This circuit is a pulse metal detector. The circuit consists of a pulse generator (first 555) which is connected to the transistor switch that commutes the metal detector coil represented by the primary coil of the transformer. The metal object is modeled by the secondary coil and a 1 Ohm resistor. The distance from the coil to the object can be modeled by varying the transformer's coupling coefficient (try changing it). The potentiometer regulates sensitivity. The rest of the circuit detects the signal reflected from the object. This signal can be seen in the tail of the voltage pulse on the detector's coil (increase the coupling to see it clearly). When an object is detected, a 455 Hz meander signal appears at the output of the second 555 timer. The blinking LED which is connected there can be replaced by a speaker in a real device.
