Quiz quizzes circuit

Shown is the schematic diagram of the answering machine. In the figure, the high-level trigger pin ⑥ and the low-level trigger pin ② of the time base circuit 555 are connected to form
a Schmitt trigger. If the voltage applied to pin ② and pin ⑥ exceeds 2ux, /3, pin ③ outputs low level; if the voltage applied to pin ② and pin ⑥
is lower than Vcr/3. ③Pin input}【{High level. When the host presses the switch S, the Schmitt trigger is powered. Since the four answering keys SBl-SB4 are
not pressed, they are in an open circuit state. There is no trigger pulse at the control terminals of the thyristors VTH1~VTH4, and VTH1-VTH4 are blocked. state.
    Next, one of SI3]-SB4, if SB1 is pressed first, acts on the control end of thyristor VTH1, VTH1 conducts, the red
light-emitting diode LED 1 emits light, and the +9V power supply acts on the ② pin of 555 through LED1 and VTH1. @ pin, ② pin and ⑥ pin become high level
, and the buzzer generates a prompt sound. Schmitt trigger flip. @ pin outputs low level. Because the output of pin ③ is low level,
when T-SB2~SB4 is pressed again, no trigger voltage is sent to the gates of VTH2~VTH4, VTH2~VTH4 remain in the off state, and
LED2-LED4 do not light up. LED1 lights up alone, indicating that the person who presses the SB1 key succeeds first.
    Finally, the host turns on the switch S and resets the thyristor. LED1 goes off. Before the next answering session, the host closes the switch S again, and the answering
machine is in a waiting state again. In order to ensure that when a certain key is pressed first, the corresponding LED will light up, and when other keys are pressed later, the other LEDs will not
light up. The circuit specially adds |_r diode VD5 between the output pin and the touch switch. ~VD8 to ensure reliable cut-off of the thyristor.