This circuit was found in a Popular Electronics of the 90s. The operational amplifier admits equivalent and the phototransistor can be of any type.
This powerful amplifier was part of commercial sound circuits from the 90s. The integrated circuit...
This circuit generates a control tone that serves to adjust amplifiers, check audio cables. The...
This circuit locks when the sensors are opened. The relay can be 6 or 12 V depending on the power...