Use of 5 V DC as high signal (digital)

Discussion in 'The Projects Forum' started by Kangsmakz, Jan 30, 2012.

  1. Kangsmakz

    Thread Starter New Member

    Jan 30, 2012
    3
    0
    I need to apply a high signal for a few seconds/minutes to the digital input of a given instrument every 48 hours.

    The simplest way of making this that I could think of consists of:
    - A mains timer (set to to turn on every 48 hours and stay on for 1 minute);
    - A mains AC to 5 V DC adapter (5 V connected to digital input of instrument).

    However, I am not sure if I can connect the 5 V DC directly to the Digital Input. Could anyone please let me know if I need to improve the above so that I do not damage the instrument?

    Thank you for your time.
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    What is the digital instrument? Does it have an internal 5VDC? Could you use a simple contact closure rather than an external power source? Does the mains timer you refer to have the ability to output for only a few seconds/minutes every 48hrs? Answers to these questions will help you get better answers to help you with your overall project.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Check the specifications of the particular digital input of the 'given instrument' in question and that will tell you precisely what voltage you need to apply to be properly recognized. Manuals are very good sources of this information.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,083
    3,021
    In addition to Bill's questions, you should specify whether your 5v adapter is regulated, for instance a USB power source or phone charger, or just an old-fashioned wall wart. The latter would not be good to connect directly to your device, because the voltage will be much higher than 5v when not loaded. The former would be fine.
     
  5. Kangsmakz

    Thread Starter New Member

    Jan 30, 2012
    3
    0
    Thank you for your replies. More information:

    > What is the digital instrument?
    It (measures and) controls the level of liquid in a tank. It usually starts filling when it measures the level of liquid to be at 10% or less (and stop filling when it reaches 90%). But it allows me to bypass the 10% check if I apply a high signal to a given digital input.

    > Does it have an internal 5VDC?
    Yes, it does.

    > Could you use a simple contact closure rather than an external power source?
    Sure, but then I would have to make or find another timer option (please see next answer).

    > Does the mains timer you refer to have the ability to output for only a few seconds/minutes every 48hrs?
    Yes, I have the following item in mind:
    http://www.clasohlson.co.uk/Product/Product.aspx?id=158191095
    Its 20 programs would allow me lots of freedom, in case I need (I will probably need some).

    > Check the specifications of the particular digital input of the 'given instrument' in question and that will tell you precisely what voltage you need to apply to be properly recognized.
    Done before asking the question. It needs a high line at a particular digital input. I usually work with solutions which have many analog and digital input and outputs. This is the first time I just need a single digital output, so I am not sure if I need to add protection such that a 5 V DC from a power supply does not damage the instrument, or if digital inputs are protected by default (provided the voltage is reasonable).

    > you should specify whether your 5v adapter is regulated
    Indeed I should. ;-)
    It is regulated. Though, since the one I have in hands is very el cheapo I intend to test it before use.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    If you can find a lines operated 48 hr timer & 5V DC wall wart, should work fine. You can limit 5 V output with simple resistor, about 1kΩ & 5 V zenor diode.
     
  7. Kangsmakz

    Thread Starter New Member

    Jan 30, 2012
    3
    0
    Good idea! If I end up using the el muy cheapo 5 VDC adapter, I will use a Zener regulator.
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I would consider adding a current limiting resistor in series between the 5v output and the input to your device. Perhaps the design of the device makes this superfluous, however.
     
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