The integrated circuit used in this 2002 project is not very common. It is powered directly by the telephone line, requiring no power supply.
This circuit, published in the 1980s, can be applied to operational amplifiers of almost any type....
This touch sensor, found in an American documentation on transistors, has gained 8 million times...
This is one of the many versions of cricket that the reader can find on this site. This circuit is...