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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. |
