One of the inductive anti-theft alarm circuits using capacitor oscillator

The self-excited multivibrator is composed of the 555 time base integrated circuit Ai, capacitors Cl, Cz, distributed capacitance Co between the metal plate (induction plate) M
and the ground, and resistors Ri and R2; it is composed of R3-Rs and C3~C5 Third
-order RC integral network; a comparator composed of CA3130 operational amplifiers Az, Re, R7 and RPi, RPz;
an amplifier circuit composed of transistors VT, R8, and R9.
    When the human body approaches the metal plate M, the capacitance of Co increases, the oscillation frequency of the multivibrator decreases, the RC network outputs
negative pulses, VT is turned on, the relay KA is closed, the alarm device is turned on, and an alarm signal is issued.
    Adjust the potentiometers RPi and RP2 to change the sensitivity of the device.