Design of state detection circuit for lighting in refrigerator

Test - uppercase JPG - memory 146K

This circuit can detect the illumination inside the refrigerator. The circuit is as shown in the figure. It is composed of 555.
555 and R2, R3, Cl form a forced multivibrator, and the forced reset end 4 is connected to the c pole of the photosensitive tube 3DU5. Under the bright illumination, the 3DU5 is turned on with low resistance, so that the 4 feet of the 555 are Low level, forced reset, the oscillator does not oscillate, the speaker does not sound, when there is no light, 3DU5 is cut off and high resistance, 4 feet is high, 555 oscillator starts, its oscillation frequency is f=1.44 ∕(R2+2R3)Cl, using the principle that the lamp is bright and vibrating can not support the grip, it can detect whether the illumination lamp in the refrigerator is extinguished after closing the door.