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 circuit that has excellent linearity using an NTC as a sensor was found in a European...
This circuit can generate signals from 100 kHz to 50 MHz depending on the crystal and the...
This circuit is from 1970 documentation. The SCRs are types of the era that can be replaced with 106...