I have two actuators, part of two different mechanisms:

  1. A cheap brushed DC motor with gearbox coupled to 2 limit switches
  2. A Firgelli L16 linear actuator with protective limit switches (not accessible)

And I want to control both of them with a 12V supply via a DPDT to change direction to limit the parts count. I can see two problems:

  1. I think the linear actuator is protected against supply cutoffs, but not the motor => inductive spikes
  2. The limit switches have to be transparent to the linear actuator since it is on a different mechanism.

Will this work? Is there any simpler? PXTY refers to the switch between pole X and trigger Y of the DPDT, LS is limit switch and CW/CCW are clockwise/counterclockwise.


simulate this circuit – Schematic created using CircuitLab

Note that the DPDT will have a center off position. Here is the "truth table" of the motor:

  • DPDT={T1} AND LSCW={open} (LSCCW don't care) => Motor OFF
  • DPDT={T1} AND LSCW={closed} (LSCCW don't care) => Motor ON CW
  • DPDT={T2} AND LSCCW={open} (LSCW don't care) => Motor OFF
  • DPDT={T2} AND LSCCW={closed} (LSCW don't care) => Motor ON CCW
  • DPDT={C} (LSCCW and LSCW don't care) => Motor OFF Linear actuator OFF
  • DPDT={T1} (LSCCW and LSCW don't care) => Linear actuator ON UP
  • DPDT={T2}(LSCCW and LSCW don't care) => Linear actuator ON DOWN

Looks okay to me (you could add 1N5401 diodes on the linear actuator left side if you want, wouldn't hurt anything) except I think the limit switches should be in series. The way you've shown them if either limit switch is closed the motor will continue to run, and you probably want the opposite (if either limit switch is open the motor will stop).

Edit: This should illustrate the problem (CW limit switch has opened, but motor continues to run and jams)

enter image description here

And the solution


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ Thanks, I had the feeling something was wrong with it. Perhaps I am too tired, but I don't see it on the schematic. Could you copy paste the schematic and edit it? To be sure we are on the same page, I wrote the truth table of the actuators in my post. \$\endgroup\$ – user42875 Jul 21 '15 at 14:41

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