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.
This circuit can generate signals in the range of 50 kHz to 500 kHz using appropriate crystal. The...
The sensor can be made using two bare wire or two screen with a piece of tisse between them. One...
This circuit was obtained from an American publication of the 80's. The integrated circuit is of the...