The metal detector circuit consists of two main components: transmitting and receiving .
The transmitting node consists of a pulse generator on a KR1006V1 chip (an analog of NE555) and a powerful key on a KP505A field-effect transistor (an analog of IRF740, IRF840). You can put a reverse-conduction bipolar transistor with a voltage K-E of at least 200V. It can be taken from an energy-saving lamp or a charger from a mobile phone. BC557 transistor is used to swing the powerful key.
The receiving node is assembled on a K157UD2 microcircuit (it is possible to assemble a OPT TL072), there are counter-parallel limiting diodes at the receiver input, there is a filter at the input of the second stage of the receiver, cutting the necessary part of the pulses, at the output of the second stage there is a VS547 transistor, its collector circuit is connected speaker 8-50 ohms. In place of T3, almost any NPN transistor can be used.
There are many different options for PIRAT metal detector circuits and modifications to them.