As shown in the figure, the doorbell is composed of a monostable timing circuit, a password switch, an AND gate circuit, an audio circuit, and the like. When the password switch is pressed by a certain program within a limited time, the doorbell circuit will not announce the sound. If the specified time limit or code number is incorrect, no audio signal will be sent. IC1 (555) and R1, C1, AN1, etc. constitute a time switch circuit, timing time td = 1.IR1C1, the graphical parameter is 11 seconds. When AN1 is pressed, 555 is set, J is pulled in, J1-1 is closed, and the power supply voltages such as VT1-VT8, IC2, and IC3 are turned on, and the ON time is 11 seconds. Four sets of electronic switches are composed of AN2~AN5, VTl~VT8, etc., and AN6~AN10 are invalid buttons, which together form a password switch disk. Taking the first group of AN2, vT1, and VT2 electronic switches as an example, when AN2 is pressed, C3 is fully charged, and v1 is saturated and turned on, and vT2 is turned off, so IC2's input pin 1 is high. The other three sets of electronic switches are similar. When AN2 is released, since the charge on C3 can only be discharged through the emitter junctions of R3 and VT1, VT1 can be maintained for an on-time of approximately 7 seconds. During the 7 seconds, the person who knows the password presses the useful password button. When the four inputs of IC2 (CD4082) are high, the output pin 6 is high level, and the integrated power switch tube will be integrated. IC3 (TWH8778) is turned on.
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