This circuit was found in the March 1984 issue of Radio Electronics magazine. The sensor is a transistor. The module is a common type at the time.
Figure bellow shows another circuit that allows adjustments in the duty cycle of the output...
This circuit is from an American publication of 1978. It can be assembled with modern transistors, since...
Found in a documentation for radio amateurs from the 90s, this circuit is suitable for RF...