limit switch (NC, NO, COM) working principle?

what is the working principal of this limit switch?
I have to use it for limiting travel of a door, window and curtain. when the object hits the plunger, the dc gear motor stops running. i know it deactivates the electrical circuit but how? plz explain the what happens actually that stops the motor?

sunnystar said:

what is the working principal of this limit switch?
I have to use it for limiting travel of a door, window and curtain. when the object hits the plunger, the dc gear motor stops running. i know it deactivates the electrical circuit but how? plz explain the what happens actually that stops the motor?

Click to expand...

See here and its associated links https://en.wikipedia.org/wiki/Limit_switch

i already refered to this link but didn't understand

It can be a switch in series with the power supply. When the object hits the plunger, the motor is disconnected from the main supply.
But they can also by connected to a digital card that acquires their status and decide which action to do.
Usually safety limit switch are in series with the supply.
NO is the normally open contact (open if the plunger is free), NC is the normally closed contact (closed if the plunger is free) and COM is the common.
Then NO normally will be open with respect to COM while NC will be shorted with COM. When the object hit the plunger, NO will be shorted with the COM while NC will be open with respect to COM

sunnystar said:

i already refered to this link but didn't understand

Click to expand...

Hi,

You can also see some working examples here **broken link removed**

It is a SPDT switch which has 3 contacts viz C (Common), NO, NC. When leaf or plunger is not pressed then C and NC are connected and NO disconnected.. When plunger is pressed C and NO connects and NC disconnects. It is used in some type of float switch. In that a metal ball presses the plunger.

https://en.wikipedia.org/wiki/Float_switch

When plunger is pressed C and NO connects and NC disconnects...but then how the electrical circuit is deactivated, wots going behind it?

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When plunger is pressed C and NO connects and NC disconnects...but then how the electrical circuit is deactivated, wots going behind it?

Hi,

See if these diagrams help.

First a simple circuit, when the switch is closed the motor runs and drives the track from left to right, you have to manually turn it off when it gets to the end.


The second shot add in a limit switch that is in the normally closed position if nothing is touching its plunger.

The third shot shows what happens when the track hits the plunger, the switch opens and breaks the circuit to the motor.

This is how it works. Instead of limit switches you can use magnetic reed switch for door close and open. If sliding door you need two switches. Another method is to calculate the time taken for the door to open. Say you need 5 sec to open the door slowly then you use a timer so that the motor rotates for 5 sec in each direction for either opening or closing the door. This way you don't need the switches. You have to manually find out at what speed the door has to open or close. You can use a single tact switch which cycles a counter in code in three steps and the 3 values of counter can select three speeds for opening and closing the door.


You can use a Bi-Directional DC motor and use PWM and Timer Interrupt to precisely open and close the door. PWM duty should slowly rise and slowly fall like 0, 10, 20, 30,...80...70, 60, 50....0. This way the moment the door opens it starts slowly opening the door and gradually speed increases and then when the door comes nearer to end point the speed falls. You need a feed back from motor shaft like if motor is rotating or not. I just found another way to control the door. use a hall sensor or IR sensor to count the no. of pulses needed for opening or closing the door. Use pressure sensors or IR sensors to detect presence of person near door which starts the opening and closing sequence. the no. of times the shaft is rotated is counted by feeding pulses to T0CKI pin of PIC. If the opening/closing of door and hence rotation of motor is forcibly stopped then there will be no pulses and the motor continues to runs after the force avoiding the opening or closing of door is removed.