Transcript 40106B Schmitt Trigger (A way of switch De

40106B Schmitt Trigger
(A way of switch De-Bouncing)
Have you ever heard a light switch buzz when you switched it on or off, or had
your finger bounce twice when jabbing at a push button. Well usually it doesn’t really
matter but if you’re making a counting circuit then that might mean you just counted two, three
or four times when you only meant to count once.
In fact whenever switches are pressed this switch “Bouncing” always occurs and
so a device called a Schmitt trigger was invented to make sure counting circuits
would only count once for each button press. i.e. “De-Bouncing” a switch.
This ordinary PTM
is the input. Each
time it is pressed
the it sends a
positive voltage
but actually
bounces on and off
a few times within
a split second!
Blah
This Triangle with a circle
on the end is actually a
NOT gate (See logic
sheet) so as well as debouncing the switch, this
I.C. also works as an
inverter
The Switch
still requires a
“Pull-Down”
resistor to
ensure that
the inputs are
“Completely
ON” or
“Completely
OFF” Never in
between!
The weird symbol in the middle
of the triangle tells us that it
contains a Schmitt trigger. That
means that even if the switch
bounces when it is pressed, the
output from the Schmitt will just
be one clean on or off signal.
VDD(+V)
Clock
1
16
Clock
inhibit
2
15
Reset
Display
Enable
3
14
Ungated C
Display
Enable Out
4
13
Carry
out
5
12
B
F
6
11
E
7
10
8
9
G
Vss (0V)
text
C
A
D
4026B Decade counter
(With De-Bounced input)
This PTM is the
input. Each time it
is pressed the
counter adds one.
The Switches
require “PullDown”
resistors to
ensure that
the inputs
are
“Completely
ON” or
“Completely
OFF” Never
in between!
This Triangle is a
Schmitt Trigger.
Is used to DeBounce the
switch. (See
Schmitt Trigger
Sheet)
VDD(+V)
Clock
1
16
Clock
inhibit
2
15
Reset
Display
Enable
3
14
Ungated C
Display
Enable Out
4
13
Carry
out
5
12
B
F
6
11
E
7
10
8
9
G
Vss (0V)
C
A
D
This PTM is the Reset
for the counter. It
returns the display to
Zero
10
1
12
13
13
9
10
8
11
6
7
2
7
11
The display
that we use is
called a “7
Segment
Display” It is
basically 7
LEDs
arranged in
the shape of a
number 8.
The 4026B
turns them on
and off in
sequence to
display the
count
The 4026B is an
integrated circuit
that counts up to
ten. Each time it
receives a signal
on the input, it
adds one to the
count. It can then
be reset by a
second input.
Seven Segment displays
like the one on the right
can be used for counters
or timers and can be
“Driven” by an integrated
circuit such as the 4026B
shown left.
Each time the input of the
4026B is triggered, it will
count up by one and
adjust the display to
match.
a
-
f
b
-
-
-
g
-
c
DP1
e
DP2
d
The tricky bit with this circuit is matching the pin numbers on the I.C. with the
(different) pin numbers on the display. Make sure you pay attention to the LETTERS