The transistor in this project found in the July 1994 issue of Electronics Now magazine can be any general-purpose type. The circuit is powered by the relay voltage up to 12 V.
November 2016 - Already available from Mouser Electronics on Texas Instruments' new DS280BR820...
This circuit is from the 80's component manual itself, but can be applied to more modern...
We found this circuit in the June 1992 issue of Radio Electronics magazine. The sensor is a common...