This circuit was found in a documentation of the 80s, having didactic purpose. For R1 values between 10k to 1M can be used. Transistors are general purpose types.
We found this Vox in a 1969 Motorola documentation. Transistors support modern equivalents and the SCR can...
Suggested by Texas Instruments in 1968 documentation, this circuit uses components from the era....
This very low frequency circuit was found in a 1964 Electronic Design. Transistors may be modern...