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Adding a manual/computer control toggle; optoisolator help

Discussion in 'Electronic Basics' started by davidd31415, Aug 2, 2005.

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  1. davidd31415

    davidd31415 Guest


    I have a circuit that controls several motors and lights (all 12VDC, 2A
    max) through push buttons and switches. I'd like to expand the circuit
    to allow for computer control through a rs232c parallel port.

    What should I be concerned about with the parallel port/opto isolator
    (input resistances, maximum currents, etc). Since I am controlling
    motors would it be least expensive to use low-current opto isolators to
    energize relays? If someone has knowledge of (or a site that
    discusses) the most significant details of wiring up a circuit such as
    this, I'd appreciate a few pointers; identification of the various opto
    isolator variables would be helpful too ('If' for example).

    I am also looking for a way to implement the "manual/automatic" mode
    switching. I'll need a total of 10-15 relays if I add one in for each
    button/switch, so suggestions on a simpler method to implement this
    would be helpful.


  2. Eric R Snow

    Eric R Snow Guest

    Looking through the Jameco catalog results in finding part #174449CF
    which is a 5 volt/125 ohm coil relay with 24 volt/2 amp contacts.
    $1.79 each. The relay draws 40 milliamps at 5 volts. I don't know if
    the computer can supply this current but I think it can.
  3. Rich Grise

    Rich Grise Guest

    Not the parallel port. But you can still get ULN2803s to drive relays
    with, and that's a darlington input, so the parallel port would
    drive it easily.

    Oh, and just FYI, rs232c is the protocol on the serial port. It and
    the parallel port are very different things.

    Good Luck!
  4. Bob Monsen

    Bob Monsen Guest

    This will be much easier with a parallel port. The serial port is
    designed to talk RS232, and uses strange voltage levels.

    If you are really using the RS232 port, you can use a chip like a MAX232
    to interface with the computer.

    It takes +5V, and generates the odd voltages required.

    Now, your serial port will want to talk async, so you'll need a uart
    chip of some kind to handle all the RS232 protocolish stuff. Programming
    a uart isn't trivial, so you will want to use a microcontroller or
    something like that. Some PICs have built-in uarts, so using one of
    those would give you a lower chip count.

    Yet another way to go would be to use a pre-built USB interface module,
    like these:

    The bottom one has up to 24 I/O points, which you can undoubtedly
    program arbitrarily.

    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
  5. davidd31415

    davidd31415 Guest

    I'll probably end up going parallel with this instead of RS232; sorry
    for the confusion in the OP. Are there certain optoisolator
    specifications I should look out for and is it as simple as just
    putting the optoisolator in line with the parallel port output or do I
    need a resistor (or other component) in there as well?
  6. Rich Grise

    Rich Grise Guest

    Have Fun!
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