This circuit was found in a 80s American documentation. The zener diodes determine the points of action of each stage and the transistors can be general purpose types.
This circuit was found in the September 1974 issue of Radio Electronics magazine. The circuit uses...
This circuit was found in the December 1983 issue of Radio Electronics magazine. The sensor is an...
We found this circuit in a 1975 publication. The circuit can be made with more modern equivalent...