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Help with simple circuit, please

Discussion in 'General Electronics Discussion' started by Mikey32230, Oct 15, 2015.

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  1. Mikey32230


    Aug 21, 2015

    I am working on my first simple circuit project. I am very new to electronics, but I have picked up a very basic understanding of general components and circuitry.

    I have a few questions today. Below is a very basic schematic of my project. Don't worry too much about specific voltage/current quantities (I made the schematic very generic).


    This is how it should (typically) work:

    - Gadget 1 will turn on for a split second, allowing current to flow through the Optoisolator and activate Gadget 3.
    - Then, Gadget 2 will turn on for a split second, allowing current to flow through the Optoisolator and activate Gadget 3.

    Additional Info / Assumptions / Possibilities:
    - Gadgets 1 & 2 can potentially power-on in any order, they could even turn on at the same time.
    - Gadgets 1 & 2 can potentially remain powered-on for indefinite periods of time (seconds, minutes, technically even hours, etc).
    Now, for my actual questions :)
    As you can see, I am unsure as to what components I should insert into the two (-) return paths after the optoisolator. Do I need to insert anything? What happens if only gadget 1 is powered-on and current flows to both gadget 1 and gadget 2's negative terminals?
    If that is an issue what components do I need to add at the (-) return paths? Simple diodes don't work because you know which battery the power is coming from at any given time. Is there a particular component that people use to solve this issue?
    *Extra question:
    Would a simple NPN transistor work as a replacement for the optocoupler? If I connect gadget 1 & 2's positive paths to the base of a NPN transistor it should allow gadget 3's current to pass across the transistor's collector and emitter. However, what happens to the the current applied to the base? I don't want that extra current flowing through the transistor into Gadget 3's sub-circuit.. do transistors allow the base current to flow out the emitter or what happens to it?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    A resistor is required to limit the currrent through the optoisolator's LED. See our ressource.

    No need to worry, current can't flow from the negative through gadget 2. Mark the path the current would have to follow with an ink pen to see that this is not possible.

    Both gadgets being on at the same time will double the current through the LED. If this is an issue, use two optoisolators with the outputs in parallel, the inputs separate, one to each gadget.

    Yes it will, if you don't need the galvanic isoation provided by the optoisolator. See our other ressource.

    No need to worry here either, see reasoning above.

    Note that in any case it is preferable to connect the "-" poles together and have the resistor in the "+" path. This is not always necessary, but an often followed convention.
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