Ladder logic, lamp & timer - simple question!

carle

Joined May 28, 2012
2
Hello!

I would appreciate if someone could help me draw this one:

"You turn on a lamp by pressing the button once, and turn it off by pressing the button twice within one second (double click). How would this ladder diagram look like?"

WBahn

Joined Mar 31, 2012
26,398
What have you done thus far?

Q1) Could you draw the logic if you were only asked to design a circuit that would turn on the lamp in response to pressing one button and turn it off in response to pressing another?

Q2) Could you draw the logic if you were only asked to design a circuit that would turn on the lamp in response to pressing a button and turn it off in response to pressing that same button (i.e., the button acts as a toggle switch)?

Q3) Could you draw the logic if you were asked to modify the circuit in Q1 so that it requires two pushes on each button, within a second, to make the desired action happen?

carle

Joined May 28, 2012
2
What have you done thus far?

Q1) Could you draw the logic if you were only asked to design a circuit that would turn on the lamp in response to pressing one button and turn it off in response to pressing another?

Q2) Could you draw the logic if you were only asked to design a circuit that would turn on the lamp in response to pressing a button and turn it off in response to pressing that same button (i.e., the button acts as a toggle switch)?

Q3) Could you draw the logic if you were asked to modify the circuit in Q1 so that it requires two pushes on each button, within a second, to make the desired action happen?

Well... no. I can turn on the lamp, but I'm not sure how to turn it off. I gave Q1 a shot, and I think you use OR if you would want to use the same switch.

http://oi49.tinypic.com/717953.jpg

Isn't it Q2) that needs to be modified? The same button will be used for turning the lamp on and off.

WBahn

Joined Mar 31, 2012
26,398
Keep in mind that the problem implies the use of momentary buttons and that they are being used only momentarily. Thus, to turn the lamp on, someone presses the button and immediately releases it. To turn off the lamp, the goal is for them to have to press and release the button twice within a one second interval.

If you can go from Q2 to a modified version of Q3 that asks you to modify the result of Q2, then go for it. The purpose of Q3 isn't to be the final result, but just to be the next step before you tackle the whole thing.

So, assume that you have a lamp, a box of momentary push-button switches (of whatever type you need: NO, NC, SPST, SPDT, MPDT, whatever), and a box of relays (ladder logic historically almost always implied relay-based implementation) and, again, any type of contact arrangement you desire.

Try Q1 again.

What kind of resources have you been shown for producing time delays? There are special time-delay relays that, once deenergised, will hold the contacts in the energized state for a short amount of time. You might assume that you have some of these that have a 1s delay.

http://www.opamp-electronics.com/tutorials/digital_theory_ch_006.htm

Last edited:

mannycc

Joined Dec 11, 2010
17
Just wondering what's the purpose of the circuit that needs to press the switch 2x in order to turn off the light? Otherwise you may use the idea of flip flops to draw equivalent circuit using the relays instead of logic gates?

WBahn

Joined Mar 31, 2012
26,398
More than likely, the purpose of the problem is to give the student practice with regards to a particular concept and the whole thing about turning lamps on and off is made-up filler to provide a context. I imagine a big point of this particular problem is to have the student deal with timing events. Unfortunately, we don't know what techniques they have available to them at this point, so it is a bit hard to offer truly relevant suggestions.