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DIGITAL LOGIC EXAMINATION QUESTION - 3

V. Ryan © 2004 - 2009

 

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1. A local systems designer has developed a system to control street lights. The street lights can be turned on manually, or by the use of a timer, so long as a light sensing unit indicates that it is dark.

1A. Below is an incomplete logic circuit for the control system. Redraw the logic circuit using the correct logic gates. Note the output of the dark/light sensor is ‘1’ (true, high, on) when it is light. The lights must be turned during the dark of night.

 

 

1B. Name the logic gates you have used:
 

GATE X .................
GATE Y .................
GATE Z ..................

   

Below is the logic circuit showing the logic states of inputs and outputs of all the gates when the street lights are ON.

   

Below is the logic circuit showing the logic states of inputs and outputs of all the gates when the street lights are OFF.

   

 

 

1C. On the logic circuit below, write the logic states of all inputs and outputs for the following:
It is night time, the manual switch is off and the timer is ‘on’. Will the street lights be on or off ?

   
   
1D. Complete the truth table for the logic circuit that controls the street lighting system.
 

   
1E. Normally a logic circuit such as the one used to control the street lights cannot power lights. In the space below name the subsystem that could be used.
   

   

 

 

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