# Logic circuit design

Discussion in 'Homework Help' started by Azimuth15, Apr 19, 2015.

1. ### Azimuth15 Thread Starter New Member

Apr 19, 2015
2
0
Hey guys,

Long time lurker having an issue with some conditions presented in this project.

I have to develop a logic circuit to control the exit of personnel from a high security processing facility. Also, I have been given a number of conditions that apply to an exit door as well as a separate search room door.

One specific condition regarding a fire alarm is that the only time both doors will unlock is when the fire alarm is engaged; however, I have encountered several other instances in which both doors will unlock when the fire alarm not engaged. I have tried to find a way to prevent this from happening but I am stuck on this issue.

So far I have developed the Boolean equations for each of the two doors and have simplified each of those equations. I have also created the truth table for these equations and have some preliminary circuits designed for them.

I will attach a pdf containing the Boolean expressions I have created for this project as well as the truth table.

I have highlighted the conditions in which the fire alarm is on in a purple highlight. Additionally, I have highlighted the conditions in which both the exit room as well as the search room doors will become unlocked when the fire alarm is not engaged.

The actual conditions are within a project document I have and are quite lengthy. If anyone would like me to expand on them I can, however, I beleive the Boolean equations outline the conditions just fine.

F-represents the fire alarm =0 when engaged =1 when not engaged

C-represents the pass-code required =1 when correct =0 when not correct

D-represents the entrance door sensor =1 if closed =0 if open

T- represents the shift time =1 if day shift =0 if night shift

P- represents the panic switch =0 if active =1 if inactive

V - represents the security over-ride. =1 when engaged =0 when not engaged

The outputs are represented as:

X for the exit room door
S for the search room door

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2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,507
512
Problem 1.
Perhaps I am being unfair, but my education in this subject says that: First, you do truth table. Second, you derive boolean equations from the truth table. This also happens to produce the most minimal boolean equations.

In conclusion. The fact that you have produced boolean equations first and truth table second sets off all kinds of alarms in my mind.

3. ### Azimuth15 Thread Starter New Member

Apr 19, 2015
2
0
Appreciate the reply. I will attempt the problem starting with the truth table and see if I produce some different results

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
It's nearly impossible (if not outright impossible) to tell if your work is correct or not. You have inputs that you don't say anything about, such as this "panic switch". What is supposed to happen when that is pressed? You also have a variable for which shift it is, yet say nothing about why or how that matters.

You talk about "both doors" yet you seem to be describing more than two doors -- you mention an exit door and a search room door, but then you have this "entrance door" sensor as one of your inputs but never describe the role it plays.

If you just want to ensure that both door are only unlocked under certain conditions, then just go down your truth table and for every condition that violates that constraint, decide which door should remain locked and change that output to locked.

If you think that the constraints are supposed to be adequate to result in this happening and that your equations must therefore have a problem, then how can we possibly help when you haven't given the full list of constraints from which your equations were derived.

5. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Often times you never do a truth table because you have too many variables involved. Designing a system based on developing the Boolean expressions that embody the system constraints is just fine -- but whichever way you do it you need to be clear on what the constraints are.