# LOGIC CIRCUIT DESIGN _II_

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```					LOGIC CIRCUIT DESIGN (II)
   The problem is to design a logic
circuit for TRAFFIC LIGHTS with
two inputs A, B and three outputs
R, Y, G.
TRAFFIC LIGHTS
   R, Y and G feed red, yellow and
green lights (e.g. LEDs), these
would flash in the order of Hong
Kong traffic signals when A and B
are supplied continuously by a two
bit binary code representing
numbers 0 to 3.
TRAFFIC LIGHTS

State   A   B   R   Y   G

0      0   0

1      0   1

2      1   0

3      1   1
TRAFFIC LIGHTS

State   A   B   R   Y   G

0      0   0   1   0   0

1      0   1   1   1   0

2      1   0   0   0   1

3      1   1   0   1   0
R = A B + A B

TRAFFIC LIGHTS        R = A (B + B)
R = A

State   A   B   R   Boolean Expression

0      0   0   1        R=A B

1      0   1   1        R=A B

2      1   0   0        R=A B

3      1   1   0        R=A B
Y = A B + A B

TRAFFIC LIGHTS        Y = B (A + A)
Y = B

State   A   B   Y   Boolean Expression

0      0   0   0        Y=A B

1      0   1   1        Y=A B

2      1   0   0        Y=A B

3      1   1   1        Y=A B
G = A B

TRAFFIC LIGHTS

State   A   B   G   Boolean Expression

0      0   0   0        G=A B

1      0   1   0        G=A B

2      1   0   1        G=A B

3      1   1   0        G=A B
TRAFFIC LIGHTS

The Boolean expression of Traffic Lights

R=A
Y=B
G = A．B
TRAFFIC LIGHTS

The Logic Circuit of Traffic Lights

R=A                A                  R
B                  Y
Y=B
G = A．B
G
TRAFFIC LIGHTS
Using NAND gates to implement the Boolean
Expression.
A                       R
R=A
B                      Y
Y=B
G = A．B
G
Question

   The problem is to design a logic circuit for A
COIN-OPERATED HOT DRINK MACHINE.
   It will provide a cup of tea or coffee under the
following conditions.
Question

The  conditions is ….
(1) The correct amount of coin is inserted (P),_____

(2) The cup is in position (Q), _____

(3) The selector is set at “TEA”(T) ___ “COFFEE”(C)

Please write down a Boolean expression for these
conditions under which a drink (Z) can be provided and
draw the logic circuit.
Question

The  conditions is ….
AND
(1) The correct amount of coin is inserted (P),_____
AND
(2) The cup is in position (Q), _____

(3) The selector is set at “TEA”(T) OR “COFFEE”(C)
___

Please write down a Boolean expression for these
conditions under which a drink (Z) can be provided and
draw the logic circuit.

```
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