Seventy-five years ago this month, the June 1945 issue of Radio Craft carried this schematic for a code oscillator sent in by one Gene Clardy of Fort Worth, Texas. According to Clardy, the circuit’s main feature was its safety, since no high voltages were present. With no possibiity of high voltages on the key or headphones, all possible shocks were avoided.

The set used a 6 volt doorbell transformer to light the filament. The secondary of the transformer was also rectified by half of the tube to supply the B+.