newbie here needing some help

[harmand]

Hi all this is my first post and if you can stop laughing long enough to help me it would be greatly appreciated. I just started class this semister and already hit with this. I know it's probably a simple solution for everybody here but not for me, lol.
I am working with logixpro and my problem is, I need to write a program to simulate this.
I need to have 3 spst switches operate a light. I also need an internal relay bit in this. Simple enough.

I did good with 2 switches but the third one threw a wrench in the gears...Thanks for any help

 

[OZEE]

What do you have? Share that with us and we will help you -- but we won't DO it for you...

 

[harmand]

I have got it to work with 2 switches but when I try and put in a third one it will not work at all. Why would I need an internal relay? I have this mental block going on.
I have switch 1 with an xio and xic and switch 2 with the same in a branch. If I add another branch the whole thing goes south. I am not asking for a solution, just someone to point me in the right direction.

 

[just_lionel]

Well there are a few different ways of doing it really.

Depends on what type of switches you have and what type of contacts there are. If you have to use an internal relay bit thats not a problem either. Remember you can use an internal bit as an output as well as an input.

 

[harmand]

From what I have read about an internal bit, it is used to extend the rung. This has me thinking that I am going to have a butt load of contacts. The first part of this was to use 2 spst switches to control the light. That was easy enough for me. i used switch 1 xic and switch 2 xio in series then branched and switch 1 xio and switch 2 xic. It worked great. However if I added another branch and tried a third switch it all went down hill. So now I am thinking that because I am supposed to use and internal bit I am not going to be using branches, at least not in the way that I did for the first part. Right?

 

[TConnolly]

Greetings harmand and welcome to the forum.

We are more than happy to help tutor you. We don't do homework mainly because some day you are going to be our coworker and we would like you to be competent.

It sounds like you have a good start but just got stuck. Post a screen shot for us of what you have, and also a screen shot of what you did that caused it all to go south. If you have a truth table post it. Along with that, give us an exact statement of the specific homework problem.

Are you trying to emulate a 3-way light switch and a 4-way light switch like you would find in a home lighting circuit?
If so, then here is how those work.
two switches:
When both switches are in the same position the light is off.
When both switches are not in the same position the light is on.
So we can write the boolean equation
Light = (Switch1 + Not Switch2) or (Not Switch1 + Not Switch2)


To add a third switch write the result of the first equation to an internal bit instead of the light.
Now can you think of a boolean equation that we can use using Switch3 and that internal bit? (hint:you already know it)

 

[bernie_carlton]

Alaric referred to boolean equations. Can you find the name of the boolean function which corresponds to what you are trying to do with 2 switches (inputs)? Now, can you find any information on how that same function can be accomplished with 3 switches (inputs)? Check around. You may not always find inspiration in PLC land.

 

[harmand]

Alaric, thanks alot. I tried alot of different things with the internal bit but using it like this wasn't one of them.

Now I wonder why I didn't come up with it before. I have been thinking about it for 2 days now. Sometimes it's easy to overlook the obvious. Thanks for your push in the right direction.

 

[Bitmore]

Alaric referred to boolean equations. Can you find the name of the boolean function which corresponds to what you are trying to do with 2 switches (inputs)? Now, can you find any information on how that same function can be accomplished with 3 switches (inputs)? Check around. You may not always find inspiration in PLC land.

Bernie they need to first to understand logic. Perhaps a graphic diagram would work (Venn).
Trying to explain boolean to someone who does not grasp the logic of "AND" or "OR" cannot grasp the output of "X" is a lost cause.

bitmore

 

[harmand]

Since I am new at this I sometimes forget to apply the theory and just jump in and try to write a program on the fly. I get ahead of myself.
I am trying to learn this from every angle and have been reading ALOT.
If anyone knows of more sites please let me know. Thanks

 

[markopo]

Hm... are You talking about xor of 3 inputs ????

 

[leitmotif]

I need to have 3 spst switches operate a light. I also need an internal relay bit in this.UNQUOTE

OK that part is easy. Now for the difficult part based on your logic you presented
AND
any switch can also turn off the light.
Is that the assignment??

Dan Bentler

 

[harmand]

I need to have 3 spst switches operate a light. I also need an internal relay bit in this.UNQUOTE

OK that part is easy. Now for the difficult part based on your logic you presented
AND
any switch can also turn off the light.
Is that the assignment??

Dan Bentler

Yes that is what I needed to do. I finally got it, look at the screen shot. Thanks