Specifications

CONSTRUCTION
frequency (of the order of 1 Hz to 4 Hz).
Its frequency can be varied with the help
of potentiometer VR1.
For proper functioning of CD4033 and
CD4017, their clock-enable (CE) pins 2
and 13 respectively must be held ‘low’.
These pins are connected to the output of
gate N3 (pin 10). If these pins are in logic
high state, the ICs are disabled from re-
ceiving clock pulses, and the Q outputs of
IC3 and segment drive outputs of IC2 re-
tain their last state before the CE pins go
‘high’.
The control clock pulses from gate
N3 also go to the base of transistor
BC547B (T1). This transistor pulls down
the common cathode of 7-segment LED
display DIS.1 to ground during the high
level of control clock pulses, to display
the number.
The control pulse also performs one
more function. After being inverted by
NAND gate N4, it resets JK flip-flop IC
CD4027 (IC4), which serves as the foul
play checker.
In nutshell, during the low state of
output of gate N3, both IC2 and IC3 are
enabled and the pulses are counted by
IC2, but the number cannot be seen in
the display because transistor T1 is re-
verse biased and cut-off.
When the output of gate N3 changes
to high state, IC2 and IC3 are disabled.
T1 gets its base voltage and pulls down
the cathode of display DIS.1, and the dis-
play shows the number (which is a ran-
dom number). At the same time, the Q
output of IC3 corresponding to the dis-
played number goes ‘high’.
Now, if one presses the correct
key corresponding to the number
shown in the display, before it vanishes,
a high-going pulse is applied to clock in-
put pin 3 of IC4. Its Q output (pin 1)
becomes ‘high’, which advances the tens
counter (IC5 of the scoreboard). It also
biases transistor T2, to drive the piezo
buzzer PZ1 for confirmation of the num-
ber shot.
Foul play checker/debouncer. Due
to bouncing, the switches produce spuri-
ous pulses and lead to erratic operation.
The player may press a switch more than
once to score more, and may keep press-
ing a switch before the respective num-
ber is displayed. This is where the foul
play checker/debouncer circuit comes
into play.
For faithful operation, the circuit re-
quirements are as follows:
Fig. 2: Circuit diagram of the digital number shooting game
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