This circuit was found in 1980's Radio Electronics. The circuit produces a 1500 Hz modulate 35 kHz. The components used are still common.
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....