fire alarm circuit

(1) Working principle of the circuit The circuit of the fire alarm is as shown in the figure. In the figure, the two parallel metal plates A and B are the two electrodes for detecting the flame
. When there is no flame, the two electrodes
are insulated. At this time, the gate of the field effect transistor
VT1 is connected to the ground through the 20MC1 resistor,
and the voltage generated by the drain current on the source resistor Rz is added to the gate
as the gate bias
(negative gate voltage) of the field effect transistor. At this time,
VTI works. In the negative gate voltage state, the drain
current is very small. Therefore, the voltage regulator tube VS does not conduct
, VT2 is cut off, the relay coil K does not
pull in, and the siren sound generator composed of VT3 ~ VT5
is powered off. There is no alarm sound.
    When there is a flame, the gas between the two electrodes is ionized at high temperature, and electricity is conducted between the plates. The drain current of field effect transistor VT1 increases
, the voltage drop on R2 increases, the voltage regulator tube breaks down and conducts, VT2 also conducts, and the relay coil K is attracted. Its normally open contact K is closed,
the sound generator is turned on, and an alarm sounds.
    VT3-VT5 form a substitute flute generator, which can produce an urgent substitute flute-like sound, giving people a feeling of tension. VT3 forms a triangle wave
generator , and its output triangle wave is added to the input end of the audio oscillator composed of VT4, vfs, etc., so that the output audio slowly rises
and falls like a siren, which is equivalent to the audio oscillator being modulated by the triangle wave.