Overtemperature alarm circuit

This is an over-temperature alarm made with a thermal bridge, as shown in the picture. RP, R, and RT, R form a
thermal ; VT1, VT2, {temporarily called a differential amplifier} VT3 and VD form an output circuit; VT4, VT5 and C form a complementary audio oscillation circuit
. When the temperature is within the controlled range, the thermal bridge is in a balanced state, and at this time ‰ = Uo. The output terminal of the differential amplifier composed of VT1 and VT2
is ‰=0, VT3 is cut off because it cannot get enough base bias current, the audio oscillator does not work, and the speaker BL is silent.
    When the external temperature exceeds the controlled temperature , the resistance of the thermistor RT decreases and the bridge balance is destroyed. The potential of point a rises, and
the potential of point d also rises -, causing the differential tube output uo to be higher than Ua, VT4 and VT5 are turned on one after another, and BL emits a long "I" with audible alarm.