This 4-tube counter was found in Radio Electronics magazine from December 1959. The circuit uses a coil as a sensor to detect the passage of metallic parts or magnets.
This metal detector was found in a 2001 Poptronics, but it can still be easily assembled, as the...
We found this circuit in a 1980 documentation. The flasher uses 3 transistors in the configuration of an...
This current meter with many scales was found in a 80s American documentation. The operational amplifier...