At any homemadewhich is done on electronic devices, as a rule, transistors are used. A transistor is an amplifying semiconductor device that uses p-n junctions. The transistors installed in the circuit can have a large gain and amplify, for example, an audio signal in a homemade receiver or a signal from a photosensor in a homemade photocell. Using powerful transistors in a homemade amplifier, you can get more power to power powerful speakers. And in a home-made induction furnace using powerful transistors, you can make a high-frequency generator to power an inductor that melts the metal.
Cost: ~ from 1