Quiz-type game shows are becoming more popular on TV these days. In these games, the earliest finger indicators (FFFI) are used to test the reaction time of the player. The number designated by the player is displayed with an audio alarm when the player presses their input button.
The circuit presented here determines which of the four contestants first pushed the button and blocks the remaining three entries. Simultaneously, an audio alarm and display of the correct decimal number of the corresponding contestant is activated.
When a contestant presses its switch, the corresponding output of the ratchet IC2 (7475) changes its logic state from 1 to 0. The combinational circuits comprising NAND gateways with 4 inputs of IC3 (7420) disable the signal.
The IC4 priority encoder (74147) encodes the active low input condition at the corresponding binary encoded decimal number (BCD) output. The IC4 outputs after inverting the inverter doors inside the 74LS04 hexagonal inverter (IC5) are coupled to a BCD-7 7-segment decoder / display controller (7447). The output of IC6 7-segment common anode LED units (DIS.1, FND507 or LT542).
The audio alarm generator comprises the clock oscillator IC7 (555), the output of which activates a loudspeaker. The frequency of the oscillator can be varied with the help of the VR1 preset. The state of logic 0 in one of the outputs of IC2 produces the condition of logical input 1 in pin 4 of IC7, allowing the audio oscillator.
IC7 needs + 12V DC power for sufficient alarm level. The remaining circuit operates with regulated supply + 5V DC, which is obtained using IC1 (7805).
Once the organizer identifies the contestant who pressed the switch first, deactivates the audio alarm and at the same time forces the digital display to '0' by pressing the reset button S5.