Circuit 94 – Single Pulse Counter
The seven-segment display will be incremented by one each time a pulse is produced by closing
and opening PTM switch.
R6 1 16 1 16
R2 c R3 c R4
VR 1 b b C1
f e f e
PTM g a g a
+ LED d d
0V C2 R8
SW1 = Toggle Switch Switch = (PTM*2)
R1 = 10k ohm R2 = 10k ohm
R3 = 10k ohm R4 = 10k ohm
R5 = 470 ohm R6 = 2200 ohm
R7 = 33k ohm R8 = 10k ohm
C1 = 470F C2 = 100F
IC1 = 555 Timer IC2 = 4026 Counter
IC3 = 4026 Counter 7 - Segment Display
In the operation of the counter, square wave pulses must be applied to the 4026 counter chip to
ensure operation. To produce the pulses the 555 timer is set up in the monostable mode.
This means when a negative pulse (this will be explained later), is applied to pin 2 of the 555
chip, the output at pin 3 will go high (9V) for a certain time (which is set by the two components
R6 and C2), and then return to 0V. Thus a square wave pulse has been created.
Time High is calculated from the formula: -
1.1 x R6 x C2 = 1.1 x 2200 x 0.000100 = 0.2 sec
Method of Triggering
For circuit to operate correctly, a fast negative pulse must be applied to pin 2 of the timer chip.
Consider the PTM switch open, no current can flow through R7 so there is no voltage drop
Therefore the voltage at the bottom of the resistor is 9V (note this is applied to pin 2).
When the switch is closed, the bottom of R7 is immediately connected to the 0V rail so the
voltage at this point is now 0V (pin 2 is also at 0V).
When the switch is released, the voltage on pin 2 immediately returns to 9V, so the voltage at pin
2 was 9V, then dropped to 0V and then returned to 9V, i.e. a negative pulse has been applied to
pin 2. With negative pulse applied to pin 2, output voltage at pin 3 will go high (9V), for a
period of 0.2 sec, and then return to 0V, until retriggered.
The voltage at pin 2 was 0V, when triggered it jumped to 9V, remained at 9V for a period of 0.2
sec, and then returned to 0V. This in effect produces a positive square wave pulse.
When inputting a pulse to a counter chip, it is not good policy to provide that pulse by using a
simple resistor and switch arrangement.
When switches are closed and opened they suffer from a fault known as “Switch Bounce”. This
produces extra pulses, which adversely affect the counting circuit. Whereas with the monostable
circuit described above, when SW1 is pressed only one “clean” pulse is produced.
This pulse is now fed to pin 1 (input pin) of IC3 (counter chip), and the output is then fed via the
various connections to the seven segment display which will register the digit 1.
When switch is depressed, the second time the digit 2 will be displayed on seven segment
display and so on (this counter IC2 will register unit values). When the count of 9 is reached, the
next pulse will be sent to IC2, which will register the value 1 and counter IC3 will reset to 0, i.e.
the value 10 is displayed on the seven segment displays. Thus the counter can count from 0 to
With the opening and closing of switches, unwanted signals “noise”, is produced in the circuit
(chips can also produce noise). For this reason, a large capacitor CI is fitted across the circuit to
filter these unwanted signals.