First of all, you need to specify what type of sensor are you looking for. That’s not a metal sensor, it was designed to detect when an object nears the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit (IC1A). As a result, the oscillator begins oscillating.
The oscillation is then rectified by D1 and the IC1B circuit reads the oscillator’s amplitude and when it reaches a specific level the output state of the IC1B changes. As the target moves away from the sensor the oscillator’s amplitude decreases, switching the IC1B output back to its original state.
The sensing surface of a capacitive sensor is formed usually by a plate (that’s not connected to the GND or other metallic objects) and a for a good result larger sensing area is recommended.