Inductive anti-theft alarm circuit 2 using capacitor oscillator

It uses a three-point capacitor oscillator (
composed of an inductor L, a capacitor Ci, a distributed capacitance Co between the induction piece M and the ground, and a transistor VTi) as the sensing element. The four-sound analog integrated circuit A drives the speaker B
to emit an alarm signal.
    When the human body (flat) approaches the sensing piece M (about 8cm), the capacitance co increases, the transistors VTi and VTz are cut off,
VT3 and VT4 are turned on, and an alarm sounds.
    Adjusting the potentiometer RPi can change the sensitivity of the device; adjusting RPz can change the size of the alarm sound.