This resistive sensor was found in a 1969 US documentation. The circuit uses transistors of the time, but current equivalents can be employed.
Suggested by a 90s Electronic Design, this circuit drains only 0.25 mA from its source. Listening...
The time the output is high (1) of a TLCSSB in the astable configuration depends on Ra and C. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...