Passive power failure alarm circuit

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In the figure, IC2 (CD4011) is a 2-input four-NAND gate CMOS integrated circuit. Its four units I to IV form two oscillators with different frequencies (I, II unit, R4, C3 form one, III, IV). Units, R5, C4 form another). After the 220V mains is connected to the power input terminal, the mains is limited by C1, D1~D4 is rectified, C2 is filtered, and after D5 is regulated, DC current of about 15V is output at both ends of C2. It is divided into three ways. One way through R1 makes the red LED light, indicating that the power supply is normal. The other way is added to the 1 pin of the photocoupler Ic1 (4N25) via R2, so that the light-emitting diode built in the photocoupler emits light, and the built-in phototransistor is turned on. The internal resistance is reduced, so that the input terminal of the I unit of the Ic2 is 8 The pin is low and the oscillator stops. The piezoelectric ceramic piece is silent. The third way is to charge C5 through D6, so that C5 is charged with about 15v DC power, as the power supply of the alarm after the grid is powered off. Once the mains supply is cut off, the stored charge of C2 will be discharged through R1 and LED, and will be discharged quickly. At this time, the electric indicator LED is off, indicating that the mains is out of power. at the same time. The LEDs embedded in IC1 are also extinguished, and the internal phototransistor is turned off by conduction, and pin 8 of IC2 is high. Then the two oscillators composed of IC2 and related components start to vibrate, and the piezoelectric ceramic piece Y will emit a "beep-beep-" alarm sound. At this time, because D6 is cut off, the stored charge of C5 will not be sent to the circuit on the left side of D6. In addition, IC2 is a CMOS circuit, which consumes electrodes, so the energy stored in C5 can maintain the sound of the ceramic film for 5 minutes. If the time is not long, increase the C5 capacity, you can also extend the vocalization time of Y. When the grid resumes power supply, the LED will light up again, IC28 pin will input low level, the oscillator will stop vibrating, and the piezoelectric ceramic piece will stop sounding. C5 is filled with electric charge, so that it will be used as power supply for the next power outage.

The photocoupler IC1 is model 4N25, which is a dual-in-line 6-pin plastic package. The 3 and 6 pins are empty feet. Only 1, 2, 4, and 5 pins are used. D5 is a 1/2W 15V Zener diode. Such as 2CW20. C1 is a CBB-400V polyphenyl capacitor. C3 and C4 are CT1 type ceramic dielectric capacitors. Withstand voltage of 100V. Y uses FT-27 or HTD27A-l piezoelectric ceramic sheets.

If you want to change the intermittent frequency of the "beep-beep" sound, you can adjust the value of resistor R4 or capacitor C3. If you want to change the pitch of the "beep-be" sound, you can adjust the value of resistor R5 or capacitor C4. Increasing or decreasing the value of C5 can prolong or shorten the duration of the alarm sound after a power outage.

Passive power failure alarm circuit: