Metal detector

I. Introduction
This circuit was designed using JLCPCB's twice-monthly free PCB fabrication service. It's a simple metal detector capable of detecting metal objects 3-5 cm in diameter.
II. Principle Analysis
A self-locking switch is used: when the switch is pressed, pins 1 and 2 are activated, the LED lights up, and the circuit begins to work.
L1 and L2 are coils drawn on the PCB, and the three transistors correspond to three functions: an oscillation circuit (9018), a detection circuit (9015), and an alarm circuit (9012).
When no metal is near, there is an LC oscillation (9018). When a metal object is near, eddy currents are generated inside the metal, causing the LC oscillator to lose its oscillation signal, thus determining the presence of metal. The detection circuit composed of 9015 captures this signal. When metal approaches, the base of 9012 goes low, the transistor conducts, and the buzzer sounds.
The adjustable resistor (R3) is used to adjust the alarm threshold. The resistor should be adjusted so that the alarm sounds when the coil is near metal, and does not sound under normal conditions. When drawing the coil on
the PCB using pin headers
, use tools and lines to make it look better; this may strain your eyes (it's recommended to drink some water to relax).
III. Function Demonstration