As shown in the figure, the circuit consists of a trigger delay circuit and a music integrated IC1. When the refrigerator opens the door, the contact switch K is turned on. Since the terminal voltage of C1 cannot be abrupt, a positive peak voltage (>2∕3VDD) is triggered at the trigger end of the 555, so that the circuit is reset and the 3 pin is low. VT1 is cut off. Capacitor C1 is charged by R1 and RP1. When the voltage of pin 6 is lower than the trigger level of 1∕3VDD, 555 is set, the 3 pin is high level, VT1 is turned on, and IC2 is energized to sound, reminding the owner to close the door. The delay time from door opening to music can be set by adjusting RP1.
As shown in the figure, the reminder consists of a monostable delay circuit consisting of 555 and Cl, RP and a KD-9300 music IC2. When the door is opened, the switch AN is pressed, and the power supply 6V is applied to 555. Since the voltage on the capacitor C1 cannot be changed, at the moment of power-on, the pin 2 has a high potential, so that 555 is low. After the delay (0-2) minutes, the circuit is set, IC2 is powered, and the music is sounded. After the door is closed, AN is released. The delay can be changed by adjusting the RP.
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