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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 question. The circuit given here permits
up to eight contestants with each one allotted a distinct number
(1 to 8). The display will show the number of the contestant
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 momentarily pressed and released, all outputs
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 encoder
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) after inversion
by hex inverter gates inside 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, 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 pressing of any other switch S1 through S8 has no effect.
Thus, the contestant who presses his switch first, jams the
display to show only his number. In the unlikely event of
simultaneous pressing (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 buzzer via pnp transistor BC158 (T1). The buzzer as well the
display can be reset (to show 0) by momentary pressing of reset
switch S9 so that next round may start. Lab Note: The original
circuit sent by the author has been modified as it did not jam the
display, and a higher number switch (higher priority), even when
pressed later, was able to change the displayed number.
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