In speed chess, a player has to make his move in 5-10 seconds. If he/she does not, he/she loses the game (depending on the agreed conventions). The ‘clock’ described here Indicates whose turn it is and who has defaulted. It may, of course, also be used or other speed games.
When player A presses S2, D3 lights to indicate that it is player B’s turn. This player returns the game to A by pressing S1. whereupon D2, lights. If a player fails to return the game to his/her opponent in time set with P1, the buzzer will sound. The circuit is built around a 556, the dual version of the well-known 555. One-half IC1b functions as a bistable (flip-flop) that ‘remembers’ which switch was Pressed last: this is indicated by the associated LED lighting. The other half operates as a monostable. It is kept in its triggered state (buzzer inactive) as long as it retriggered regularly, and timely, by switches S1 and S2.
The trigger input is made low by S1 with the aid of T), and by S2 with the aid of D1. Every time IC1a is retriggered. C5 is discharged via pin 1 (DIS) of IC1a. The time set with P1 then starts afresh. If either of the switches is not pressed in time, the voltage across C5 becomes so high that IC1a is reset. Its output is then low and the buzzer sounds.