A home security system has 15 sensors that detect open doors and windows.

Thread Starter

Ansh Kumar

Joined Oct 28, 2018
5
Hey all,
I'm having trouble with this homework problem from my digital logic class.

A home security system has 15 sensors that detect open doors and windows. Each sensor produces a digital signal 0 when the window or door is closed and 1 when the window or door is open. The control for the security system is a microcontroller with eight digital input/output bits available. Each bit can be programmed to be either an input or an output. Design a logic circuit using dual 8-to-1-line multiplexer that repeatedly checks each of the 15 sensor values by connecting the sensor output to a microcontroller input/output that is programmed to be an input.

I would love to show you all the progress I have made, but I can't even figure out where to start. If someone could just point me in the right direction that would be fantastic.
 

ericgibbs

Joined Jan 29, 2010
11,111
hi Ansh,
Welcome to AAC.
As it is a Homework problem, we would like to see your attempt at solving the problem, then we can check your work.
E
 

jpanhalt

Joined Jan 18, 2008
10,506
" Design a logic circuit using dual 8-to-1-line multiplexer that repeatedly checks each of the 15 sensor values by connecting the sensor output to a microcontroller input/output that is programmed to be an input."

If I understand that correctly, you not only have to know that at least one sensor is high, but you have to know which one(s) and can use only one input? If true, then you need a synchronizing/timing clock. If you can use 4 of the 8 available MCU inputs, it becomes easier. Please clarify.
 

jpanhalt

Joined Jan 18, 2008
10,506
I thought this homework was due "tomorrow". Have you got an extension?

Here's a 4:1 implementation -- perhaps from your textbook, but it is a standard arrangement:
upload_2018-11-1_12-43-53.png

Can you write the truth table for it? Can you extend that to give an 8:1, which will use 3 outputs of the MCU and one input? Do you also have to write the code for the MCU?
 
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