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How to Build Electronic Limit Switches

A limit switch is a simple on/off switch that is connected in an electrical circuit to stop the movement of some motorized object. For example, a motor may move a door to open it, and a limit switch can be placed at the point where the door is to stop. When the door hits the switch, the voltage to the motor will shut off, and the door movement will stop, preventing the door from travelling too far in that direction. Limit switches have a thousand applications, from detecting when an elevator has reached the top floor, to stopping the turning of a valve beyond a specified point.

Things You'll Need

Pushbutton normally-closed momentary switch
Hook-up wire
Solder gun or pencil
Mounting hardware

Instructions

- 1
  Mount a momentary contact switch with normally-closed contacts at the location at which the moving part of your device must stop. The moving device must touch the pushbutton knob and depress it. You can use a microswitch or a pushbutton switch.
- 2
  Wire the switch contacts in series with the motor and its associated circuity. The circuit can be AC or DC, and if using DC, there is no need to be concerned about polarity at the switch contacts, since these switches are not polarity-sensitive.
- 3
  Connect the two limit switch contacts to other circuity, rather than in series with the motor if you have an application where the motor's direction needs to be reversed once the limit switch has been activated. This will require the use of a DC motor, where a reversal of polarity on the motor's connections will cause the armature to rotate in the opposite direction.

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