We found this circuit that converts a direct voltage into a proportional frequency signal. The circuit is from an Electronics Now from April 1993. The JFET can be of any type.
This sensitive electroscope uses a MOSFET 3N128 and can detect electrostatic charges of objects....
This circuit can use both separate LEDs and a common cathode display when one segment indicates...
This circuit dates to 1999, when most vehicles did not yet have electronic ignition and used...