Employing the XR2206 IC, this circuit was found in the May 1977 issue of Radio Electronics magazine. The formula next to the diagram gives the component values as a function of frequency.
This circuit for cars of the time was found in a Popular Electronics magazine from the 90s. The...
IDT ZSSC3230 Capacitive Sensor Signal Conditioner Available from Mouser Electronics stock, this IDT...
According to the article in Radio Electronics from April 1986, it would be possible to detect...