We found this circuit in an old documentation, but it is still current. General purpose transistors can be used, and the SCR can be the TIC106. The bell is a transducer with an oscillator (buzzer).
This is a commercial radio circuit from the 1960s. The circuit was found in a Japanese publication from...
The integrated circuit used in this 80's differential amplifier is a quadruple version of the 741...
This touch or proximity sensor circuit was found in a Popular Electronics from 1992. In the...