is a factor contributing to your problem by darkening the room and testing the performance of the sensor. If this is the problem you can use a technique referred to as aperturing. Aperturing involves placing a short piece of tubing between the phototransistor and the LED. The tubing is placed over the phototransistor so that it lines up with light source.