888 safety box and refrigerator door-opening over-time sound alarm circuit

Published:2011/6/7 3:53:00 Author:TaoXi | Keyword: 888, safety box, refrigerator, door-opening, over-time, sound alarm | From:SeekIC

The SB in the figure is the refrigerator door linkage switch, when the door opens, the SB automatically shuts off, when the light turns on, the circuit gets power. The alternating current is reduced by C1 and R1, then it is rectified by the full bridge rectifier QU and is filtered by C2 to output the +9V voltage. The VT4 is the photoelectric diode, the photoelectric switch is composed of the VT1, R3, R4 and the VT4. When the refrigerator door is opening, the light H turns on. VT4 has the low resistance because of the illumination (tens of thousands of ohms), VT1 gets the positive bias to conduct, the voltage between c and e is lower than 0.4V to make the IC1-n(1/2 556)'s trigger port has low electric potential, if you set the IC1-n, the pin-5 outputs the high electrical level to conduct the VT2.



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