The rear lights of a car are to be controlled by digital logic. There is a single lamp in each of the rear lights.
Inputs
LT left turn switch—causes blinking of left side lamp
RT right turn switch—causes blinking of right side lamp
EM emergency flasher switch—causes blinking of both lamps
BR brake applied switch—causes both lamps to be on
BL blinking signal with 1 Hz frequency
Outputs
LR power control for left rear lamp
RR power control for right rear lamp
(a) Write the equations for LR and RR. Assume that BR overrides EM and that LT and RT override BR.
(b) Implement each function LR (BL, BR, EM, LT) and RR (BL, BR, EM, RT) with a 4–to–16-line decoder and external OR gates.
Consider the following data:
Inputs of a car rear lights:
• LT- left turn switch – cause blinking of left side lamp
• RT- right turn switch – cause blinking of right side lamp
• EM- emergency flasher switch – cause blinking of both lamps
• BR- brake applied switch – cause both lamps to be on
• BL- blinking signal with 1Hz frequency
Outputs:
• LR- power control for left rear lamp
• RR- power control for right rear lamp
(a)
Consider that, \(B R\) overrides EM. \(L T\) and \(R T\) override \(B R\).
It is already known that, EM(Emergency Lights) triggers \(B L\) (Blinking light) and both \(L T\) and \(R T\) also trigger \(B L\).
The equation for both \(L R\) and \(R R\) are derived as follows:
- As EM triggers \(B L\), the expression is: \(E M \cdot B L\)
For left lamp LR :
- As \(L T\) triggers \(B L\), the expression is: \(L T . B L\)
- As \(L T\) overrides \(B R\), the expression is: \(\overline{L T} \cdot B R\)
For right lamp RR :
- As \(R T\) triggers \(B L\), the expression is: \(R T . B L\)
- As \(R T\) overrides \(B R\), the expression is: \(\overline{R T} . B R\)
On combining all the expressions of \(L R\) with expression of emergency flasher the equation of \(L R\) is obtained as follows:
\(L R=E M . B L+L T . B L+\overline{L T} . B R\)
On combining all the expressions of \(R R\) with expression of emergency flasher the equation of \(R R\) is obtained as follows:
\(R R=E M . B L+R T . B L+\overline{R T}, B R\)
(b)
To implement each function, we first must make a truth table of all possible combinations for both LR and RR.
Using LT , EM , RR , and BL as inputs the truth tables for both LR and RR are as follows:
Truth table for LR :
Truth table for RR :
Now, directly apply the truth values of LR and RR to the 4x16 decoder. The outputs of the decoder will be wired into OR gates to output LR and RR.
The logic diagram for output LR is as follows:
The logic diagram for output RR is as follows: