This circuit was obtained from the Encyclopedia of Electronic Circuits volume 5 of 1995. The circuit uses the 4001 CMOS to obtain hysteresis in the sensor feedback.
This simple test for general purpose NPN and PNP transistors was found in a 1974 publication....
This 300 Hz to 3.4 kHz filter was found in the May 1989 Radio Electronics magazine. The circuit...
Found in an English documentation from 1989, this circuit uses components common today. The...