This circuit was obtained from 1967 documentation. It uses a sensor to determine the trigger point. The components are of the time, difficulty to obtain these days.
The sensor for this Intersil circuit (1983 databook) is a common silicon transistor. The trimpot...
This circuit is from a 1980 publication. The circuit has high gain with weak signals and lower gain...
The integrated circuit used in this 1987 project is not very common these days. However, if found...