This circuit was obtained from the July 1970 issue of Radio Electronics magazine. Zeners are power types that can provide a good current to the circuit. Selection is made by rotary switch.
This filter for interference generated by motors via the power grid was found in the June 1972...
The signals from this circuit obtained from the 1965 Radio Shack documentation are picked up from...
This digital instrument input circuit was found in the September 1989 issue of Radio Electronics...