Elenco Understanding Logic Gates and Circuits - Manual (Page 26)

U15
U16 U20
Project 16: Half Subtractor
This circuit demonstrates how a half
subtractor works. Turn the slide switch (S1)
on. The green LED represents the Difference
(D) of input A minus input B and the red LED
represents the Borrow (B). When the loose
ends of the red and black wires are both
connected to 0 (0-0) or to 1 (1-1), both LEDs
are off (D=B=0). When the loose end of the
red wire is connected to 1 and the loose end
of the black wire is connected to 0 (1-0), the
green LED is on and the red LED is off (D=1,
B=0). When the loose end of the red is
connected to 0 and the loose end of the black
wire is connected to 1 (0-1), the green and red
LEDs are both on (D=1, B=1), indicating a
borrow condition.
Subtractors are often implemented with adders since the incremental cost is low (one
additional NOT gate).
1
1
1 1
1
1
2
2
2
1
2
2
2
2
1
2
2
2
3
D
B
Represents the
Difference
Represents the
Borrow
1
2
1
2
Input (A) Input (B) Output (D) Output (B)
0 0 0 0
0 1 1 1
1 0 1 0
1 1 0 0
A
B
A
3
2
4
26
2
2
2
1
2
3
2
3
1
2
B
2
4
3 1
2
1
2
3
1
0