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.
The first circuit, shown in the figure, generates a stepped signal whose frequency depends on the...
I found this circuit in a Popular Electronics from November 1999. The magazine is no longer...
The operational used is like the 741 and the voltage gain is unitary. The circuit is from the...