This circuit is from a 60's documentation. The capacitance C1 can be between 20 pF and 0.01 uF depending on the generated frequency. Equivalent transistors may be employed.
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...