Reversing dc gear motor timer circuit needed

Discussion in 'The Projects Forum' started by kahuna20, Dec 6, 2007.

  1. kahuna20

    Thread Starter New Member

    Dec 6, 2007
    I'm trying to design a circuit that will automaticaly reverse the polarity of a small 12v dc gear motor drawing anywhere from 1 to 2 amps at user defined times. This circuit will be applied to raise a small scaled down R/R crossing gate at timed intervals as defined by the time needed for the motor to pull a cable across a series of pulleys, stop and then reverse to allow the gate to drop back down.

    Any help would be greatly appreciated.:)
  2. recca02

    Senior Member

    Apr 2, 2007
    maybe u can use a comparator with some reference voltage and a 555 timer output as
    another input for changing polarity at user defined times.
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    I've seen it done with relays and switches but I'm not finding a schematic and an too fuzzy to figure it out just now.
  4. kahuna20

    Thread Starter New Member

    Dec 6, 2007
    I have a Christmas "Reindeer" R/R crossing gate which I need to apply a circuit that will run a 12v gearmotor @ 2 amps at timed intervals to allow the gate to raise and lower every 30 seconds or so by cable off the motor. If anyone has applied this circuit or has knowledge of one or an alternative idea please advise of such.

    Thanks for the help ! :)
  5. HarveyH42

    Active Member

    Jul 22, 2007
    Attached is simple drawing you can use with a DPDT relay. It'll reverse the motor. You can probably find a timer circuit using a 555 or something, pretty common. Figure this would be quickest.
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    Harvey, the attachment didn't take.:confused:
  7. SgtWookie


    Jul 17, 2007
    You'll need:
    1) Two normally-closed single-pole switches (to detect the limits of the crossing gates' travel, prevents powering the motor in a "stall" condition.)
    2) A double-pole double-throw relay (to reverse the direction of current through the motor)
    3) A timer circuit (an NE555 or 1/2 of an NE556 IC) to drive the relay
    4) A diode to protect the NE555 from the reverse EMF of the relay's coil.

    I threw together a schematic of just the motor, switches and relay part; it's attached. Didn't have a DPDT relay in the library, so used two SPDT relays.

    You place one switch so that it gets pushed when the gate is almost down, the other so that it gets pushed when the gate is almost up. If you put the switches in the wrong place, just reverse the wires on the motor.

    Diode across the relay coil is not shown, nor is the timer circuit.

    However, simply energizing the relay's coil(s) will cause the gate to change it's position from open to closed, or vice versa. If you're using it for a model train, why not just use a weight-sensing switch under the tracks?

    Added a 2nd version; this one has a protection diode across the relay coil(s) and another switch; S3. Opening or closing S3 will change the position of the gate.

    If you don't have any other (cheaper) place to get components, Radio Shack carries a 12V DPDT relay as Catalog #:275-249 for $5.49, contacts are rated 125VDC @ 5A.
    They also carry a couple of microswitches that might work for S1 and S2 under Catalog #: 275-016 and Catalog #: 275-017, $2.79 each; one has a roller, one doesn't.