# help with combinational logic problem

#### Coefficient

Joined Sep 5, 2012
75
A car ignition system is designed to function such that an alarm comes on whenever any of the following occur;
a)when the engine is running while the door open.
b)the door is open while the headlamp is on.
c) the key is in the ignition while the door is open.
how can i design a combinational logic circuit whose output goes high to trigger the alarm system whenever either of the conditions come up?
state all asumptions.
kindly assist.

#### AlbertHall

Joined Jun 4, 2014
12,387
a) is irrelevant because of c)
For the engine to be running the key must be in the ignition.

#### dl324

Joined Mar 30, 2015
17,022
how can i design a combinational logic circuit whose output goes high to trigger the alarm system whenever either of the conditions come up?
Do you know how to draw a truth table and use Kmaps?

#### Coefficient

Joined Sep 5, 2012
75
Do you know how to draw a truth table and use Kmaps?
i just started learning this topic and a lecturer gave me this to try out..well i know how to draw truth table but dont know to use kmap yet

#### Coefficient

Joined Sep 5, 2012
75
a) is irrelevant because of c)
For the engine to be running the key must be in the ignition.
sir Albert,
whats the better way to solve this?,,,,,,,,i will be grateful

#### AlbertHall

Joined Jun 4, 2014
12,387
i just started learning this topic and a lecturer gave me this to try out..well i know how to draw truth table but dont know to use kmap yet
There is good stuff on karnaugh maps here.

#### eetech00

Joined Jun 8, 2013
4,017
a) is irrelevant because of c)
For the engine to be running the key must be in the ignition.
Just to clarify.....

0. Engine electrical is off with key in "off" postion
1. Engine electrical is on with key in "on" position.
2. Engine electrical is on and Engine starts with key in "start" position.
3. Engine electrical is on and Engine runs with key in "run" position

Accessory power may or may not be dependent on Ignition key position.
For example, headlamps or map lights usually don't depend on ignition key position.

eT

Last edited:

#### Coefficient

Joined Sep 5, 2012
75

#### Coefficient

Joined Sep 5, 2012
75
Just to clarify.....

0. Engine electrical is off with key in "off" postion
1. Engine electrical is on with key in "on" position.
2. Engine electrical is on and Engine starts with key in "start" position.
3. Engine electrical is on and Engine runs with key in "run" position

Accessory power may or may not be dependent on Ignition key position.
For example, headlamps or map lights usually don't depend on ignition key position.

eT
apparently,does it mean the question is not correct or unsolvable?

#### eetech00

Joined Jun 8, 2013
4,017
apparently,does it mean the question is not correct or unsolvable?
No...just means there might be more logic states to consider...

#### djsfantasi

Joined Apr 11, 2010
9,185
apparently,does it mean the question is not correct or unsolvable?
It also means, take s shot and document what you've tried here.

It can be solved without knowing K-maps. Start simply. Can you tell us how many variables there are and what they are?

And for extra credit, what is the output of your logic?

#### Coefficient

Joined Sep 5, 2012
75
It also means, take s shot and document what you've tried here.

It can be solved without knowing K-maps. Start simply. Can you tell us how many variables there are and what they are?

And for extra credit, what is the output of your logic?
attached is my first step and i dont know if i am at the right direction,how do i begin from here

#### Attachments

• 15.2 KB Views: 10

#### djsfantasi

Joined Apr 11, 2010
9,185
That's a good start. But I also asked if you could distinguish between inputs and outputs. Add another column to your table with this data.

#### dl324

Joined Mar 30, 2015
17,022
i just started learning this topic and a lecturer gave me this to try out..
The Lecturer must have intended for you to use what you've been taught, or should have learned in previous training/classes, to solve the problem.

#### dl324

Joined Mar 30, 2015
17,022
how do i begin from here
You show that you have 4 variables.

How many different combinations can you have with 4 variables? Which should trigger the alarm. With that information in a table, you should be able to write the equation that will trigger the alarm.

Using a Kmap is a convenient way to minimize the logic.

#### djsfantasi

Joined Apr 11, 2010
9,185
The Lecturer must have intended for you to use what you've been taught, or should have learned in previous training/classes, to solve the problem.
@Coefficient what dl324 says is exactly what I meant when I said that you don't need to use a kmap to solve this. Waiting your reply to my question.

P.S. If you want to get a head start, my next question is going to be, of the original conditions defined, are all of them necessary? That is, which conditions are you going to use in solving this problem? A, b and/or c?

#### Coefficient

Joined Sep 5, 2012
75
@Coefficient what dl324 says is exactly what I meant when I said that you don't need to use a kmap to solve this. Waiting your reply to my question.

P.S. If you want to get a head start, my next question is going to be, of the original conditions defined, are all of them necessary? That is, which conditions are you going to use in solving this problem? A, b and/or c?
You show that you have 4 variables.

How many different combinations can you have with 4 variables? Which should trigger the alarm. With that information in a table, you should be able to write the equation that will trigger the alarm.

Using a Kmap is a convenient way to minimize the logic.
well.
he treated Boolean algebra,demorgan theorem so far with some calculations,but this paritcular assisgnment is an applied question different from what he taught.

from what i undestand so far on the assignment is;
· The key is in the ignition (1) AND the door is open (1), OR may be

· The headlamp is on(1) AND the door is open(1), OR

· The alarm sounds(1) when the car is running and the door is open (1)
i can really place it and i hope i am not fooling around anyways

#### djsfantasi

Joined Apr 11, 2010
9,185
So close!

And you may find that this applied question is NOT different from what he taught.

But I am getting ahead of myself.

Someone previously in this thread made a comment about what conditions may NOT be necessary. Can you identify it/them?

If so, rewrite your description that you gave with only the necessary conditions. Your instructor threw in a curveball, but I am confident that you can find it by answering my question.

#### djsfantasi

Joined Apr 11, 2010
9,185
Oh, and you still haven't a column to your table identifying the inputs and output(s).

#### Coefficient

Joined Sep 5, 2012
75
So close!

And you may find that this applied question is NOT different from what he taught.

But I am getting ahead of myself.

Someone previously in this thread made a comment about what conditions may NOT be necessary. Can you identify it/them?

If so, rewrite your description that you gave with only the necessary conditions. Your instructor threw in a curveball, but I am confident that you can find it by answering my question.
yea,he said for the engine to running,the key must be in the ignition.
but funny enough,the lecturer didn`t state if the car will be in motion or not not OR that the engine will be off when the key is in the ignition