Latching a contact

cue2671

Member
Join Date
Jul 2005
Location
USA
Posts
5
Hello, I am a student working on a college final and I need to latch a N.O. contact closed for 5 seconds and then release. I can not figure it out although it has to be simple. Your advice would be appreciated
 
I just need to latch the contact for 5 seconds only.
I understand that, but first you have to know how to latch it forever.
Can you draw the ladder rung that will make a normally open contact latch, and remain latched?
 
yes, there will be a n.o. contact with a branch around it and a n.o. contact on the branch. the no contact on the branch will have the same address as the output energize on the end of the rung. I do not know how to draw on this.
 
would I use a off delay timer?. So, does the contact that needs to energized have the same address as the timer?
 
Im sorry for asking but..


This is for a final? I didnt think college was in session til August? Is it a summer class? If so, and you cant do this by final time I would suggest asking for a refund? Am I wrong?
 
The timer will have its own address. Usually T1 or T2.

You have the timer turn on with the output contact. Put a normally closed contact with the timers address in series with the contact and the output. When you close the first contact, start, the output energizes, seals in and holds its self on. the timer starts and times for 5 seconds then it opens its contact turning the output off. It will remain off until the input is once again started.
 
Personally I'd stay away from off delay timer.
Just have an output input start a timer in the first rung.
now on the next rung ---| |--- there should be a bit like T4:0/tt.
Now on the right side of this rung just energize ---( )--- your output.
If you have to use a latch, I guess you can add another rung to unlatch the bit when the timer is done.
Hope this is helping.
 
Last edited:
Small steps, George. Small steps. He will learn more this way...

cue2671 said:
yes, there will be a n.o. contact with a branch around it and a n.o. contact on the branch. the no contact on the branch will have the same address as the output energize on the end of the rung. I do not know how to draw on this.
OK, so far, so good. You now have:

| START
|----] [----+-----------( OUTPUT )
| |
| OUTPUT |
|----] [----+


This will, as George stated, latch forever (er, as long as the rung is being scanned). That is the correct starting point. Now, how could we unlatch this?

Don't worry about the timer just yet. Let's say you have a STOP pushbutton connected to another input. This wil be another contact in your rung. For now, pretend that YOU are the timer, and you'll press the STOP button after 5 seconds. Where would you put this STOP contact to unlatch the output?

Hint: You can use a normally-open (N.O.) or a normally-closed (N.C.) contact... ;)

🍻

-Eric
 
I can not use a stop button. I am programming a coke machine. And when I press the coke select button, a little picture flashes a picture of a coke signifying a purchase. I have to have this "picture" remain on for 5 seconds after I press the coke select button. So, when I press the coke button once, I need the "picture" to latch for 5 sec. and then unlatch itself with no stop button or OTU instruction.

Thanks,
 
Yes we know you can't use a N/C stop button, but if you could, where would you put it?

Look at Eric's sketch.
Try and draw one yourself.
 
Last edited:
Don't worry, there won't be an actual stop button in the end, but I feel it will help you understand if you ignore the timer part for now.

As per my hint, you'll use a normally-closed contact (---]/[---) for the stop button. In other words, this contact is closed (true) when the button is NOT pressed, and open (false) when the button IS pressed. More info about this HERE (see the LoadBar section)... đź““

Where should you put this contact in my 'forever latch' rung to unlatch the output when you press the STOP button?

We'll get to the timer once you understand this part. I promise.

🍻

-Eric

P.S. You don't have to draw the ladder in your reply. A text answer is fine... :nodi:
 

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