This is the traditional voltage follower configuration with the 741 operational amplifier. The circuit is from 1976 documentation and the power supply must be symmetrical from 6 to 15 V.
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....