This high impedance step suggested by an English publication can operate up to 1 MHz with 1 V rms output. Input impedance is 1 M ohm.
This circuit is from an old American documentation using LEDs in the rectification. The circuit...
This source only uses passive components (without transistors or integrated circuits) and even so it...
This four-digit circuit was found in a 1990 documentation. The sensor can be a contact or reed...