Limit switches are used for rollup curtains and moveable vents such as roof vents to stop the motor operation at the desired positions for fully open or fully closed vents.
Advancing Alternatives provides two types of limit switches. The first type, referred to as the “internal limit switch”, is integral in the LVM line of low voltage DC motors. This arrangement utilizes an assembly attached to the motor gearbox that contains two micro switches. As the motor turns, it drives two slowly moving cams with lobes that eventually contact the micro switches, stopping the motor. Each of the two cams is attached to an adjustment knob that turns the cam to adjust the stop point of the motor. One cam is for the open position, and the other is for the closed position. The limit switch has a clear cover so that the operation of the assembly can be visibly observed. A Phillips head screw in the center of each adjustment knob can be loosened to allow adjustment.
The second type, referred to as the “external limit switch”, is a unit separate from the motor that is mounted such that when a vent reaches its desired stop point it presses against the switch actuator, tripping the switch. An external limit switch is important when a precise stop is needed, such as a roof vent or a roll-lock curtain vent. This is because the metal frame of the greenhouse expands and contracts with temperature, causing the exact stopping point of the internal limit switches to drift. When installing an external limit switch, it is important to observe the polarity of the connection (typically a white and black wire). This is because the switch contains a diode to allow the motor to run in the reverse direction after it is stopped by opening its connection.
Additional detailed information is available on the Advancing Alternatives website to assist in using both internal and external limit switches. A two-minute video is also available which provides helpful instruction on setting the internal motor limit switches.