Power outage sound and light alarm circuit

The power outage sound and light alarm circuit is as shown in the figure. The circuit consists of AC detection (left half) and audio oscillator (right half).
The AC 220V is rectified by VD and filtered by C1 to obtain 300V DC high voltage. One DC path passes through Ri to make the light-emitting diode LED 1 emit light, and the other
path passes through Rz and is sent to the ① pin of the optical coupler 4N25, causing the internal LED to light up. The light shines on the phototransistor, and the phototransistor shows low current
The resistor is turned on, pulling the base potential of VT1 to the ground potential, and the audio oscillator cannot work normally. There is no current flowing through the speaker and the light-emitting tube, and they neither sound
nor emit light.
    Once there is a sudden power outage, the detection voltage at both ends of c] disappears, LED1 goes out, and the internal light-emitting tube in the optocoupler also goes out.
The phototransistor is not exposed to light and shows high resistance, and the base of VT1 is no longer short-circuited by the optocoupler. arrived. The audio oscillator composed of VT1, VT2 and other components meets
the oscillation conditions and starts to oscillate, making the speaker and LED2 sound and light up. When the mains power is restored, LED I lights up.