This circuit is from an old Mullard (Philips) manual with applications using the 555. Choosing appropriately the resistors and the timing capacitors, the ranges obtained can exceed days.
Using a capacitor as a sensor, this circuit was found in the 1989 Radio Electronics Experimenter magazine....
Maxim MAX20416 Step Down Converters Available through Mouser Electronics these 2.2 MHz converters feature...
This precision circuit was suggested by General Electric in 1980s documentation. The circuit can be...