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Pull down resistor for ultrasound sensor

Discussion in 'General Electronics' started by Brian, Jul 7, 2003.

  1. Brian

    Brian Guest

    Hello,

    my robot from robowatch (www.robowatch.com)has several interfaces,
    where I can connect different sensors. I have connected an ultrasound
    sensor. The robot's manual mentions a pull down resistor, which is
    supposed to be connected to the ground. Why is this necessary? I find
    this information in connection with many different sensors.

    Many thanks for an explanation.

    Brian
     
  2. Some Guy

    Some Guy Guest

    In most TTL logic, an open input acts like a high. Some sensors and logic
    chips will effectively open their outputs when they are not actively making
    them high, or when they are not "enabled". That is, the system may be
    powering down the sensor whenever it is not actively using it. A powered
    down sensor would appear open. This open state will be seen by the next
    chip down the line as a logic high, when it is actually intended to be a
    low. The pull down resistor forces the line between the open output and the
    input to be held low. (If the input is unused, we just tie it directly to
    ground) When the output goes active, it will no longer be open, but will
    now have a positive voltage. The resistor now drops enough voltage
    (effectively all of it) to keep the line high.

    So the pull down resistor is there to keep errors from creeping into the
    logic.
    A pull up resistor does essentially the same thing, but holds the line high
    unless it is being made actively low...
     
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