CIRCUIT IDEAS
ELECTRONICS FOR YOUJUNE '99
A.P.S
. DHI
LLON
This jam circuit can be used in
quiz contests wherein any par-
ticipant who presses his button
(switch) before the other contestants,
gets the first chance to answer a ques-
tion. The circuit given here permits up
to eight contestants with each one al-
lotted a distinct number (1 to 8). The
display will show the number of the con-
testant pressing his button before the
others. Simultaneously, a buzzer will
also sound. Both, the display as well as
the buzzer have to be reset manually
using a common reset switch.
Initially, when reset switch S9 is mo-
mentarily pressed and released, all out-
puts of 74LS373 (IC1) transparent latch
go ‘high’ since all the input data lines
are returned to Vcc via resistors R1
through R8. All eight outputs of IC1
are connected to inputs of priority en-
coder 74LS147 (IC2) as well as 8-input
NAND gate 74LS30 (IC3). The output
of IC3 thus becomes logic 0 which, after
inversion by NAND gate N2, is applied
to latch-enable pin 11 of IC1. With all
input pins of IC2 being logic 1, its BCD
output is 0000, which is applied to 7-
segment decoder/driver 74LS47 (IC6) af-
ter inversion by hex inverter gates in-
side 74LS04 (IC5). Thus, on reset the
display shows 0.
When any one of the push-to-on
switches—S1 through S8—is pressed,
the corresponding output line of IC1 is
latched at logic 0 level and the display
indicates the number associated with
the specific switch. At the same time,
Electronic Jam
RAJESH K.P.
output pin 8 of IC3 becomes high, which
causes outputs of both gates N1 and
N2 to go to logic 0 state. Logic 0 output
of gate N2 inhibits IC1, and thus press-
ing of any other switch S1 through S8
has no effect. Thus, the contestant who
presses his switch first, jams the dis-
play to show only his number. In the
unlikely event of simultaneous press-
ing (within few nano-seconds difference)
of more than one switch, the higher
priority number (switch no.) will be
displayed. Simultaneously, the logic 0
output of gate N1 drives the bu