The circuit shown was found in an American documentation from 1964. The circuit uses transistors of the time and the indicators are small lamps.
This small amplifier, with output less than 1 W, was found in a 1960 transistor manual from...
We find the original circuit in a technical publication of 1978. Changing the components, such as...
This 3-step level indicator uses only passive components. LEDs are common and trimpots can be 4k7...