AR40B schematics

Discussion in 'General Electronics Chat' started by vick5821, Feb 14, 2012.

  1. vick5821

    Thread Starter Member

    Jan 27, 2012
    54
    0
    Hey there,
    recently I read up AR40B schematic diagram and I got some doubt.
    My doubt is about the sensor part.Why is there a pull up resistor (4.7k) there?

    [​IMG]

    Can I just eliminate it ? or do not have the 5V there?
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    Perhaps to keep the SEN data lines high if no sensor is fitted?
     
  3. vick5821

    Thread Starter Member

    Jan 27, 2012
    54
    0
    How about if I nop connect the resistor ?
     
  4. Andreas

    Active Member

    Jan 26, 2009
    68
    4
    I agree with Paultreg and it prevents the sensor's output from floating with no connection especially. What sensors can it use, are they specific to the controller board or generic types?
    Note how the two encoder outputs don't need pull-ups.
     
  5. vick5821

    Thread Starter Member

    Jan 27, 2012
    54
    0
    If I didnt use the 4.7k resistor ? Ok ?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The sensors are most likely have open-collector or open-drain outputs. This means that they can sink current, but cannot source current.

    They frequently use open-collector outputs so that the output voltage levels can be different from the supply voltage, like yours is.

    You need the 4.7k resistors in order to have an output that is between 0v and 5v. If you leave the resistors off, the outputs of the sensors will always be at/near 0v, and the circuit will not work as it is supposed to.
     
  7. vick5821

    Thread Starter Member

    Jan 27, 2012
    54
    0
    why it will be 0/near 0 ?
     
  8. Andreas

    Active Member

    Jan 26, 2009
    68
    4
    Let's suppose the output from the sensor has an NPN Bipolar Junction Transistor as its final output drive stage. When the required base voltage is reached, current flows through the device (Collector / Emitter pathway) causing it to conduct and shunt the collectors voltage to ground. Thus the final output you will see is zero volts. When the transistor is not conducting due to no base voltage, the collector will be at the rails potential, in this case 5V. The resistor is there to limit the current through the transistor when it conducts.
    Refer to the AAC VOL III - Semiconductor pages.
     
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