We found this circuit in a 1996 publication. The circuit uses components that are still common. The timing depends on C1, which can be changed, and the triac can be of any type.
This sophisticated RPM counter circuit with discrete integrated circuits was obtained in a 1993...
The circuit shown is a Step-Down converter that can provide a 5 A output with 4 to 18 V inputs. The...
The operational amplifier for this tone control found in a 1977 Signetics manual may be the 741....