http://www.allaboutcircuits.com/worksheets/relay3.html

Question #13...

I believe the circuit, AS DEPICTED, could be constructed by utilizing a NOTC (on delay) timer.

I understand the author's intent of demonstrating a particular limitation of timers, but perhaps was too clever by half on this one.

 

The intention of the author is to demonstrate that one cannot plan ahead of a non-causal event.

Let us define the length of the "pulse" as Tp.

In this case, the engineer wants a circuit that will light a lamp for a standard amount of time of length Tp and that that amount ends exactly when the pushbutton is released.

One cannot know Tp amount of time before the button is released about this event. Therefore, this implementation is unfeasible.

 

The schematic indicates a specific time delay for the transition to the ON-state. It also indicates a nonspecific ON-state duration, that is user defined... by the length of time the button is depressed. Unless, of course, that red addendum to the schematic indicates a specific duration.

If that be the case, it is poorly depicted because the end of the (inferred) time coincides exactly with the release of the user controlled button. However, as an extension of that vague notion, the utilization of a one-shot timer in the circuit would render that concept feasible, and not a laughable situation... no?

Drawing "convention"(s) differ from place to place, and from time to time. Interpreting the desired outcome is the real challenge.

 

It seems that you have not understood what the question was trying to convey.

Notice how the pushbutton pulse isn't defined in time, hence its duration is not fixed nor measurable until it is over. This is the user input.

On the contrary, the engineer asked for a lamp to be lit for a constant amount of time. We know that it is constant because it is defined in space by the "dimension" arrows. The duration of the lamp pulse is application related and not given.

What makes the exercise impossible is that the engineer asks for the pushbutton be released and the lamp be switched off at the same time. This cannot be realized, since the circuit would have to know when an arbitrary duration user input pulse would end.

Is that clear?

 

Geo: "It seems that you have not understood what the question was trying to convey."

You were absolutely correct in your assertion. Now that you have explained the concept, all I can say is: They should seek out a new Engineer!

Design flaws happen... We often work with the Engineers in those situations, and hammer out the flaws when they occur. As I mentioned earlier, drawing conventions often present an obstacle to clear communication of intent. What I attempted to do here was... Build a circuit that I thought was intended, since the "as drawn" was vague to me, based on my own convention.

Having beaten this horse to death, I would only add: Since the workbooks are probably directed toward beginners, it may be helpful to change the circuit flaw to something more common in nature, (lord knows there are plenty to choose from) that would be clear and easier for one to grasp. No Engineer, regardless of how incompetent they may be, would ever design a circuit as ridiculous as the one cited.

Thanks for your time.

 

No problem. Have a nice day.