1. UCCLES Design and Technology (Technology) 1988
(a) Two commonly available IC logic families are the TTL 74
series and the CMOS 4000 series. Make a simple comparison, in tabular form, of
at least six of the common characteristics of these two families.
(b) The diagram above shows a conveyor system
along which three different sizes of metal casings pass. A logic circuit is
required to sort the metal casings as follows:
All small casings are deflected by D. All
medium casings are deflected by E. All large casings continue to collection
point F. The problem of the time delay between sensing and activation of the
deflectors should be ignored.
(i) Write down and simplify three Boolean
expressions which describe how the sensors can distinguish between each size
of casing.
(ii) Draw a logic circuit which will activate
the deflectors as described, using AND, OR, NOT and XOR gates only.
(iii) Draw and simplify a logic circuit to
perform the same function as above using 2 input NAND gates only.
(iv) State briefly the advantages of
designing circuits using only one type of gate.
(v) How appropriate are fixed logic circuit
solutions if the basis for sorting the components is likely to change from
time to time ?
2. Use De-Morgans theorem to find
P if:
a) P = A . (B + C)
b) P = A + B . C
c) P = A . B + C
. D
d) P = (A + B) . (C + D)
3. Show that:
a) (U + V) . (U + V)
= U
b) 
c) (W . Z + W) = W
4. Work out a Boolean expression to
describe each of the following logic systems.
5. Part of an automated sequence for
packing rice in sealed plastic bags is represented by the truth table shown
below.
| count |
Q0
Hold bag
open |
Q1
start
motor |
Q2
Hopper
opened |
Q3
Seal
bag |
| A |
B |
| 0 |
0 |
1 |
1 |
0 |
0 |
| 1 |
0 |
1 |
1 |
1 |
0 |
| 0 |
1 |
1 |
0 |
1 |
0 |
| 1 |
1 |
0 |
0 |
0 |
1 |
a) Derive a Boolean expression for each of the
output requirements Q0 to Q3.
b) Draw a complete logic system incorporating
all four output functions. Use any of the following gates AND, OR, NOT, NAND,
NOR, XOR.
6. In the circuit shown below, the LED
lights up when certain switches are closed.
a) Draw up a truth table for the system.
b) What is the Logic level at the output when
the LED lights ?
c) What is the logic level at A when (i) the
switch is closed (ii) the switch is open ?
d) State the combination of switches, which
when pressed, light the LED.
