This circuit for low frequency data transmission was found in the May 1977 issue of Radio Electronics magazine. The circuit modulates the signal at two frequencies for bits 0 and 1.
Found in American documentation this circuit uses transistors of the time. The circuit can be made...
This flasher with two power transistors was found in a Radio Electronics magazine from September 1961....
Found in American documentation from 1969, this circuit switches signals with a FET of the time....