This circuit was found in a 1967 American documentation. The circuit uses components of the time, applying to analog lines.
This circuit is from a 1965 General Electric publication. It uses a thermistor to fire a reed...
This circuit came out in a publication called Electronics Handbook of 1993. The circuit shows how...
This circuit uses an LM339 and was found in the July 1995 Electronics Now magazine. The LED lights...