This filter configuration used in telecommunications uses 4 Jaumnann crystals that are interesting for low and medium frequencies. The circuit is from an 80's documentation.
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...