Found in the July 1970 Radio Electronics magazine, this circuit has an adjustable activation time that depends on capacitor C1. The SCR may be a 106 series equivalent.
This circuit was found in a 90s Wels' Think Tank. The frequency depends on C2 and R3. The handset...
Found in an Australian documentation from 1991, this circuit was sold in a kit at the time. It is a...
This unusual circuit from a Popular Electronics of the 90s uses LED indicators, operating as a...