This circuit is from old documentation, using a component that cannot be found easily today. The indicator is a galvanometer, and the input voltage is in the order of 1 V.
This receiver for listening to headphones and 2-battery power was found in Japanese documentation from the...
This circuit was found in a Texas Instruments optoelectronics manual and can work with any...
This circuit activates a relay when raindrops fall on the sensor. The circuit is from 1960...