This differential amplifier, found in a 1964 Electronic Design, has a gain of 30. The setting can be applied to modern equivalent transistors.
This 80's circuit, suggested in Texas Insrtruments documentation, uses a Hall effect sensor. The...
This circuit, from a publication of the 1970s, can compare the intensity of two light sources,...
This circuit uses a photocell as sensor and a sensitive relay according the power supply voltage....