Need advice - reversing DC motor direction and timer

Discussion in 'The Projects Forum' started by ChrisN, Apr 21, 2011.

  1. ChrisN

    Thread Starter New Member

    Apr 15, 2011
    3
    0
    Hi folks. I hope it's OK to post this question as my first post to the forum. I have no electronics skills (that will be quickly apparent!) but I do have a basic understanding of DC circuits and can manage a soldering iron.

    My main hobby is traditional (darkroom) photography and I need a timing device that will power a gadget to rotate my film developing tank for me. I need to be able to reverse the direction of rotation at set intervals, and I need to allow the motor to come to a complete stop before reversing the direction.

    The gadget is built, and works, powered by a 12v 4rpm geared motor that according to the specs draws 200 ma. I've tested reversing the motor manually with a DPDT centre-off switch wired for reversing polarity. Now I wish to automate the process of reversing the polarity.



    The cycle of operations I want is as follows:
    • Switch on
    • motor runs clockwise for 30 seconds (or thereabouts)
    • motor stops for 2 seconds
    • motor runs anti-clockwise for 30 seconds
    • motor stops - and the cycle repeats for as long as I leave the whole thing turned on, usually 20 minutes to two hours (manually controlled, because I have to be there to empty the tank immediately the time period is up).
    I've looked around this forum, and found some promising leads but I lack the electronics knowledge to follow through. I am confident I can build a kit, and have found a Jaycar kit KC5464 Flexi/Interval timer that appears to offer the basics I need:
    • ability to set a suitable time interval independently for ON time and OFF time;
    • ability to reprogram different times for both ON and OFF cycles as I experiment with the gadget and different films and developers; and
    • ability to set the ON - OFF cycle to repeat continuously.
    The best I can come up with is shown in the attached diagram (apologies for any conventions I have abused!) The SPDT relay controls the polarity change, by directing power alternately to either Timer 2 or Timer 3 depending on whether the relay coil is powered. Depending on the state (ON/OFF) of Timer 1, the SPDT relay will direct power to either Timer 2 or Timer 3 for the appropriate interval.

    Timers 2 & 3 are configured to deliver 12v DC to the motor in different polarity, thus reversing the direction of rotation. The motor needs a 2-second OFF time between changes in direction. Timers 2 & 3 are each set to cycle ON (27 seconds) - OFF (5 seconds) - ON - OFF - continuously while powered.

    Timer 1 controls switching between Timers 2 & 3 through the SPDT relay. Timer 1 is set to cycle ON (30 seconds) - OFF (30 seconds) - ON - OFF continuously while powered. Note that this ON cycle is shorter than the total ON-OFF cycle time of the downstream Timers. Timer 2 will complete its 27-second ON time and 3 seconds of its OFF time before it loses power as Timer 1 switches power across to Timer 3, which repeats the same cycles but outputs with reversed polarity.

    Now my questions:

    First - will this work? (Assuming I have understood what the timer does.)

    Second - do I need to put a diode into each of the + wires between the timers and the motor, to protect the dormant timer from + current flowing into the timer from the wrong direction? If yes can you please suggest a value or specification (or better yet a Jaycar part numbr :) ).

    I understand there must be smarter / more elegant / cheaper ways to do this, but due to my total lack of electronics knowledge I have to make use of parts I understand and can put together to suit my needs.

    Sorry for the long post - thanks to anyone who can take a look and advise me. Cheers!

    Chris.
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    The only 'practical' approach for this is a microcontroller.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    It will look very much like this. However, after re-reading your post, (No electronics background & lots of variable requirements) I think this will be far beyond you. This project will require the ability to read a schematic and to program a uC. I guess that's why I'm the only idiot that replied.
     
    Last edited: Apr 22, 2011
  4. ChrisN

    Thread Starter New Member

    Apr 15, 2011
    3
    0
    Yes, you're absolutely right. :)

    I've built one of the timers and tests show it will do the job.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Best of luck!;)
     
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