Natural gas (gas) stove flameout alarm

After the gas stove is ignited, close the power switch S and the circuit is in working condition. The photosensitive transistor VT1 is
irradiated by the flame and is in a low-resistance state. The transistor VT2: is saturated and conducts.
The trigger terminal of the music integrated chip connected to the collector of VT3 is at a low potential , the IC
does not work, and the speaker BL does not produce sound.
  If the gas stove suddenly goes out for some reason during the combustion process, VT1 loses strong
light illumination, its internal resistance becomes larger, VT2 exits saturation and tends to cut off,
the trigger end of 1C is at a high potential , Ic works, and the music signal is sent through
the transistor VT3 amplifies and drives BL to sound a bird chirping alarm to remind the owner to close the gas
valve.
    Unless noted in the figure, transistors VT1 and VT2 are ~500.