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Digital IO board help!! — Parallax Forums

Digital IO board help!!

HoangTran83HoangTran83 Posts: 52
edited 2010-06-25 23:13 in General Discussion
I set the Digital IO board input voltage range for 12 volts by installing 1k resistors instead of the 1.5k resistors. However, when i tested the inputs, they are triggering at a different voltage levels. When i connect 3.3 volts to the logic power, the inputs trigger at about 2 volts. But when i connect 5 volts to the logic power the inputs trigger at about 8 volts. Shouldn't the trigger voltage be at about 12 voltages?? Please help.. Thanks

Hoang

Comments

  • TubularTubular Posts: 4,718
    edited 2010-06-24 01:23
    Is this the digital IO board with optocouplers? Do you have a link to the product you're referring to?
  • HoangTran83HoangTran83 Posts: 52
    edited 2010-06-24 03:58
    Yes, this is the digital IO board kit that parallax offer.

    Hoang
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2010-06-24 19:27
    Hello,

    The inputs will always trigger at a range of voltages depending on the resistor value. The LED in the opto only needs to have a certain amount of current flowing for the transistor to conduct. It sounds like you don't want it to trigger until 12V? If that is the case you'll have to understand two things...first, you'll want to set the resistors to approximately the lowest value that will trigger the input, but also understand your range is now higher as well.

    The second thing to understand is that due to component tolerances if you try to cut it too close, some channel inputs may work whle others don't, hence the range of input voltages. You can find the sweet spot though by placing a potentiometer in place of the 1K resistor and dialing it in. You will find there will be some variance between channels, but you may be able to find a standard value that does work for you.

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    Chris Savage

    Parallax Engineering
    ·
  • HoangTran83HoangTran83 Posts: 52
    edited 2010-06-24 20:35
    Chris,

    Thanks for the helpful reply, i will try what you've suggested.

    Hoang
  • skylightskylight Posts: 1,915
    edited 2010-06-25 19:33
    would using 741's as Differential Amplifiers referenced to a stable accurate 12v source work as gates to the inputs?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2010-06-25 23:13
    You could use comparators for inputs. The idea here is eletrical isolation. You could however feed the output of the comparator (op-amp) into the opto if you like, using a small resistor for the 12V input range.

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    Chris Savage

    Parallax Engineering
    ·
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