This sensitive circuit was found in the August 1971 issue of Radio Electronics magazine. The SCR is equivalent, and the transistor may be a BC548. The load may be a relay.
Analog Devives EVAL AD5941 Electrochemical Evaluation Board This board provided by Mouser...
Found in a Popular Electronics from the 90s, this Wien bridge oscillator uses a single-source...
Found in documentation from the 1980s, this circuit uses an operational amplifier from that time....