Here is a small and simple circuit to catch the first to punch the button in a quiz game. It is basically designed for two but can be expanded to accommodate more contestants. Present circuit works straightaway on mains but can be modified for use at low and less risky AC voltages also.
Silicon Controlled Rectifier (SCR) is much similar to a conventional rectifier except that the conduction is controlled by a gate signal. SCR does not normally conduct until a signal applied at the gate. Once it is turned on, it continues to be in the ON state even after removal of the gate signal, as long as a minimum holding current is maintained or removed. The symbol of SCR is given in Figure 34.
In a quiz game, we need to know, who pressed the switch first and a corresponding light bulb should glow indicating it or a buzzer should go on. Immediately all other switches should be disabled.
At the instance of power on, both SCRs will be in off state as both gates are at the ground level because of Rl and R2. Therefore, both the SCRs will not conduct, and both lamps will be off. Now let us say, if SI is pressed first, the gate of corresponding SCR1 is pulled up, SCR1 conducts and latches on. LI bulb glows and stays on because of SCR action. Now this SCR also makes diode Dl to conduct and pull down the gate of SCR2 disabling it. Hence it can not come on even if S2 is pulled up. Power up again to restart. Similar action is ensured when S2 is pressed. The circuit is shown in Schematic 27.