This interesting benchtop circuit was found in the May 1977 issue of Radio Electronics magazine. The circuit has frequency adjustment, and the waveform selected by a switch.
This source of constant current was obtained in an American documentation of 1973. The transistors can be...
When light fails to focus on the sensor, a common photo-transistor, the relay closes its contacts....
This low impedance circuit was found in an Electronic Design from the 90's. Operational amplifiers admit...