Human body proximity alarm circuit

Circuit working principle: The oscillation circuit uses FET field effect tube, inductor L, capacitor
C1, and variable capacitor Cp to form a Hartley-type oscillator. The gate of VT1 is connected to the antenna,
and C2 is a high-frequency bypass capacitor. When the hand is close to the antenna, r distributed capacitance increases on the FET gate capacitance , so that when the LC resonance stops, the high-frequency bias voltage of the gate disappears and the drain current (IDSS) increases compared with normal oscillation.