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Speed controller for motorised shutters

I have an extension to my house which has motorised shutters in the roof. The motor looks like those used in car windscreen wipers. A control unit applying +12V or -12V to the motor performs opening and closing the shutters. The system works fine except that the motor is too fast, resulting in the shutters crashing closed.

Possible solutions include changing the gear ratios in the motor mechanism or using an electronic controller to the motor. The second option would seem to be preferable.

Your 12V 10A Motor Speed Controller (SILICON CHIP, June 1997, Jaycar KC5225) would appear to do the job well, however the motor needs to be able to operate in the forward and reverse direction so that the shutters open and close. Obviously I cannot simply reverse the supply polarity to the Motor Speed Controller unit.

Is there a simple solution to this problem? Can the output polarity of the Motor Speed Controller be reversed? Is there another circuit better suited to this situation? (G. B., via email).

The output from the Motor Speed Controller cannot be reversed but the connections to the motor can. Presumably the control unit switches off power once the shutters are fully open or closed and power is off after partially opening or closing the shutters.

Depending on whether the control unit for the shutters is electro-mechanical (ie, just switches and a relay) you should be able to supply the controller with power via the Motor Speed Controller. The control unit for the shutters will then provide the supply reversal for the motor. The reverse diode for the motor used in the Motor Speed Controller should be connected across the output terminals of the Motor Speed Controller and not the motor.

If the control unit for the shutters is electronic then it will need to be powered directly via the 12V supply. The Motor Speed Controller would then have to be powered by connecting it to the + and - terminals of a bridge rectifier (rated at 35A) and the AC terminals of the bridge rectifier connected to the control unit outputs that normally connect to the motor.

The Motor Speed Controller outputs would then connect to a DPDT relay rated for the motor. You could use the Jaycar SY-4008 cradle relay. Connect the NO (normally open) contact of the first pole to the NC (normally closed) contact of the second pole and this connects to one output of the shutter controller. Connect the NC contact of the first pole to the NO contact of the second pole and connect to the other shutter controller output. The common relay terminals connect to the motor.

The relay coil connects across the output of the control unit for the shutters (like the AC terminals for the diode bridge do). As before, the reverse diode for the motor should be connected across the output terminals of the Motor Speed Controller and not the motor itself.