Gas flameout alarm circuit

When using a gas stove to boil water or cook, sometimes the boiling water overflows and the fire is extinguished. If not detected in time, a large amount of gas will overflow, resulting in
the risk of poisoning.
  The flameout alarm circuit introduced in this example will sound a loud alarm when the fire goes out. The flameout alarm circuit is shown in Figure 13-27.
The photodiode and transistors VT1 and VT2 form a light-controlled electronic switch, and VT3 and VT4 form a complementary audio oscillator. When the fire is burning,
the photodiode is subject to strong infrared radiation and the internal resistance decreases. At this
time, VT1 and VF2 are both cut off, and the oscillators VT3 and VT4 do not work.
When the fire goes out, the infrared radiation basically disappears, and
the resistance of the photosensitive diode immediately increases, so that VT1 and VT2 are quickly turned on. The oscillators
VT3 and VT4 receive power through VT2, and the speakers emit
sound.
    Photosensitive diode can be used in 2CU or 2AU series photosensitive tubes.
VT1 and VT4 can use ordinary 3DG8 silicon transistors with a sound value of
about 100. VT2 can use 9013 silicon transistors with a sound value greater than 50.
VT3 can use 3AX31B germanium transistors with a sound value greater than 30.
RP can use a trimmer potentiometer. Other components such as circuits