We found this circuit in the May 1989 Radio Electronics magazine. The circuit can have its values changed to operate at other frequencies and the power supply must be symmetrical.
This circuit is from Popular Electronics of the 90's. It makes use of NO and NC sensors. The relay...
Based on TTL logic this circuit can vary both the frequency and the pulse width of a rectangular signal....
This circuit was found in an old English publication. The circuit makes use of general-purpose transistors....