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.
This simple polarity indicator for continuous voltages can operate with voltages from 3 to 15 V...
This circuit with RIAA equalization is from 1985 US documentation. The circuit has a flat response...
This circuit, suggested by an Electronics World of 1969, keeps a blinking LED for a while when a...