RF or even switching transistors like the 2N2222 can be used in this circuit. The capacitors are ceramic and the frequency is determined by the crystal.
This alarm with the 555 integrated circuit is from an EDN from the 90s. The circuit directly...
Found in a General Electric documentation from the 1980s, this circuit makes use of a FET...
This circuit was suggested by Siliconix in a 1980s publication. The circuit uses components from the time...