First-Response Monitor with SCR (Quiz Alarm)

A pair of cross-coupled SCRs can be used to build a first-response monitor circuit, as shown in the schematic diagram below.  First-response circuit is popular in quiz application where the first contestant who press the button will prevent the other activate the button/alarm. The purpose of this circuit in quiz application is to ease the identification of the first one when two contestants appears to press the button simultaneously. Here is the schematic diagram of the circuit:


This circuit works by turning of other SCR triggering voltage when an SCR is triggered, prevent the other party to be triggered later. S3 is a reset switch, which restore the initial state: both SCR is inactive. When both SCR is in inactive state, both voltage source for triggering the SCR is available. The voltage source for triggering SCR1 ia running through LED2, R2, D2, R4, and S1, while the path for triggering SCR2 is running through LED1, R1, D1, R3, and S2. When one SCR is triggered first, the voltage path to trigger the other SCR will be shorted by the first activated SCR, prevent the other SCR to be triggered later.