This circuit can be used as a welcome doorbell, as well as a dog barking for the door to deter the thief. The circuit consists of a language doorbell circuit, a monostable trigger, a delay controller, and a simulated canine circuit, as shown in the figure.
In the figure, SA is the doorbell button switch. When SA is pressed, the music integrated circuit IC1 is triggered to issue a "Welcome!" statement. The AG is a normally open reed switch. When the owner hears the doorbell, he opens the door. When the door is open, the AG is disconnected, the power supply of the latter circuit is disconnected, and the IC2 and IC3 are not powered and do not work. If it is a thief, in order to test whether there is someone in the room, they will first press the doorbell button. When the owner is not at home, during the ringing of the doorbell, VT2 is turned on, so that the set trigger end of the time base circuit IC2 turns to a low potential, and its 5 feet Turned to a high level. Its one-shot operation time depends on the charging time constant of R3 and C3, and its one-shot time is tl=1.1R3C3.
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