Electronic circuits > Fun Circuits > JAM(Just A Minute) Circuit
JAM(Just A Minute) CircuitThis 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.
Published on: 2004-08-21 (3552 reads)
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