60-second acousto-optic digital timer circuit for competition

Photocoupler

As shown, the display terminal of the timer uses a 838 calculator, which consists of a 60 second timer, a second signal generator, a photocoupler, and an audio circuit. Used for quiz 1 minute rush timing or other occasions.
ICl (555) and RP1, R1, C1 form a single stable circuit, the monostable transient stability time td = 1.1 (R1 + RP1) C1, adjust RP1, set at 60 seconds. Press AN and the timing starts.
IC2 and R2, RP2, and C2 form a second signal oscillator with an oscillation frequency of f=1.44/(R2+RP2)C2. Adjust RP2 so that the oscillation frequency is at 1 Hz. IC3 and R4, R5, C3 form an audio oscillator with a frequency of about 1300 Hz. Its reset end is controlled by the output of IC2. Each time a pulse signal is sent, a "beep--" sound is emitted, and the LED emits light. At the end of the timer, the 3 pin of IC2 turns to a low level, so that IC3 is in the forced reset state and stops vibration.
Each time a second pulse is emitted, the optocoupler couples it and adds it to the 838 "=" end, counting once, until the end of 60 seconds.