The Fastest Finger First Indicator circuit determines as to which of the four contestants pressed the button first and locks out the entries from the remaining three contestants. 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 the latch IC 7475(IC2) changes its logic state from 1 to 0. The combinational circuitry comprising dual 4-input NAND gates of 7420 (IC3) locks out subsequent entries by producing the appropriate latch-disable signal. Priority encoder IC 74147(IC4) 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 7447(IC6). The output of IC6 drives common common-anode 7-segment LED display (DIS.1, FND507 or LT543).

The audio alarm generator comprises of the frequency oscillator NE555 (IC7), 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 organizer 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 push button S5.