1. 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!
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This Triangle with a circle on the end is actually a NOT gate (See logic sheet) so as well as de-bouncing 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 C
Carry out 5 12 B F 6 11 E G 7 10 A
Vss (0V) 8 9 D
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2. 4026B Decade counter (With De-Bounced input)
This PTM is the Reset for the counter. It returns the display to Zero
This PTM is the input. Each time it is pressed the counter adds one. 10 12 13 1 13 10
The Switches require “Pull-Down” resistors to ensure that the inputs are “Completely ON” or “Completely OFF” Never in between! 9 11 8 7 6 7 2 11
This Triangle is a Schmitt Trigger. Is used to De-Bounce the switch. (See Schmitt Trigger Sheet)
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.
VDD(+V) a -
Clock 1 16
Clock inhibit f b 2 15
Reset -
Display Enable - 3 14
Ungated C
Display Enable Out C - 4 13 g
Carry out - c 5 12 B
DP1
DP2 F 6 11 E e G 7 10 A d
Vss (0V) 8 9 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