This circuit uses an unusual sensor nowadays, but the configuration can be adapted to more modern types, as well as the operational amplifier. The circuit is from an 80's documentation.
In the circuit that uses a PIC16F84, the audio present at the input will have its level controlled by...
August 2017 - STMicroelectronics' new T850 switches with triacs up to 8 A can be obtained from...
This circuit was found in an American documentation from 1969. The circuit is a multi-indicator with...