This circuit delivers at its output a voltage that proportional to the amplitude of the input signal in a range of 36 dB. We found it in an American documentation from 1966.
This fast trigger circuit allows the counting of up to 300 pulses per minute. The circuit is from...
This intercom uses transistors from the 60s. It was found in a Japanese publication at the time....
This circuit is from an old publication from the 90s, but it can be adapted to operate with LEDs....