06-05-2011, 02:27 PM
Quiz-type game shows are increasinglybecoming popular on televisionthese days. In such games,fastest finger first indicators (FFFIs) areused to test the player’s reaction time. Theplayer’s designated number is displayedwith an audio alarm when the playerpresses his entry button.The circuit presented here determines asto which of the four contestants first pressedthe button and locks out the remaining threeentries. Simultaneously, an audio alarm andthe correct decimal number display of the correspondingcontestant 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 comprisingdual 4-input NAND gates of IC3 (7420)locks out subsequent entries by producingthe appropriate latch-disable signal.Priority encoder IC4 (74147) encodesthe active-low input condition into thecorresponding binary coded decimal (BCD)number output. The outputs of IC4 afterinversion by inverter gates inside hexinverter 74LS04 (IC5) are coupled to BCDto-7-segment decoder/display driver IC6(7447). The output of IC6 drives commonanode7-segment LED display (DIS.1,FND507 or LT542).The audio alarm generator comprisesclock oscillator IC7 (555), whose outputdrives a loudspeaker. The oscillator frequencycan be varied with the help of presetVR1. Logic 0 state at one of the outputsof IC2 produces logic 1 input condition atpin 4 of IC7, thereby enabling the audiooscillator.IC7 needs +12V DC supply for sufficientalarm level. The remaining circuitoperates on regulated +5V DC supply,which is obtained using IC1 (7805).Once the organiser identifies the contestantwho pressed the switch first, hedisables the audio alarm and at the sametime forces the digital display to ‘0’ bypressing reset pushbutton S5.With a slight modification, thiscircuit can accommodate more than fourcontestants.
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http://www.electronicsforuefylinux/circu...icator.pdf