Both the contestants have a trigger push switch and a corresponding LED. Whenever any of the contestants press the switch, it triggers the bistable output (pin 3), the LED lights up and the beeper sounds due to which the other players switch stops working and it becomes easy to find that which switch was first pressed. In this a diode is used to link the output with the trigger line. Now the third switch comes into picture which is used to reset the circuit and it is with the quiz master. As soon as that switch is pressed the beeper gets cancelled and the LED turns off so that the circuit becomes ready for the next question.
Filed Under: 555 Timers, Electronic Projects