Fastest Finger First Indicator

Quiz-type game shows are
increasingly becoming popular on television these days. In such games,
fastest finger first indicators (FFFIs) are
used to test the player’s reaction time. The player’s designated number
is displayed with an audio alarm when the player presses his entry
button. The circuit presented here determines as to which of the four
contestants first pressed the button and locks out the remaining three
entries. Simultaneously, an audio alarm and the correct decimal number
display of the corresponding contestant are activated. When a contestant
presses his switch, the corresponding output of latch IC2 (7475)
changes its logic state from 1 to 0. The combinational circuitry
comprising dual 4-input NAND gates of IC3
(7420) locks out subsequent entries by producing the appropriate
latch-disable signal. Priority encoder IC4 (74147) encodes the
active-low input condition into the corresponding binary coded decimal (BCD) number output.

The outputs of IC4 after inversion by inverter gates inside hex inverter 74LS04 (IC5) are coupled to BCD-to-7-segment decoder/display driver IC6 (7447). The output of IC6 drives common-anode 7-segment LED display (DIS.1,
FND507 or LT542). The audio alarm generator comprises clock oscillator
IC7 (555), whose output drives a loudspeaker. The oscillator frequency
can be varied with the help of preset VR1. Logic 0 state at one of the
outputs of IC2 produces logic 1 input condition at pin 4 of IC7, thereby
enabling the audio oscillator. IC7 needs +12V DC supply for sufficient
alarm level. The remaining circuit operates on regulated +5V DC supply,
which is obtained using IC1 (7805). Once the organiser identifies the
contestant who pressed the switch first, he disables the audio alarm and
at the same time forces the digital display to ‘0’ by pressing reset
pushbutton S5. With a slight modification, this circuit can accommodate
more than four contestants.

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